From 492f904772ba452db3b60003e8cf935c9bbeec39 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Mon, 16 Dec 2019 10:31:36 -0600
Subject: [PATCH 01/24] DOC/TST: Indexing with NA raises

---
 asv_bench/benchmarks/indexing.py          |  4 ++
 doc/source/user_guide/boolean.rst         | 23 +++++++
 pandas/core/arrays/boolean.py             | 13 +++-
 pandas/tests/indexing/test_loc.py         |  3 +
 pandas/tests/indexing/test_na_indexing.py | 79 +++++++++++++++++++++++
 5 files changed, 119 insertions(+), 3 deletions(-)
 create mode 100644 pandas/tests/indexing/test_na_indexing.py

diff --git a/asv_bench/benchmarks/indexing.py b/asv_bench/benchmarks/indexing.py
index c78c2fa92827e..f057ab5d6da31 100644
--- a/asv_bench/benchmarks/indexing.py
+++ b/asv_bench/benchmarks/indexing.py
@@ -131,6 +131,7 @@ def setup(self):
         self.col_scalar = columns[10]
         self.bool_indexer = self.df[self.col_scalar] > 0
         self.bool_obj_indexer = self.bool_indexer.astype(object)
+        self.boolean_indexer = (self.df[self.col_scalar] > 0).astype("boolean")
 
     def time_loc(self):
         self.df.loc[self.idx_scalar, self.col_scalar]
@@ -144,6 +145,9 @@ def time_boolean_rows(self):
     def time_boolean_rows_object(self):
         self.df[self.bool_obj_indexer]
 
+    def time_boolean_rows_boolean(self):
+        self.df[self.bool_obj_indexer]
+
 
 class DataFrameNumericIndexing:
     def setup(self):
diff --git a/doc/source/user_guide/boolean.rst b/doc/source/user_guide/boolean.rst
index e0f676d3072fc..46695917bf15b 100644
--- a/doc/source/user_guide/boolean.rst
+++ b/doc/source/user_guide/boolean.rst
@@ -14,6 +14,29 @@ Nullable Boolean Data Type
 
 .. versionadded:: 1.0.0
 
+
+.. _boolean.indexing:
+
+Indexing with NA values
+-----------------------
+
+pandas does not allow indexing with NA values. Attempting to do so
+will raise a ``ValueError``.
+
+.. ipython:: python
+   :okexcept:
+
+   s = pd.Series([1, 2, 3])
+   mask = pd.array([True, False, None])
+   s[mask]
+
+The missing values will need to be explicitly filled with True or False prior
+to using the array as a mask.
+
+.. ipython:: python
+
+   s[mask.fillna(False)]
+
 .. _boolean.kleene:
 
 Kleene Logical Operations
diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index 743d45e1fa400..2ed26be01b978 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -289,6 +289,13 @@ def _from_factorized(cls, values, original: "BooleanArray"):
     def _formatter(self, boxed=False):
         return str
 
+    @property
+    def _hasnans(self):
+        # Note: this is expensive right now! The hope is that we can
+        # make this faster by having an optional mask, but not have to change
+        # source code using it..
+        return self._mask.any()
+
     def __getitem__(self, item):
         if is_integer(item):
             if self._mask[item]:
@@ -311,7 +318,7 @@ def _coerce_to_ndarray(self, dtype=None, na_value: "Scalar" = libmissing.NA):
         if dtype is None:
             dtype = object
         if is_bool_dtype(dtype):
-            if not self.isna().any():
+            if not self._hasnans:
                 return self._data
             else:
                 raise ValueError(
@@ -485,7 +492,7 @@ def astype(self, dtype, copy=True):
 
         if is_bool_dtype(dtype):
             # astype_nansafe converts np.nan to True
-            if self.isna().any():
+            if self._hasnans:
                 raise ValueError("cannot convert float NaN to bool")
             else:
                 return self._data.astype(dtype, copy=copy)
@@ -497,7 +504,7 @@ def astype(self, dtype, copy=True):
             )
         # for integer, error if there are missing values
         if is_integer_dtype(dtype):
-            if self.isna().any():
+            if self._hasnans:
                 raise ValueError("cannot convert NA to integer")
         # for float dtype, ensure we use np.nan before casting (numpy cannot
         # deal with pd.NA)
diff --git a/pandas/tests/indexing/test_loc.py b/pandas/tests/indexing/test_loc.py
index 6f20ec649b200..16a093872cf62 100644
--- a/pandas/tests/indexing/test_loc.py
+++ b/pandas/tests/indexing/test_loc.py
@@ -373,6 +373,9 @@ def test_loc_index(self):
         result = df.loc[mask.values]
         tm.assert_frame_equal(result, expected)
 
+        result = df.loc[pd.array(mask, dtype="boolean")]
+        tm.assert_frame_equal(result, expected)
+
     def test_loc_general(self):
 
         df = DataFrame(
diff --git a/pandas/tests/indexing/test_na_indexing.py b/pandas/tests/indexing/test_na_indexing.py
new file mode 100644
index 0000000000000..550082ca8f53c
--- /dev/null
+++ b/pandas/tests/indexing/test_na_indexing.py
@@ -0,0 +1,79 @@
+import pytest
+
+import pandas as pd
+import pandas.util.testing as tm
+
+
+@pytest.mark.parametrize(
+    "values, dtype",
+    [
+        ([1, 2, 3], "int64"),
+        ([1.0, 2.0, 3.0], "float64"),
+        (["a", "b", "c"], "object"),
+        (["a", "b", "c"], "string"),
+        ([1, 2, 3], "datetime64[ns]"),
+        ([1, 2, 3], "datetime64[ns, CET]"),
+        ([1, 2, 3], "timedelta64[ns]"),
+        (["2000", "2001", "2002"], "Period[D]"),
+        ([1, 0, 3], "Sparse"),
+        ([pd.Interval(0, 1), pd.Interval(1, 2), pd.Interval(3, 4)], "interval"),
+    ],
+)
+@pytest.mark.parametrize(
+    "mask", [[True, False, False], [True, True, True], [False, False, False]]
+)
+@pytest.mark.parametrize("box_mask", [True, False])
+@pytest.mark.parametrize("frame", [True, False])
+def test_series_mask_boolean(values, dtype, mask, box_mask, frame):
+    ser = pd.Series(values, dtype=dtype, index=["a", "b", "c"])
+    if frame:
+        ser = ser.to_frame()
+    mask = pd.array(mask, dtype="boolean")
+    if box_mask:
+        mask = pd.Series(mask, index=ser.index)
+
+    expected = ser[mask.astype("bool")]
+
+    result = ser[mask]
+    tm.assert_equal(result, expected)
+
+    if not box_mask:
+        # Series.iloc[Series[bool]] isn't allowed
+        result = ser.iloc[mask]
+        tm.assert_equal(result, expected)
+
+    result = ser.loc[mask]
+    tm.assert_equal(result, expected)
+
+    # empty
+    mask = mask[:0]
+    ser = ser.iloc[:0]
+    expected = ser[mask.astype("bool")]
+    result = ser[mask]
+    tm.assert_equal(result, expected)
+
+    if not box_mask:
+        # Series.iloc[Series[bool]] isn't allowed
+        result = ser.iloc[mask]
+        tm.assert_equal(result, expected)
+
+    result = ser.loc[mask]
+    tm.assert_equal(result, expected)
+
+
+@pytest.mark.parametrize("frame", [True, False])
+def test_indexing_with_na_raises(frame):
+    s = pd.Series([1, 2, 3], name="name")
+
+    if frame:
+        s = s.to_frame()
+    mask = pd.array([True, False, None], dtype="boolean")
+    match = "cannot index with vector containing NA / NaN values"
+    with pytest.raises(ValueError, match=match):
+        s[mask]
+
+    with pytest.raises(ValueError, match=match):
+        s.loc[mask]
+
+    with pytest.raises(ValueError, match=match):
+        s.iloc[mask]

From 53f4f634cba2fcd6deff46b81238aa629b617918 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 09:06:11 -0600
Subject: [PATCH 02/24] Handle BooleanArray in all EAs

---
 pandas/api/extensions/__init__.py       |  2 ++
 pandas/core/arrays/boolean.py           |  8 ++++++
 pandas/core/arrays/categorical.py       | 13 ++++++---
 pandas/core/arrays/integer.py           |  6 +++++
 pandas/core/arrays/numpy_.py            |  5 ++++
 pandas/core/arrays/sparse/array.py      |  5 +++-
 pandas/core/indexing.py                 | 35 ++++++++++++++++++++-----
 pandas/tests/extension/base/getitem.py  | 31 ++++++++++++++++++++++
 pandas/tests/extension/decimal/array.py |  8 +++++-
 pandas/tests/extension/json/array.py    | 12 ++++++---
 10 files changed, 108 insertions(+), 17 deletions(-)

diff --git a/pandas/api/extensions/__init__.py b/pandas/api/extensions/__init__.py
index 573d700dac43d..d76e4903c4e78 100644
--- a/pandas/api/extensions/__init__.py
+++ b/pandas/api/extensions/__init__.py
@@ -11,3 +11,5 @@
 )
 from pandas.core.algorithms import take  # noqa: F401
 from pandas.core.arrays import ExtensionArray, ExtensionScalarOpsMixin  # noqa: F401
+from pandas.core.common import is_bool_indexer  # noqa: F401
+from pandas.core.indexing import check_bool_array_indexer  # noqa: F401
diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index 2ed26be01b978..b48024da2bcaf 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -29,6 +29,7 @@
 from pandas.core import nanops, ops
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
+from pandas.core.common import is_bool_indexer
 
 if TYPE_CHECKING:
     from pandas._typing import Scalar
@@ -297,10 +298,17 @@ def _hasnans(self):
         return self._mask.any()
 
     def __getitem__(self, item):
+        # import here to avoid circular import. Probably need to restructure
+        from pandas.core.indexing import check_bool_array_indexer
+
         if is_integer(item):
             if self._mask[item]:
                 return self.dtype.na_value
             return self._data[item]
+
+        elif is_bool_indexer(item):
+            item = check_bool_array_indexer(self, item)
+
         return type(self)(self._data[item], self._mask[item])
 
