From a0b8c4cf205623a76f0e9e37bc20c73ee49e96af Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Wed, 20 Mar 2019 09:18:09 -0500
Subject: [PATCH 01/13] #25790 Updated type hints to Python3 style in files
 outside core/arrays/

---
 pandas/core/base.py                 |  6 ++----
 pandas/core/common.py               |  3 +--
 pandas/core/dtypes/base.py          | 17 ++++++-----------
 pandas/core/frame.py                | 14 ++++++++------
 pandas/core/groupby/groupby.py      | 14 ++++++--------
 pandas/core/indexes/base.py         |  3 +--
 pandas/core/indexes/datetimelike.py |  5 ++---
 pandas/core/indexes/period.py       |  2 +-
 pandas/core/internals/blocks.py     | 12 +++++++-----
 pandas/core/internals/managers.py   |  4 ++--
 10 files changed, 36 insertions(+), 44 deletions(-)

diff --