     def _coerce_to_ndarray(self, dtype=None, na_value: "Scalar" = libmissing.NA):
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index 6b422adef2d68..3581c70305013 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -1990,16 +1990,21 @@ def __getitem__(self, key):
         """
         Return an item.
         """
+        from pandas.core.indexing import check_bool_array_indexer
+
         if isinstance(key, (int, np.integer)):
             i = self._codes[key]
             if i == -1:
                 return np.nan
             else:
                 return self.categories[i]
-        else:
-            return self._constructor(
-                values=self._codes[key], dtype=self.dtype, fastpath=True
-            )
+
+        elif com.is_bool_indexer(key):
+            key = check_bool_array_indexer(self, key)
+
+        return self._constructor(
+            values=self._codes[key], dtype=self.dtype, fastpath=True
+        )
 
     def __setitem__(self, key, value):
         """
diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index 3f5a4ca49702f..a3f821b0fff1d 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -26,6 +26,8 @@
 from pandas.core import nanops, ops
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
+from pandas.core.common import is_bool_indexer
+from pandas.core.indexing import check_bool_array_indexer
 from pandas.core.ops import invalid_comparison
 from pandas.core.ops.common import unpack_zerodim_and_defer
 from pandas.core.tools.numeric import to_numeric
@@ -371,6 +373,10 @@ def __getitem__(self, item):
             if self._mask[item]:
                 return self.dtype.na_value
             return self._data[item]
+
+        elif is_bool_indexer(item):
+            item = check_bool_array_indexer(self, item)
+
         return type(self)(self._data[item], self._mask[item])
 
     def _coerce_to_ndarray(self):
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index deec30dfe34ff..4e50bdae636d8 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -17,7 +17,9 @@
 from pandas import compat
 from pandas.core import nanops
 from pandas.core.algorithms import searchsorted, take, unique
+from pandas.core.common import is_bool_indexer
 from pandas.core.construction import extract_array
+from pandas.core.indexing import check_bool_array_indexer
 from pandas.core.missing import backfill_1d, pad_1d
 
 from .base import ExtensionArray, ExtensionOpsMixin
@@ -234,6 +236,9 @@ def __getitem__(self, item):
         if isinstance(item, type(self)):
             item = item._ndarray
 
+        elif is_bool_indexer(item):
+            item = check_bool_array_indexer(self, item)
+
         result = self._ndarray[item]
         if not lib.is_scalar(item):
             result = type(self)(result)
diff --git a/pandas/core/arrays/sparse/array.py b/pandas/core/arrays/sparse/array.py
index 548f2bf702e60..fbfcdd3591a1b 100644
--- a/pandas/core/arrays/sparse/array.py
+++ b/pandas/core/arrays/sparse/array.py
@@ -42,6 +42,7 @@
 from pandas.core.base import PandasObject
 import pandas.core.common as com
 from pandas.core.construction import sanitize_array
+from pandas.core.indexing import check_bool_indexer
 from pandas.core.missing import interpolate_2d
 import pandas.core.ops as ops
 from pandas.core.ops.common import unpack_zerodim_and_defer
@@ -766,7 +767,9 @@ def __getitem__(self, key):
                 else:
                     key = np.asarray(key)
 
-            if com.is_bool_indexer(key) and len(self) == len(key):
+            if com.is_bool_indexer(key):
+                key = check_bool_indexer(self, key)
+
                 return self.take(np.arange(len(key), dtype=np.int32)[key])
             elif hasattr(key, "__len__"):
                 return self.take(key)
diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index b31973de5bca0..f7f37cbae27b0 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -21,6 +21,7 @@
 from pandas.core.dtypes.generic import ABCDataFrame, ABCMultiIndex, ABCSeries
 from pandas.core.dtypes.missing import _infer_fill_value, isna
 
+from pandas._typing import AnyArrayLike
 import pandas.core.common as com
 from pandas.core.indexers import is_list_like_indexer, length_of_indexer
 from pandas.core.indexes.api import Index, InvalidIndexError
@@ -2282,6 +2283,32 @@ def convert_to_index_sliceable(obj, key):
     return None
 
 
+def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndarray:
+    """
+    Check wither `mask` is a valid boolean indexer for `array`.
+
+    `array` and `mask` are checked to have the same length.
+
+    Parameters
+    ----------
+    array : array
+        The array that's being masked.
+    mask : array
+        The boolean array that's masking.
+
+    Returns
+    -------
+    numpy.ndarray
+        The validated boolean mask.
+
+    """
+    result = np.asarray(mask, dtype=bool)
+    # GH26658
+    if len(result) != len(array):
+        raise IndexError(f"Item wrong length {len(result)} instead of {len(array)}.")
+    return result
+
+
 def check_bool_indexer(index: Index, key) -> np.ndarray:
     """
     Check if key is a valid boolean indexer for an object with such index and
@@ -2322,13 +2349,7 @@ def check_bool_indexer(index: Index, key) -> np.ndarray:
     else:
         if is_sparse(result):
             result = result.to_dense()
-        result = np.asarray(result, dtype=bool)
-
-        # GH26658
-        if len(result) != len(index):
-            raise IndexError(
-                "Item wrong length {} instead of {}.".format(len(result), len(index))
-            )
+        result = check_bool_array_indexer(index, result)
 
     return result
 
diff --git a/pandas/tests/extension/base/getitem.py b/pandas/tests/extension/base/getitem.py
index 0e4a86ed44529..007b4e768f608 100644
--- a/pandas/tests/extension/base/getitem.py
+++ b/pandas/tests/extension/base/getitem.py
@@ -121,6 +121,37 @@ def test_getitem_mask(self, data):
         assert len(result) == 1
         assert result.dtype == data.dtype
 
+    def test_getitem_boolenarray_mask(self, data):
+        mask = pd.array(np.zeros(data.shape, dtype="bool"), dtype="boolean")
+        result = data[mask]
+        assert len(result) == 0
+        assert isinstance(result, type(data))
+
+        result = pd.Series(data)[mask]
+        assert len(result) == 0
+        assert result.dtype == data.dtype
+
+        mask[:5] = True
+        expected = data.take([0, 1, 2, 3, 4])
+        result = data[mask]
+        self.assert_extension_array_equal(result, expected)
+
+        expected = pd.Series(expected)
+        result = pd.Series(data)[mask]
+        self.assert_series_equal(result, expected)
+
+    def test_getitem_boolenarray_mask_raises(self, data):
+        mask = pd.array(np.zeros(data.shape, dtype="bool"), dtype="boolean")
+        mask[:2] = pd.NA
+        match = ""
+        with pytest.raises(ValueError, match=match):
+            data[mask]
+
+        s = pd.Series(data)
+
+        with pytest.raises(ValueError, match=match):
+            s[mask]
+
     def test_getitem_slice(self, data):
         # getitem[slice] should return an array
         result = data[slice(0)]  # empty
diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index 74f1e3cfbaf20..f81aae7db0f9e 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -8,7 +8,11 @@
 from pandas.core.dtypes.base import ExtensionDtype
 
 import pandas as pd
-from pandas.api.extensions import register_extension_dtype
+from pandas.api.extensions import (
+    check_bool_array_indexer,
+    is_bool_indexer,
+    register_extension_dtype,
+)
 from pandas.core.arrays import ExtensionArray, ExtensionScalarOpsMixin
 
 
@@ -109,6 +113,8 @@ def __getitem__(self, item):
         if isinstance(item, numbers.Integral):
             return self._data[item]
         else:
+            if is_bool_indexer(item):
+                item = check_bool_array_indexer(self, item)
             return type(self)(self._data[item])
 
     def take(self, indexer, allow_fill=False, fill_value=None):
diff --git a/pandas/tests/extension/json/array.py b/pandas/tests/extension/json/array.py
index 46ca7bd8f760a..9cb5b07c9b910 100644
--- a/pandas/tests/extension/json/array.py
+++ b/pandas/tests/extension/json/array.py
@@ -19,9 +19,12 @@
 
 import numpy as np
 
-from pandas.core.dtypes.base import ExtensionDtype
-
-from pandas.core.arrays import ExtensionArray
+from pandas.api.extensions import (
+    ExtensionArray,
+    ExtensionDtype,
+    check_bool_array_indexer,
+    is_bool_indexer,
+)
 
 
 class JSONDtype(ExtensionDtype):
@@ -76,7 +79,8 @@ def _from_factorized(cls, values, original):
     def __getitem__(self, item):
         if isinstance(item, numbers.Integral):
             return self.data[item]
-        elif isinstance(item, np.ndarray) and item.dtype == "bool":
+        elif is_bool_indexer(item):
+            item = check_bool_array_indexer(self, item)
             return self._from_sequence([x for x, m in zip(self, item) if m])
         elif isinstance(item, abc.Iterable):
             # fancy indexing

From 3bbf868eb1033b9aace90921cef4d8a460ecfa85 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 09:09:48 -0600
Subject: [PATCH 03/24] update

---
 doc/source/reference/extensions.rst | 3 +++
 pandas/core/common.py               | 6 ++++++
 pandas/core/indexing.py             | 3 +++
 3 files changed, 12 insertions(+)

diff --git a/doc/source/reference/extensions.rst b/doc/source/reference/extensions.rst
index 4b1a99da7cd4c..117049ff621eb 100644
--- a/doc/source/reference/extensions.rst
+++ b/doc/source/reference/extensions.rst
@@ -18,6 +18,8 @@ objects.
    api.extensions.register_series_accessor
    api.extensions.register_index_accessor
    api.extensions.ExtensionDtype
+   api.extensions.is_bool_indexer
+   api.extensions.check_bool_array_indexer
 
 .. autosummary::
    :toctree: api/
@@ -26,6 +28,7 @@ objects.
    api.extensions.ExtensionArray
    arrays.PandasArray
 
+
 .. We need this autosummary so that methods and attributes are generated.
 .. Separate block, since they aren't classes.
 
diff --git a/pandas/core/common.py b/pandas/core/common.py
index 9017584171850..186bfb573a6b2 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -110,12 +110,18 @@ def is_bool_indexer(key: Any) -> bool:
     Returns
     -------
     bool
+        Whether `key` is a valid boolean indexer.
 
     Raises
     ------
     ValueError
         When the array is an object-dtype ndarray or ExtensionArray
         and contains missing values.
+
+    See Also
+    --------
+    api.extensions.check_bool_array_indexer : Check that `key`
+        is a valid mask for an array, and convert to an ndarary.
     """
     na_msg = "cannot index with vector containing NA / NaN values"
     if isinstance(key, (ABCSeries, np.ndarray, ABCIndex)) or (
diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index f7f37cbae27b0..f8c698bee98c1 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -2301,6 +2301,9 @@ def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndar
     numpy.ndarray
         The validated boolean mask.
 
+    See Also
+    --------
+    api.extensions.is_bool_indexer : Check if `key` is a boolean indexer.
     """
     result = np.asarray(mask, dtype=bool)
     # GH26658

From a5ac45714cb86b4e639b350913424579119bf518 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 09:13:33 -0600
Subject: [PATCH 04/24] fixups

---
 doc/source/user_guide/boolean.rst | 2 +-
 pandas/core/arrays/boolean.py     | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/doc/source/user_guide/boolean.rst b/doc/source/user_guide/boolean.rst
index 46695917bf15b..6af49695962c6 100644
--- a/doc/source/user_guide/boolean.rst
+++ b/doc/source/user_guide/boolean.rst
@@ -27,7 +27,7 @@ will raise a ``ValueError``.
    :okexcept:
 
    s = pd.Series([1, 2, 3])
-   mask = pd.array([True, False, None])
+   mask = pd.array([True, False, pd.NA], dtyep="boolean")
    s[mask]
 
 The missing values will need to be explicitly filled with True or False prior
diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index b48024da2bcaf..c716c6dd43c65 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -291,7 +291,7 @@ def _formatter(self, boxed=False):
         return str
 
     @property
-    def _hasnans(self):
+    def _hasna(self):
         # Note: this is expensive right now! The hope is that we can
         # make this faster by having an optional mask, but not have to change
         # source code using it..
@@ -326,7 +326,7 @@ def _coerce_to_ndarray(self, dtype=None, na_value: "Scalar" = libmissing.NA):
         if dtype is None:
             dtype = object
         if is_bool_dtype(dtype):
-            if not self._hasnans:
+            if not self._hasna:
                 return self._data
             else:
                 raise ValueError(
@@ -500,7 +500,7 @@ def astype(self, dtype, copy=True):
 
         if is_bool_dtype(dtype):
             # astype_nansafe converts np.nan to True
-            if self._hasnans:
+            if self._hasna:
                 raise ValueError("cannot convert float NaN to bool")
             else:
                 return self._data.astype(dtype, copy=copy)
@@ -512,7 +512,7 @@ def astype(self, dtype, copy=True):
             )
         # for integer, error if there are missing values
         if is_integer_dtype(dtype):
-            if self._hasnans:
+            if self._hasna:
                 raise ValueError("cannot convert NA to integer")
         # for float dtype, ensure we use np.nan before casting (numpy cannot
         # deal with pd.NA)

From 0dfe761ee2255e07470a2cb69df1a0b6ceacf4a4 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 09:56:00 -0600
Subject: [PATCH 05/24] type

---
 pandas/core/arrays/boolean.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index c716c6dd43c65..76d93d7a9b486 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -291,7 +291,7 @@ def _formatter(self, boxed=False):
         return str
 
     @property
-    def _hasna(self):
+    def _hasna(self) -> bool:
         # Note: this is expensive right now! The hope is that we can
         # make this faster by having an optional mask, but not have to change
         # source code using it..

From dac111d0b871d71f42490696b5b2365bbb99f5ec Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 11:25:46 -0600
Subject: [PATCH 06/24] fix benchmark

---
 asv_bench/benchmarks/indexing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/asv_bench/benchmarks/indexing.py b/asv_bench/benchmarks/indexing.py
index f057ab5d6da31..6453649b91270 100644
--- a/asv_bench/benchmarks/indexing.py
+++ b/asv_bench/benchmarks/indexing.py
@@ -146,7 +146,7 @@ def time_boolean_rows_object(self):
         self.df[self.bool_obj_indexer]
 
     def time_boolean_rows_boolean(self):
-        self.df[self.bool_obj_indexer]
+        self.df[self.boolean_indexer]
 
 
 class DataFrameNumericIndexing:

From d1f08d98bc19d45389afe5c394a4949ffeda853b Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 11:40:08 -0600
Subject: [PATCH 07/24] fixup

---
 pandas/core/arrays/integer.py      | 4 +++-
 pandas/core/arrays/numpy_.py       | 4 +++-
 pandas/core/arrays/sparse/array.py | 4 +++-
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index a3f821b0fff1d..67427d9aa9a28 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -27,7 +27,6 @@
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
 from pandas.core.common import is_bool_indexer
-from pandas.core.indexing import check_bool_array_indexer
 from pandas.core.ops import invalid_comparison
 from pandas.core.ops.common import unpack_zerodim_and_defer
 from pandas.core.tools.numeric import to_numeric
@@ -369,6 +368,9 @@ def fmt(x):
         return fmt
 
     def __getitem__(self, item):
+        # Importing this at the top-level causes many unrelated(?) mypy failures
+        from pandas.core.indexing import check_bool_array_indexer
+
         if is_integer(item):
             if self._mask[item]:
                 return self.dtype.na_value
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index 4e50bdae636d8..cc9825f9cbbd0 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -19,7 +19,6 @@
 from pandas.core.algorithms import searchsorted, take, unique
 from pandas.core.common import is_bool_indexer
 from pandas.core.construction import extract_array
-from pandas.core.indexing import check_bool_array_indexer
 from pandas.core.missing import backfill_1d, pad_1d
 
 from .base import ExtensionArray, ExtensionOpsMixin
@@ -233,6 +232,9 @@ def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
     # Pandas ExtensionArray Interface
 
     def __getitem__(self, item):
+        # Avoid mypy failures when importing at the top-level
+        from pandas.core.indexing import check_bool_array_indexer
+
         if isinstance(item, type(self)):
             item = item._ndarray
 
diff --git a/pandas/core/arrays/sparse/array.py b/pandas/core/arrays/sparse/array.py
index fbfcdd3591a1b..adf10642f337a 100644
--- a/pandas/core/arrays/sparse/array.py
+++ b/pandas/core/arrays/sparse/array.py
@@ -42,7 +42,6 @@
 from pandas.core.base import PandasObject
 import pandas.core.common as com
 from pandas.core.construction import sanitize_array
-from pandas.core.indexing import check_bool_indexer
 from pandas.core.missing import interpolate_2d
 import pandas.core.ops as ops
 from pandas.core.ops.common import unpack_zerodim_and_defer
@@ -739,6 +738,9 @@ def value_counts(self, dropna=True):
     # --------
 
     def __getitem__(self, key):
+        # avoid mypy issues when importing at the top-level
+        from pandas.core.indexing import check_bool_indexer
+
         if isinstance(key, tuple):
             if len(key) > 1:
                 raise IndexError("too many indices for array.")

From 3dd59ca020c6c1d168baa0afa792a385b8da9ae7 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Wed, 18 Dec 2019 11:53:28 -0600
Subject: [PATCH 08/24] typo

---
 doc/source/user_guide/boolean.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/user_guide/boolean.rst b/doc/source/user_guide/boolean.rst
index 6af49695962c6..5276bc6142206 100644
--- a/doc/source/user_guide/boolean.rst
+++ b/doc/source/user_guide/boolean.rst
@@ -27,7 +27,7 @@ will raise a ``ValueError``.
    :okexcept:
 
    s = pd.Series([1, 2, 3])
-   mask = pd.array([True, False, pd.NA], dtyep="boolean")
+   mask = pd.array([True, False, pd.NA], dtype="boolean")
    s[mask]
 
 The missing values will need to be explicitly filled with True or False prior

From 151bdfecb5dfcf3994b43e427e6fae1915eee938 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 19 Dec 2019 10:15:46 -0600
Subject: [PATCH 09/24] updates

---
 doc/source/whatsnew/v1.0.0.rst     |  1 +
 pandas/core/arrays/datetimelike.py |  4 +++-
 pandas/core/common.py              | 19 ++++++-------------
 pandas/core/frame.py               |  4 ----
 pandas/core/indexes/base.py        |  4 +++-
 pandas/core/indexes/datetimes.py   |  2 ++
 pandas/core/indexes/multi.py       |  3 ++-
 pandas/core/indexing.py            |  3 +++
 pandas/core/series.py              |  1 +
 pandas/tests/indexes/common.py     |  5 +++++
 10 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index e44fec112c5c1..06f2e95316c62 100755
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -706,6 +706,7 @@ Datetimelike
 - Bug in :attr:`Timestamp.resolution` being a property instead of a class attribute (:issue:`29910`)
 - Bug in :func:`pandas.to_datetime` when called with ``None`` raising ``TypeError`` instead of returning ``NaT`` (:issue:`30011`)
 - Bug in :func:`pandas.to_datetime` failing for `deques` when using ``cache=True`` (the default) (:issue:`29403`)
+- Bug in datetimelike indexes and arrays not validating that the length of a boolean mask matches the array (:issue:`30308`)
 - Bug in :meth:`Series.item` with ``datetime64`` or ``timedelta64`` dtype, :meth:`DatetimeIndex.item`, and :meth:`TimedeltaIndex.item` returning an integer instead of a :class:`Timestamp` or :class:`Timedelta` (:issue:`30175`)
 -
 
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index f5d1e62f44fd0..90e56930bf13b 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -416,7 +416,9 @@ def __getitem__(self, key):
             return self._box_func(val)
 
         if com.is_bool_indexer(key):
-            key = np.asarray(key, dtype=bool)
+            from pandas.core.indexing import check_bool_indexer
+
+            key = check_bool_indexer(self, key)
             if key.all():
                 key = slice(0, None, None)
             else:
diff --git a/pandas/core/common.py b/pandas/core/common.py
index 186bfb573a6b2..dbf0339c5b009 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -112,18 +112,18 @@ def is_bool_indexer(key: Any) -> bool:
     bool
         Whether `key` is a valid boolean indexer.
 
-    Raises
-    ------
-    ValueError
-        When the array is an object-dtype ndarray or ExtensionArray
-        and contains missing values.
+    Notes
+    -----
+    This function is inexpensive for `bool` and `BooleanDtype`.
+    It is expensive for object-dtype backed arrays. In this case
+    a scan of the data to check that all the values are bool is
+    needed.
 
     See Also
     --------
     api.extensions.check_bool_array_indexer : Check that `key`
         is a valid mask for an array, and convert to an ndarary.
     """
-    na_msg = "cannot index with vector containing NA / NaN values"
     if isinstance(key, (ABCSeries, np.ndarray, ABCIndex)) or (
         is_array_like(key) and is_extension_array_dtype(key.dtype)
     ):
@@ -131,16 +131,9 @@ def is_bool_indexer(key: Any) -> bool:
             key = np.asarray(values_from_object(key))
 
             if not lib.is_bool_array(key):
-                if isna(key).any():
-                    raise ValueError(na_msg)
                 return False
             return True
         elif is_bool_dtype(key.dtype):
-            # an ndarray with bool-dtype by definition has no missing values.
-            # So we only need to check for NAs in ExtensionArrays
-            if is_extension_array_dtype(key.dtype):
-                if np.any(key.isna()):
-                    raise ValueError(na_msg)
             return True
     elif isinstance(key, list):
         try:
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index b699961cf07e8..fd3a67fa73620 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2799,10 +2799,6 @@ def _setitem_slice(self, key, value):
     def _setitem_array(self, key, value):
         # also raises Exception if object array with NA values
         if com.is_bool_indexer(key):
-            if len(key) != len(self.index):
-                raise ValueError(
-                    f"Item wrong length {len(key)} instead of {len(self.index)}!"
-                )
             key = check_bool_indexer(self.index, key)
             indexer = key.nonzero()[0]
             self._check_setitem_copy()
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 5abd049b9564c..e370417c8a0fe 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -3998,6 +3998,8 @@ def __getitem__(self, key):
         corresponding `Index` subclass.
 
         """
+        from pandas.core.indexing import check_bool_indexer
+
         # There's no custom logic to be implemented in __getslice__, so it's
         # not overloaded intentionally.
         getitem = self._data.__getitem__
@@ -4013,7 +4015,7 @@ def __getitem__(self, key):
             return promote(getitem(key))
 
         if com.is_bool_indexer(key):
-            key = np.asarray(key, dtype=bool)
+            key = check_bool_indexer(self, key)
 
         key = com.values_from_object(key)
         result = getitem(key)
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 523c434cb7377..1fb2ec181605b 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -1115,6 +1115,8 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
     _has_same_tz = ea_passthrough(DatetimeArray._has_same_tz)
 
     def __getitem__(self, key):
+        # if com.is_bool_indexer(key):
+        #     breakpoint()
         result = self._data.__getitem__(key)
         if is_scalar(result):
             return result
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 9e434d0f5f704..58e227f7364fc 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -42,6 +42,7 @@
     ensure_index,
 )
 from pandas.core.indexes.frozen import FrozenList
+from pandas.core.indexing import check_bool_indexer
 import pandas.core.missing as missing
 from pandas.core.sorting import (
     get_group_index,
@@ -1934,7 +1935,7 @@ def __getitem__(self, key):
             return tuple(retval)
         else:
             if com.is_bool_indexer(key):
-                key = np.asarray(key, dtype=bool)
+                key = check_bool_indexer(self, key)
                 sortorder = self.sortorder
             else:
                 # cannot be sure whether the result will be sorted
diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index f8c698bee98c1..c0edd5e9dffaa 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -1595,6 +1595,7 @@ def _validate_key(self, key, axis: int):
             return
 
         if com.is_bool_indexer(key):
+            # XXX: do we need to verify no NA here?
             return
 
         if not is_list_like_indexer(key):
@@ -1681,6 +1682,7 @@ def _getitem_axis(self, key, axis: int):
             self._validate_key(key, axis)
             return self._get_slice_axis(key, axis=axis)
         elif com.is_bool_indexer(key):
+            # check_bool_indexer is called in getbool_axis
             return self._getbool_axis(key, axis=axis)
         elif is_list_like_indexer(key):
 
@@ -2030,6 +2032,7 @@ def _getitem_axis(self, key, axis: int):
             key = np.asarray(key)
 
         if com.is_bool_indexer(key):
+            # check_bool_indexer is called in _getbool_axis
             self._validate_key(key, axis)
             return self._getbool_axis(key, axis=axis)
 
diff --git a/pandas/core/series.py b/pandas/core/series.py
index 54c163330e6ee..c634fc52bee29 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -879,6 +879,7 @@ def __getitem__(self, key):
             elif key is Ellipsis:
                 return self
             elif com.is_bool_indexer(key):
+                # We check later on.
                 pass
             else:
 
diff --git a/pandas/tests/indexes/common.py b/pandas/tests/indexes/common.py
index 102949fe3f05e..a6103cff1f016 100644
--- a/pandas/tests/indexes/common.py
+++ b/pandas/tests/indexes/common.py
@@ -220,6 +220,11 @@ def test_get_indexer_consistency(self, indices):
         assert isinstance(indexer, np.ndarray)
         assert indexer.dtype == np.intp
 
+    def test_getitem_mask_wrong_length(self, indices):
+        mask = np.array([True])
+        with pytest.raises(IndexError, match="Item wrong length 1"):
+            indices[mask]
+
     def test_ndarray_compat_properties(self):
         idx = self.create_index()
         assert idx.T.equals(idx)

From d57b0ac41f2b84e00f51a4cf1b0ab48cb91c741d Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 19 Dec 2019 12:01:31 -0600
Subject: [PATCH 10/24] Revert "updates"

This reverts commit 151bdfecb5dfcf3994b43e427e6fae1915eee938.
---
 doc/source/whatsnew/v1.0.0.rst     |  1 -
 pandas/core/arrays/datetimelike.py |  4 +---
 pandas/core/common.py              | 19 +++++++++++++------
 pandas/core/frame.py               |  4 ++++
 pandas/core/indexes/base.py        |  4 +---
 pandas/core/indexes/datetimes.py   |  2 --
 pandas/core/indexes/multi.py       |  3 +--
 pandas/core/indexing.py            |  3 ---
 pandas/core/series.py              |  1 -
 pandas/tests/indexes/common.py     |  5 -----
 10 files changed, 20 insertions(+), 26 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index 06f2e95316c62..e44fec112c5c1 100755
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -706,7 +706,6 @@ Datetimelike
 - Bug in :attr:`Timestamp.resolution` being a property instead of a class attribute (:issue:`29910`)
 - Bug in :func:`pandas.to_datetime` when called with ``None`` raising ``TypeError`` instead of returning ``NaT`` (:issue:`30011`)
 - Bug in :func:`pandas.to_datetime` failing for `deques` when using ``cache=True`` (the default) (:issue:`29403`)
-- Bug in datetimelike indexes and arrays not validating that the length of a boolean mask matches the array (:issue:`30308`)
 - Bug in :meth:`Series.item` with ``datetime64`` or ``timedelta64`` dtype, :meth:`DatetimeIndex.item`, and :meth:`TimedeltaIndex.item` returning an integer instead of a :class:`Timestamp` or :class:`Timedelta` (:issue:`30175`)
 -
 
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 90e56930bf13b..f5d1e62f44fd0 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -416,9 +416,7 @@ def __getitem__(self, key):
             return self._box_func(val)
 
         if com.is_bool_indexer(key):
-            from pandas.core.indexing import check_bool_indexer
-
-            key = check_bool_indexer(self, key)
+            key = np.asarray(key, dtype=bool)
             if key.all():
                 key = slice(0, None, None)
             else:
diff --git a/pandas/core/common.py b/pandas/core/common.py
index dbf0339c5b009..186bfb573a6b2 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -112,18 +112,18 @@ def is_bool_indexer(key: Any) -> bool:
     bool
         Whether `key` is a valid boolean indexer.
 
-    Notes
-    -----
-    This function is inexpensive for `bool` and `BooleanDtype`.
-    It is expensive for object-dtype backed arrays. In this case
-    a scan of the data to check that all the values are bool is
-    needed.
+    Raises
+    ------
+    ValueError
+        When the array is an object-dtype ndarray or ExtensionArray
+        and contains missing values.
 
     See Also
     --------
     api.extensions.check_bool_array_indexer : Check that `key`
         is a valid mask for an array, and convert to an ndarary.
     """
+    na_msg = "cannot index with vector containing NA / NaN values"
     if isinstance(key, (ABCSeries, np.ndarray, ABCIndex)) or (
         is_array_like(key) and is_extension_array_dtype(key.dtype)
     ):
@@ -131,9 +131,16 @@ def is_bool_indexer(key: Any) -> bool:
             key = np.asarray(values_from_object(key))
 
             if not lib.is_bool_array(key):
+                if isna(key).any():
+                    raise ValueError(na_msg)
                 return False
             return True
         elif is_bool_dtype(key.dtype):
+            # an ndarray with bool-dtype by definition has no missing values.
+            # So we only need to check for NAs in ExtensionArrays
+            if is_extension_array_dtype(key.dtype):
+                if np.any(key.isna()):
+                    raise ValueError(na_msg)
             return True
     elif isinstance(key, list):
         try:
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index fd3a67fa73620..b699961cf07e8 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2799,6 +2799,10 @@ def _setitem_slice(self, key, value):
     def _setitem_array(self, key, value):
         # also raises Exception if object array with NA values
         if com.is_bool_indexer(key):
+            if len(key) != len(self.index):
+                raise ValueError(
+                    f"Item wrong length {len(key)} instead of {len(self.index)}!"
+                )
             key = check_bool_indexer(self.index, key)
             indexer = key.nonzero()[0]
             self._check_setitem_copy()
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index e370417c8a0fe..5abd049b9564c 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -3998,8 +3998,6 @@ def __getitem__(self, key):
         corresponding `Index` subclass.
 
         """
-        from pandas.core.indexing import check_bool_indexer
-
         # There's no custom logic to be implemented in __getslice__, so it's
         # not overloaded intentionally.
         getitem = self._data.__getitem__
@@ -4015,7 +4013,7 @@ def __getitem__(self, key):
             return promote(getitem(key))
 
         if com.is_bool_indexer(key):
-            key = check_bool_indexer(self, key)
+            key = np.asarray(key, dtype=bool)
 
         key = com.values_from_object(key)
         result = getitem(key)
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 1fb2ec181605b..523c434cb7377 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -1115,8 +1115,6 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
     _has_same_tz = ea_passthrough(DatetimeArray._has_same_tz)
 
     def __getitem__(self, key):
-        # if com.is_bool_indexer(key):
-        #     breakpoint()
         result = self._data.__getitem__(key)
         if is_scalar(result):
             return result
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 58e227f7364fc..9e434d0f5f704 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -42,7 +42,6 @@
     ensure_index,
 )
 from pandas.core.indexes.frozen import FrozenList
-from pandas.core.indexing import check_bool_indexer
 import pandas.core.missing as missing
 from pandas.core.sorting import (
     get_group_index,
@@ -1935,7 +1934,7 @@ def __getitem__(self, key):
             return tuple(retval)
         else:
             if com.is_bool_indexer(key):
-                key = check_bool_indexer(self, key)
+                key = np.asarray(key, dtype=bool)
                 sortorder = self.sortorder
             else:
                 # cannot be sure whether the result will be sorted
diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index c0edd5e9dffaa..f8c698bee98c1 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -1595,7 +1595,6 @@ def _validate_key(self, key, axis: int):
             return
 
         if com.is_bool_indexer(key):
-            # XXX: do we need to verify no NA here?
             return
 
         if not is_list_like_indexer(key):
@@ -1682,7 +1681,6 @@ def _getitem_axis(self, key, axis: int):
             self._validate_key(key, axis)
             return self._get_slice_axis(key, axis=axis)
         elif com.is_bool_indexer(key):
-            # check_bool_indexer is called in getbool_axis
             return self._getbool_axis(key, axis=axis)
         elif is_list_like_indexer(key):
 
@@ -2032,7 +2030,6 @@ def _getitem_axis(self, key, axis: int):
             key = np.asarray(key)
 
         if com.is_bool_indexer(key):
-            # check_bool_indexer is called in _getbool_axis
             self._validate_key(key, axis)
             return self._getbool_axis(key, axis=axis)
 
diff --git a/pandas/core/series.py b/pandas/core/series.py
index c634fc52bee29..54c163330e6ee 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -879,7 +879,6 @@ def __getitem__(self, key):
             elif key is Ellipsis:
                 return self
             elif com.is_bool_indexer(key):
-                # We check later on.
                 pass
             else:
 
diff --git a/pandas/tests/indexes/common.py b/pandas/tests/indexes/common.py
index a6103cff1f016..102949fe3f05e 100644
--- a/pandas/tests/indexes/common.py
+++ b/pandas/tests/indexes/common.py
@@ -220,11 +220,6 @@ def test_get_indexer_consistency(self, indices):
         assert isinstance(indexer, np.ndarray)
         assert indexer.dtype == np.intp
 
-    def test_getitem_mask_wrong_length(self, indices):
-        mask = np.array([True])
-        with pytest.raises(IndexError, match="Item wrong length 1"):
-            indices[mask]
-
     def test_ndarray_compat_properties(self):
         idx = self.create_index()
         assert idx.T.equals(idx)

From 36be0f6ec7777c6901d3741e99cae3c8dbb87afd Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 20 Dec 2019 08:52:34 -0600
Subject: [PATCH 11/24] examples

---
 pandas/core/indexing.py | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index f8c698bee98c1..8ee3800901023 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -2285,9 +2285,10 @@ def convert_to_index_sliceable(obj, key):
 
 def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndarray:
     """
-    Check wither `mask` is a valid boolean indexer for `array`.
+    Check if `mask` is a valid boolean indexer for `array`.
 
-    `array` and `mask` are checked to have the same length.
+    `array` and `mask` are checked to have the same length, and the
+    dtype is validated.
 
     Parameters
     ----------
@@ -2301,9 +2302,42 @@ def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndar
     numpy.ndarray
         The validated boolean mask.
 
+    Raises
+    ------
+    IndexError
+        When the lengths don't match.
+    ValueError
+        When `mask` cannot be converted to a bool-dtype ndarray.
+
     See Also
     --------
     api.extensions.is_bool_indexer : Check if `key` is a boolean indexer.
+
+    Examples
+    --------
+    A boolean ndarray is returned when the arguments are all valid.
+
+    >>> mask = pd.array([True, False])
+    >>> arr = pd.Series([1, 2])
+    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
+    array([ True, False])
+
+    An IndexError is raised when the lengths don't match.
+
+    >>> mask = pd.array([True, False, True])
+    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
+    Traceback (most recent call last):
+    ...
+    IndexError: Item wrong length 3 instead of 2.
+
+    A ValueError is raised when the mask cannot be converted to
+    a bool-dtype ndarray.
+
+    >>> mask = pd.array([True, pd.NA])
+    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
+    Traceback (most recent call last):
+    ...
+    ValueError: cannot convert to bool numpy array in presence of missing values
     """
     result = np.asarray(mask, dtype=bool)
     # GH26658

From 7bd6c2fe8971e5a72934b8b8efee5615807d277f Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 20 Dec 2019 09:00:46 -0600
Subject: [PATCH 12/24] restore datetime fix

---
 doc/source/whatsnew/v1.0.0.rst         |  1 +
 pandas/core/arrays/datetimelike.py     |  4 +++-
 pandas/tests/extension/base/getitem.py | 14 +++++++++++---
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index e44fec112c5c1..61a6f50ef3ecb 100755
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -703,6 +703,7 @@ Datetimelike
 - Bug in :func:`pandas._config.localization.get_locales` where the ``locales -a`` encodes the locales list as windows-1252 (:issue:`23638`, :issue:`24760`, :issue:`27368`)
 - Bug in :meth:`Series.var` failing to raise ``TypeError`` when called with ``timedelta64[ns]`` dtype (:issue:`28289`)
 - Bug in :meth:`DatetimeIndex.strftime` and :meth:`Series.dt.strftime` where ``NaT`` was converted to the string ``'NaT'`` instead of ``np.nan`` (:issue:`29578`)
+- Bug in masking datetime-like arrays with a boolean mask of an incorrect length not raising an ``IndexError`` (:issue:`30308`)
 - Bug in :attr:`Timestamp.resolution` being a property instead of a class attribute (:issue:`29910`)
 - Bug in :func:`pandas.to_datetime` when called with ``None`` raising ``TypeError`` instead of returning ``NaT`` (:issue:`30011`)
 - Bug in :func:`pandas.to_datetime` failing for `deques` when using ``cache=True`` (the default) (:issue:`29403`)
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index f5d1e62f44fd0..6fc8040455a1e 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -416,7 +416,9 @@ def __getitem__(self, key):
             return self._box_func(val)
 
         if com.is_bool_indexer(key):
-            key = np.asarray(key, dtype=bool)
+            from pandas.core.indexing import check_bool_array_indexer
+
+            key = check_bool_array_indexer(self, key)
             if key.all():
                 key = slice(0, None, None)
             else:
diff --git a/pandas/tests/extension/base/getitem.py b/pandas/tests/extension/base/getitem.py
index 007b4e768f608..a5b2bfee8287b 100644
--- a/pandas/tests/extension/base/getitem.py
+++ b/pandas/tests/extension/base/getitem.py
@@ -121,6 +121,15 @@ def test_getitem_mask(self, data):
         assert len(result) == 1
         assert result.dtype == data.dtype
 
+    def test_getitem_mask_raises(self, data):
+        mask = np.array([True, False])
+        with pytest.raises(IndexError):
+            data[mask]
+
+        mask = pd.array(mask, dtype="boolean")
+        with pytest.raises(IndexError):
+            data[mask]
+
     def test_getitem_boolenarray_mask(self, data):
         mask = pd.array(np.zeros(data.shape, dtype="bool"), dtype="boolean")
         result = data[mask]
@@ -143,13 +152,12 @@ def test_getitem_boolenarray_mask(self, data):
     def test_getitem_boolenarray_mask_raises(self, data):
         mask = pd.array(np.zeros(data.shape, dtype="bool"), dtype="boolean")
         mask[:2] = pd.NA
-        match = ""
-        with pytest.raises(ValueError, match=match):
+        with pytest.raises(ValueError):
             data[mask]
 
         s = pd.Series(data)
 
-        with pytest.raises(ValueError, match=match):
+        with pytest.raises(ValueError):
             s[mask]
 
     def test_getitem_slice(self, data):

From 505112eb3d3b45e3fd19b178022ff2e94537ed3d Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Sat, 28 Dec 2019 10:47:27 -0600
Subject: [PATCH 13/24] update error message

---
 pandas/core/common.py                  | 2 +-
 pandas/tests/extension/base/getitem.py | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/common.py b/pandas/core/common.py
index 186bfb573a6b2..2dabc098517f7 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -123,7 +123,7 @@ def is_bool_indexer(key: Any) -> bool:
     api.extensions.check_bool_array_indexer : Check that `key`
         is a valid mask for an array, and convert to an ndarary.
     """
-    na_msg = "cannot index with vector containing NA / NaN values"
+    na_msg = "cannot mask with array containing NA / NaN values"
     if isinstance(key, (ABCSeries, np.ndarray, ABCIndex)) or (
         is_array_like(key) and is_extension_array_dtype(key.dtype)
     ):
diff --git a/pandas/tests/extension/base/getitem.py b/pandas/tests/extension/base/getitem.py
index 639489ae591e9..dc1f62c4c97c5 100644
--- a/pandas/tests/extension/base/getitem.py
+++ b/pandas/tests/extension/base/getitem.py
@@ -130,7 +130,7 @@ def test_getitem_mask_raises(self, data):
         with pytest.raises(IndexError):
             data[mask]
 
-    def test_getitem_boolenarray_mask(self, data):
+    def test_getitem_boolean_array_mask(self, data):
         mask = pd.array(np.zeros(data.shape, dtype="bool"), dtype="boolean")
         result = data[mask]
         assert len(result) == 0
@@ -149,7 +149,7 @@ def test_getitem_boolenarray_mask(self, data):
         result = pd.Series(data)[mask]
         self.assert_series_equal(result, expected)
 
-    def test_getitem_boolenarray_mask_raises(self, data):
+    def test_getitem_boolean_array_mask_raises(self, data):
         mask = pd.array(np.zeros(data.shape, dtype="bool"), dtype="boolean")
         mask[:2] = pd.NA
         with pytest.raises(ValueError):

From c73ae8e2048260e05615599b780b3c0aecaca83b Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Sat, 28 Dec 2019 10:53:06 -0600
Subject: [PATCH 14/24] checks

---
 ci/code_checks.sh             | 2 +-
 pandas/core/arrays/boolean.py | 4 ++--
 pandas/core/arrays/integer.py | 4 ++--
 pandas/core/arrays/numpy_.py  | 4 ++--
 4 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/ci/code_checks.sh b/ci/code_checks.sh
index 46ace2dd9d70e..4a4887124885c 100755
--- a/ci/code_checks.sh
+++ b/ci/code_checks.sh
@@ -121,7 +121,7 @@ if [[ -z "$CHECK" || "$CHECK" == "patterns" ]]; then
     # Check for imports from pandas.core.common instead of `import pandas.core.common as com`
     # Check for imports from collections.abc instead of `from collections import abc`
     MSG='Check for non-standard imports' ; echo $MSG
-    invgrep -R --include="*.py*" -E "from pandas.core.common import" pandas
+    invgrep -R --include="*.py*" --exclude="__init__.py" -E "from pandas.core.common import" pandas
     RET=$(($RET + $?)) ; echo $MSG "DONE"
     invgrep -R --include="*.py*" -E "from pandas.core import common" pandas
     RET=$(($RET + $?)) ; echo $MSG "DONE"
diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index 66f85a9c934b2..d998b64690670 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -29,7 +29,7 @@
 from pandas.core import nanops, ops
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
-from pandas.core.common import is_bool_indexer
+import pandas.core.common as com
 
 if TYPE_CHECKING:
     from pandas._typing import Scalar
@@ -324,7 +324,7 @@ def __getitem__(self, item):
                 return self.dtype.na_value
             return self._data[item]
 
-        elif is_bool_indexer(item):
+        elif com.is_bool_indexer(item):
             item = check_bool_array_indexer(self, item)
 
         return type(self)(self._data[item], self._mask[item])
diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index 67427d9aa9a28..7fdd31979d0a5 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -26,7 +26,7 @@
 from pandas.core import nanops, ops
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
-from pandas.core.common import is_bool_indexer
+import pandas.core.common as com
 from pandas.core.ops import invalid_comparison
 from pandas.core.ops.common import unpack_zerodim_and_defer
 from pandas.core.tools.numeric import to_numeric
@@ -376,7 +376,7 @@ def __getitem__(self, item):
                 return self.dtype.na_value
             return self._data[item]
 
-        elif is_bool_indexer(item):
+        elif com.is_bool_indexer(item):
             item = check_bool_array_indexer(self, item)
 
         return type(self)(self._data[item], self._mask[item])
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index cc9825f9cbbd0..79c0a1a7f8df5 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -17,7 +17,7 @@
 from pandas import compat
 from pandas.core import nanops
 from pandas.core.algorithms import searchsorted, take, unique
-from pandas.core.common import is_bool_indexer
+import pandas.core.common as com
 from pandas.core.construction import extract_array
 from pandas.core.missing import backfill_1d, pad_1d
 
@@ -238,7 +238,7 @@ def __getitem__(self, item):
         if isinstance(item, type(self)):
             item = item._ndarray
 
-        elif is_bool_indexer(item):
+        elif com.is_bool_indexer(item):
             item = check_bool_array_indexer(self, item)
 
         result = self._ndarray[item]

From f94483fe037ae56c983f7238fe24d034e117221a Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Mon, 30 Dec 2019 06:32:21 -0600
Subject: [PATCH 15/24] update for error message

---
 pandas/tests/indexing/test_na_indexing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/indexing/test_na_indexing.py b/pandas/tests/indexing/test_na_indexing.py
index 550082ca8f53c..4b92df581d164 100644
--- a/pandas/tests/indexing/test_na_indexing.py
+++ b/pandas/tests/indexing/test_na_indexing.py
@@ -68,7 +68,7 @@ def test_indexing_with_na_raises(frame):
     if frame:
         s = s.to_frame()
     mask = pd.array([True, False, None], dtype="boolean")
-    match = "cannot index with vector containing NA / NaN values"
+    match = "cannot mask with array containing NA / NaN values"
     with pytest.raises(ValueError, match=match):
         s[mask]
 

From 8b1e56788b5a8832d7e41c123e7d66626df38d28 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Mon, 30 Dec 2019 09:05:04 -0600
Subject: [PATCH 16/24] update isort

---
 .pre-commit-config.yaml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index b34f5dfdd1a83..88548f6c2f678 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -11,7 +11,7 @@ repos:
         language: python_venv
         additional_dependencies: [flake8-comprehensions>=3.1.0]
 -   repo: https://github.com/pre-commit/mirrors-isort
-    rev: v4.3.20
+    rev: v4.3.21
     hooks:
     -   id: isort
         language: python_venv

From f317c64d1afa9eda64222073ef2376b0acb199ba Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Mon, 30 Dec 2019 09:06:17 -0600
Subject: [PATCH 17/24] isort

---
 pandas/core/indexing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 62fc1e3397c11..6fc04b7025e7f 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -4,6 +4,7 @@
 
 from pandas._libs.indexing import _NDFrameIndexerBase
 from pandas._libs.lib import item_from_zerodim
+from pandas._typing import AnyArrayLike
 from pandas.errors import AbstractMethodError
 from pandas.util._decorators import Appender
 
@@ -21,7 +22,6 @@
 from pandas.core.dtypes.generic import ABCDataFrame, ABCMultiIndex, ABCSeries
 from pandas.core.dtypes.missing import _infer_fill_value, isna
 
-from pandas._typing import AnyArrayLike
 import pandas.core.common as com
 from pandas.core.indexers import is_list_like_indexer, length_of_indexer
 from pandas.core.indexes.api import Index, InvalidIndexError

From c6562921aaeffb4f03622906925cddb937bb9907 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Mon, 30 Dec 2019 09:58:04 -0600
Subject: [PATCH 18/24] fixup

---
 pandas/tests/series/indexing/test_boolean.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/series/indexing/test_boolean.py b/pandas/tests/series/indexing/test_boolean.py
index c2912cf3ce53f..925d657d7dd04 100644
--- a/pandas/tests/series/indexing/test_boolean.py
+++ b/pandas/tests/series/indexing/test_boolean.py
@@ -75,7 +75,7 @@ def test_getitem_boolean_object(string_series):
 
     # nans raise exception
     omask[5:10] = np.nan
-    msg = "cannot index with vector containing NA / NaN values"
+    msg = "cannot mask with array containing NA / NaN values"
     with pytest.raises(ValueError, match=msg):
         s[omask]
     with pytest.raises(ValueError, match=msg):

From 37ea95e569ff43c4e5d9e3c579df6797832efd23 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Tue, 31 Dec 2019 14:12:00 -0600
Subject: [PATCH 19/24] fixup

---
 ci/code_checks.sh                       |  2 +-
 doc/source/reference/extensions.rst     |  3 --
 pandas/api/extensions/__init__.py       |  2 -
 pandas/core/arrays/boolean.py           |  2 +-
 pandas/core/arrays/categorical.py       |  2 +-
 pandas/core/arrays/datetimelike.py      |  2 +-
 pandas/core/arrays/integer.py           |  2 +-
 pandas/core/arrays/numpy_.py            |  2 +-
 pandas/core/common.py                   |  4 +-
 pandas/core/indexers.py                 | 65 +++++++++++++++++++++++
 pandas/core/indexing.py                 | 70 ++-----------------------
 pandas/tests/extension/decimal/array.py |  8 ++-
 pandas/tests/extension/json/array.py    |  9 ++--
 13 files changed, 84 insertions(+), 89 deletions(-)

diff --git a/ci/code_checks.sh b/ci/code_checks.sh
index 4a4887124885c..46ace2dd9d70e 100755
--- a/ci/code_checks.sh
+++ b/ci/code_checks.sh
@@ -121,7 +121,7 @@ if [[ -z "$CHECK" || "$CHECK" == "patterns" ]]; then
     # Check for imports from pandas.core.common instead of `import pandas.core.common as com`
     # Check for imports from collections.abc instead of `from collections import abc`
     MSG='Check for non-standard imports' ; echo $MSG
-    invgrep -R --include="*.py*" --exclude="__init__.py" -E "from pandas.core.common import" pandas
+    invgrep -R --include="*.py*" -E "from pandas.core.common import" pandas
     RET=$(($RET + $?)) ; echo $MSG "DONE"
     invgrep -R --include="*.py*" -E "from pandas.core import common" pandas
     RET=$(($RET + $?)) ; echo $MSG "DONE"
diff --git a/doc/source/reference/extensions.rst b/doc/source/reference/extensions.rst
index 117049ff621eb..4b1a99da7cd4c 100644
--- a/doc/source/reference/extensions.rst
+++ b/doc/source/reference/extensions.rst
@@ -18,8 +18,6 @@ objects.
    api.extensions.register_series_accessor
    api.extensions.register_index_accessor
    api.extensions.ExtensionDtype
-   api.extensions.is_bool_indexer
-   api.extensions.check_bool_array_indexer
 
 .. autosummary::
    :toctree: api/
@@ -28,7 +26,6 @@ objects.
    api.extensions.ExtensionArray
    arrays.PandasArray
 
-
 .. We need this autosummary so that methods and attributes are generated.
 .. Separate block, since they aren't classes.
 
diff --git a/pandas/api/extensions/__init__.py b/pandas/api/extensions/__init__.py
index d76e4903c4e78..573d700dac43d 100644
--- a/pandas/api/extensions/__init__.py
+++ b/pandas/api/extensions/__init__.py
@@ -11,5 +11,3 @@
 )
 from pandas.core.algorithms import take  # noqa: F401
 from pandas.core.arrays import ExtensionArray, ExtensionScalarOpsMixin  # noqa: F401
-from pandas.core.common import is_bool_indexer  # noqa: F401
-from pandas.core.indexing import check_bool_array_indexer  # noqa: F401
diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index cc3a954b0489f..807fb4547a261 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -317,7 +317,7 @@ def _hasna(self) -> bool:
 
     def __getitem__(self, item):
         # import here to avoid circular import. Probably need to restructure
-        from pandas.core.indexing import check_bool_array_indexer
+        from pandas.core.indexers import check_bool_array_indexer
 
         if is_integer(item):
             if self._mask[item]:
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index 016257340b47f..c0b04e4edb163 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -1990,7 +1990,7 @@ def __getitem__(self, key):
         """
         Return an item.
         """
-        from pandas.core.indexing import check_bool_array_indexer
+        from pandas.core.indexers import check_bool_array_indexer
 
         if isinstance(key, (int, np.integer)):
             i = self._codes[key]
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index ff230fc1d79c4..3c7d90084d633 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -437,7 +437,7 @@ def __getitem__(self, key):
             return type(self)(val, dtype=self.dtype)
 
         if com.is_bool_indexer(key):
-            from pandas.core.indexing import check_bool_array_indexer
+            from pandas.core.indexers import check_bool_array_indexer
 
             key = check_bool_array_indexer(self, key)
             if key.all():
diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index 8a0caa79fd576..49447a5099706 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -366,7 +366,7 @@ def _from_factorized(cls, values, original):
 
     def __getitem__(self, item):
         # Importing this at the top-level causes many unrelated(?) mypy failures
-        from pandas.core.indexing import check_bool_array_indexer
+        from pandas.core.indexers import check_bool_array_indexer
 
         if is_integer(item):
             if self._mask[item]:
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index 79c0a1a7f8df5..b75d47515cf78 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -233,7 +233,7 @@ def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
 
     def __getitem__(self, item):
         # Avoid mypy failures when importing at the top-level
-        from pandas.core.indexing import check_bool_array_indexer
+        from pandas.core.indexers import check_bool_array_indexer
 
         if isinstance(item, type(self)):
             item = item._ndarray
diff --git a/pandas/core/common.py b/pandas/core/common.py
index a2fb399e7ff43..f0fcb736586d6 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -121,8 +121,8 @@ def is_bool_indexer(key: Any) -> bool:
 
     See Also
     --------
-    api.extensions.check_bool_array_indexer : Check that `key`
-        is a valid mask for an array, and convert to an ndarary.
+    check_bool_array_indexer : Check that `key`
+        is a valid mask for an array, and convert to an ndarray.
     """
     na_msg = "cannot mask with array containing NA / NaN values"
     if isinstance(key, (ABCSeries, np.ndarray, ABCIndex)) or (
diff --git a/pandas/core/indexers.py b/pandas/core/indexers.py
index f75087ca3b505..b6d4f5b04e86c 100644
--- a/pandas/core/indexers.py
+++ b/pandas/core/indexers.py
@@ -3,6 +3,8 @@
 """
 import numpy as np
 
+from pandas._typing import AnyArrayLike
+
 from pandas.core.dtypes.common import is_list_like
 from pandas.core.dtypes.generic import ABCIndexClass, ABCSeries
 
@@ -240,3 +242,66 @@ def length_of_indexer(indexer, target=None) -> int:
     elif not is_list_like_indexer(indexer):
         return 1
     raise AssertionError("cannot find the length of the indexer")
+
+
+def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndarray:
+    """
+    Check if `mask` is a valid boolean indexer for `array`.
+
+    `array` and `mask` are checked to have the same length, and the
+    dtype is validated.
+
+    Parameters
+    ----------
+    array : array
+        The array that's being masked.
+    mask : array
+        The boolean array that's masking.
+
+    Returns
+    -------
+    numpy.ndarray
+        The validated boolean mask.
+
+    Raises
+    ------
+    IndexError
+        When the lengths don't match.
+    ValueError
+        When `mask` cannot be converted to a bool-dtype ndarray.
+
+    See Also
+    --------
+    api.extensions.is_bool_indexer : Check if `key` is a boolean indexer.
+
+    Examples
+    --------
+    A boolean ndarray is returned when the arguments are all valid.
+
+    >>> mask = pd.array([True, False])
+    >>> arr = pd.Series([1, 2])
+    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
+    array([ True, False])
+
+    An IndexError is raised when the lengths don't match.
+
+    >>> mask = pd.array([True, False, True])
+    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
+    Traceback (most recent call last):
+    ...
+    IndexError: Item wrong length 3 instead of 2.
+
+    A ValueError is raised when the mask cannot be converted to
+    a bool-dtype ndarray.
+
+    >>> mask = pd.array([True, pd.NA])
+    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
+    Traceback (most recent call last):
+    ...
+    ValueError: cannot convert to bool numpy array in presence of missing values
+    """
+    result = np.asarray(mask, dtype=bool)
+    # GH26658
+    if len(result) != len(array):
+        raise IndexError(f"Item wrong length {len(result)} instead of {len(array)}.")
+    return result
diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 32bffb8e31cfa..b15d91240e7bb 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -4,7 +4,6 @@
 
 from pandas._libs.indexing import _NDFrameIndexerBase
 from pandas._libs.lib import item_from_zerodim
-from pandas._typing import AnyArrayLike
 from pandas.errors import AbstractMethodError
 from pandas.util._decorators import Appender
 
@@ -23,7 +22,11 @@
 from pandas.core.dtypes.missing import _infer_fill_value, isna
 
 import pandas.core.common as com
-from pandas.core.indexers import is_list_like_indexer, length_of_indexer
+from pandas.core.indexers import (
+    check_bool_array_indexer,
+    is_list_like_indexer,
+    length_of_indexer,
+)
 from pandas.core.indexes.api import Index, InvalidIndexError
 
 
@@ -2270,69 +2273,6 @@ def convert_to_index_sliceable(obj, key):
     return None
 
 
-def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndarray:
-    """
-    Check if `mask` is a valid boolean indexer for `array`.
-
-    `array` and `mask` are checked to have the same length, and the
-    dtype is validated.
-
-    Parameters
-    ----------
-    array : array
-        The array that's being masked.
-    mask : array
-        The boolean array that's masking.
-
-    Returns
-    -------
-    numpy.ndarray
-        The validated boolean mask.
-
-    Raises
-    ------
-    IndexError
-        When the lengths don't match.
-    ValueError
-        When `mask` cannot be converted to a bool-dtype ndarray.
-
-    See Also
-    --------
-    api.extensions.is_bool_indexer : Check if `key` is a boolean indexer.
-
-    Examples
-    --------
-    A boolean ndarray is returned when the arguments are all valid.
-
-    >>> mask = pd.array([True, False])
-    >>> arr = pd.Series([1, 2])
-    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
-    array([ True, False])
-
-    An IndexError is raised when the lengths don't match.
-
-    >>> mask = pd.array([True, False, True])
-    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
-    Traceback (most recent call last):
-    ...
-    IndexError: Item wrong length 3 instead of 2.
-
-    A ValueError is raised when the mask cannot be converted to
-    a bool-dtype ndarray.
-
-    >>> mask = pd.array([True, pd.NA])
-    >>> pd.api.extensions.check_bool_array_indexer(arr, mask)
-    Traceback (most recent call last):
-    ...
-    ValueError: cannot convert to bool numpy array in presence of missing values
-    """
-    result = np.asarray(mask, dtype=bool)
-    # GH26658
-    if len(result) != len(array):
-        raise IndexError(f"Item wrong length {len(result)} instead of {len(array)}.")
-    return result
-
-
 def check_bool_indexer(index: Index, key) -> np.ndarray:
     """
     Check if key is a valid boolean indexer for an object with such index and
diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index f81aae7db0f9e..94695bb3d306d 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -8,12 +8,10 @@
 from pandas.core.dtypes.base import ExtensionDtype
 
 import pandas as pd
-from pandas.api.extensions import (
-    check_bool_array_indexer,
-    is_bool_indexer,
-    register_extension_dtype,
-)
+from pandas.api.extensions import register_extension_dtype
 from pandas.core.arrays import ExtensionArray, ExtensionScalarOpsMixin
+from pandas.core.common import is_bool_indexer
+from pandas.core.indexers import check_bool_array_indexer
 
 
 @register_extension_dtype
diff --git a/pandas/tests/extension/json/array.py b/pandas/tests/extension/json/array.py
index a37f28c114fca..dabcd02a76c76 100644
--- a/pandas/tests/extension/json/array.py
+++ b/pandas/tests/extension/json/array.py
@@ -19,12 +19,9 @@
 
 import numpy as np
 
-from pandas.api.extensions import (
-    ExtensionArray,
-    ExtensionDtype,
-    check_bool_array_indexer,
-    is_bool_indexer,
-)
+from pandas.api.extensions import ExtensionArray, ExtensionDtype
+from pandas.core.common import is_bool_indexer
+from pandas.core.indexers import check_bool_array_indexer
 
 
 class JSONDtype(ExtensionDtype):

From 21fd589d5003578340314e884b4cdcbbb0d6b71d Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 2 Jan 2020 07:42:38 -0600
Subject: [PATCH 20/24] update arrayo

---
 pandas/tests/extension/decimal/array.py | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index 94695bb3d306d..770b703def87d 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -10,7 +10,6 @@
 import pandas as pd
 from pandas.api.extensions import register_extension_dtype
 from pandas.core.arrays import ExtensionArray, ExtensionScalarOpsMixin
-from pandas.core.common import is_bool_indexer
 from pandas.core.indexers import check_bool_array_indexer
 
 
@@ -111,8 +110,13 @@ def __getitem__(self, item):
         if isinstance(item, numbers.Integral):
             return self._data[item]
         else:
-            if is_bool_indexer(item):
-                item = check_bool_array_indexer(self, item)
+            # array, slice.
+            if pd.api.types.is_list_like(item):
+                if not pd.api.types.is_array_like(item):
+                    item = pd.array(item)
+                dtype = item.dtype
+                if pd.api.types.is_bool_dtype(dtype):
+                    item = check_bool_array_indexer(self, item)
             return type(self)(self._data[item])
 
     def take(self, indexer, allow_fill=False, fill_value=None):

From 36370705735430601892bd67145ca22072233c96 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 2 Jan 2020 07:55:03 -0600
Subject: [PATCH 21/24] doc

---
 doc/source/reference/extensions.rst     | 9 +++++++++
 pandas/api/indexers/__init__.py         | 1 +
 pandas/core/indexers.py                 | 2 ++
 pandas/tests/extension/decimal/array.py | 3 +--
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/doc/source/reference/extensions.rst b/doc/source/reference/extensions.rst
index 4b1a99da7cd4c..6d4eff8c97052 100644
--- a/doc/source/reference/extensions.rst
+++ b/doc/source/reference/extensions.rst
@@ -59,3 +59,12 @@ objects.
       api.extensions.ExtensionArray.nbytes
       api.extensions.ExtensionArray.ndim
       api.extensions.ExtensionArray.shape
+
+Additionally, we have some utility methods for ensuring your object
+behaves correctly.
+
+.. autosummary::
+  :toctree: api/
+
+  api.indexers.check_bool_array_indexer
+
diff --git a/pandas/api/indexers/__init__.py b/pandas/api/indexers/__init__.py
index a5d6bc07da3eb..64383d8ecbd24 100644
--- a/pandas/api/indexers/__init__.py
+++ b/pandas/api/indexers/__init__.py
@@ -1,2 +1,3 @@
 """Public API for Rolling Window Indexers"""
+from pandas.core.indexers import check_bool_array_indexer  # noqa: F401
 from pandas.core.window.indexers import BaseIndexer  # noqa: F401
diff --git a/pandas/core/indexers.py b/pandas/core/indexers.py
index b6d4f5b04e86c..0f932f7b849e3 100644
--- a/pandas/core/indexers.py
+++ b/pandas/core/indexers.py
@@ -251,6 +251,8 @@ def check_bool_array_indexer(array: AnyArrayLike, mask: AnyArrayLike) -> np.ndar
     `array` and `mask` are checked to have the same length, and the
     dtype is validated.
 
+    .. versionadded:: 1.0.0
+
     Parameters
     ----------
     array : array
diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index 770b703def87d..af3775c8b174d 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -10,7 +10,6 @@
 import pandas as pd
 from pandas.api.extensions import register_extension_dtype
 from pandas.core.arrays import ExtensionArray, ExtensionScalarOpsMixin
-from pandas.core.indexers import check_bool_array_indexer
 
 
 @register_extension_dtype
@@ -116,7 +115,7 @@ def __getitem__(self, item):
                     item = pd.array(item)
                 dtype = item.dtype
                 if pd.api.types.is_bool_dtype(dtype):
-                    item = check_bool_array_indexer(self, item)
+                    item = pd.api.indexers.check_bool_array_indexer(self, item)
             return type(self)(self._data[item])
 
     def take(self, indexer, allow_fill=False, fill_value=None):

From 61599f2aa762bcaf83095f2277a96d44d36e7a8c Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 2 Jan 2020 07:56:14 -0600
Subject: [PATCH 22/24] integer

---
 pandas/tests/extension/decimal/array.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index af3775c8b174d..570cdf5f29d00 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -116,6 +116,8 @@ def __getitem__(self, item):
                 dtype = item.dtype
                 if pd.api.types.is_bool_dtype(dtype):
                     item = pd.api.indexers.check_bool_array_indexer(self, item)
+                elif pd.api.types.is_integer_dtype(dtype):
+                    item = np.asarray(item, dtype="int")
             return type(self)(self._data[item])
 
     def take(self, indexer, allow_fill=False, fill_value=None):

From e62282645b9794730ef1281006cf3debbe526f0c Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 2 Jan 2020 08:07:38 -0600
Subject: [PATCH 23/24] fixup

---
 doc/source/reference/extensions.rst | 1 -
 pandas/core/arrays/boolean.py       | 4 +---
 pandas/core/arrays/categorical.py   | 3 +--
 pandas/core/arrays/datetimelike.py  | 3 +--
 pandas/core/arrays/integer.py       | 4 +---
 pandas/core/arrays/numpy_.py        | 4 +---
 6 files changed, 5 insertions(+), 14 deletions(-)

diff --git a/doc/source/reference/extensions.rst b/doc/source/reference/extensions.rst
index 6d4eff8c97052..16a84b5d2ecaf 100644
--- a/doc/source/reference/extensions.rst
+++ b/doc/source/reference/extensions.rst
@@ -67,4 +67,3 @@ behaves correctly.
   :toctree: api/
 
   api.indexers.check_bool_array_indexer
-
diff --git a/pandas/core/arrays/boolean.py b/pandas/core/arrays/boolean.py
index 807fb4547a261..102150b1cbce1 100644
--- a/pandas/core/arrays/boolean.py
+++ b/pandas/core/arrays/boolean.py
@@ -30,6 +30,7 @@
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
 import pandas.core.common as com
+from pandas.core.indexers import check_bool_array_indexer
 
 if TYPE_CHECKING:
     from pandas._typing import Scalar
@@ -316,9 +317,6 @@ def _hasna(self) -> bool:
         return self._mask.any()
 
     def __getitem__(self, item):
-        # import here to avoid circular import. Probably need to restructure
-        from pandas.core.indexers import check_bool_array_indexer
-
         if is_integer(item):
             if self._mask[item]:
                 return self.dtype.na_value
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index e00ddc8589b84..8c78676834786 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -49,6 +49,7 @@
 from pandas.core.base import NoNewAttributesMixin, PandasObject, _shared_docs
 import pandas.core.common as com
 from pandas.core.construction import array, extract_array, sanitize_array
+from pandas.core.indexers import check_bool_array_indexer
 from pandas.core.missing import interpolate_2d
 from pandas.core.ops.common import unpack_zerodim_and_defer
 from pandas.core.sorting import nargsort
@@ -1990,8 +1991,6 @@ def __getitem__(self, key):
         """
         Return an item.
         """
-        from pandas.core.indexers import check_bool_array_indexer
-
         if isinstance(key, (int, np.integer)):
             i = self._codes[key]
             if i == -1:
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 156fa006bf351..2bdd9acaeb70f 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -40,6 +40,7 @@
 from pandas.core import missing, nanops
 from pandas.core.algorithms import checked_add_with_arr, take, unique1d, value_counts
 import pandas.core.common as com
+from pandas.core.indexers import check_bool_array_indexer
 from pandas.core.ops.common import unpack_zerodim_and_defer
 from pandas.core.ops.invalid import make_invalid_op
 
@@ -436,8 +437,6 @@ def __getitem__(self, key):
             return type(self)(val, dtype=self.dtype)
 
         if com.is_bool_indexer(key):
-            from pandas.core.indexers import check_bool_array_indexer
-
             key = check_bool_array_indexer(self, key)
             if key.all():
                 key = slice(0, None, None)
diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index 49447a5099706..0922f4ac6f71d 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -27,6 +27,7 @@
 from pandas.core.algorithms import take
 from pandas.core.arrays import ExtensionArray, ExtensionOpsMixin
 import pandas.core.common as com
+from pandas.core.indexers import check_bool_array_indexer
 from pandas.core.ops import invalid_comparison
 from pandas.core.ops.common import unpack_zerodim_and_defer
 from pandas.core.tools.numeric import to_numeric
@@ -365,9 +366,6 @@ def _from_factorized(cls, values, original):
         return integer_array(values, dtype=original.dtype)
 
     def __getitem__(self, item):
-        # Importing this at the top-level causes many unrelated(?) mypy failures
-        from pandas.core.indexers import check_bool_array_indexer
-
         if is_integer(item):
             if self._mask[item]:
                 return self.dtype.na_value
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index b75d47515cf78..a114be9a21c6c 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -19,6 +19,7 @@
 from pandas.core.algorithms import searchsorted, take, unique
 import pandas.core.common as com
 from pandas.core.construction import extract_array
+from pandas.core.indexers import check_bool_array_indexer
 from pandas.core.missing import backfill_1d, pad_1d
 
 from .base import ExtensionArray, ExtensionOpsMixin
@@ -232,9 +233,6 @@ def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
     # Pandas ExtensionArray Interface
 
     def __getitem__(self, item):
-        # Avoid mypy failures when importing at the top-level
-        from pandas.core.indexers import check_bool_array_indexer
-
         if isinstance(item, type(self)):
             item = item._ndarray
 

From 5004d91cfa3c897e0748de6bca8f2b9e7d92b329 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 2 Jan 2020 09:02:57 -0600
Subject: [PATCH 24/24] fixup

---
 pandas/tests/extension/json/array.py | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/pandas/tests/extension/json/array.py b/pandas/tests/extension/json/array.py
index dabcd02a76c76..17bc2773aad19 100644
--- a/pandas/tests/extension/json/array.py
+++ b/pandas/tests/extension/json/array.py
@@ -19,9 +19,8 @@
 
 import numpy as np
 
+import pandas as pd
 from pandas.api.extensions import ExtensionArray, ExtensionDtype
-from pandas.core.common import is_bool_indexer
-from pandas.core.indexers import check_bool_array_indexer
 
 
 class JSONDtype(ExtensionDtype):
@@ -76,18 +75,21 @@ def _from_factorized(cls, values, original):
     def __getitem__(self, item):
         if isinstance(item, numbers.Integral):
             return self.data[item]
-        elif is_bool_indexer(item):
-            item = check_bool_array_indexer(self, item)
-            return self._from_sequence([x for x, m in zip(self, item) if m])
-        elif isinstance(item, abc.Iterable):
-            # fancy indexing
-            return type(self)([self.data[i] for i in item])
         elif isinstance(item, slice) and item == slice(None):
             # Make sure we get a view
             return type(self)(self.data)
-        else:
+        elif isinstance(item, slice):
             # slice
             return type(self)(self.data[item])
+        else:
+            if not pd.api.types.is_array_like(item):
+                item = pd.array(item)
+            dtype = item.dtype
+            if pd.api.types.is_bool_dtype(dtype):
+                item = pd.api.indexers.check_bool_array_indexer(self, item)
+                return self._from_sequence([x for x, m in zip(self, item) if m])
+            # integer
+            return type(self)([self.data[i] for i in item])
 
     def __setitem__(self, key, value):
         if isinstance(key, numbers.Integral):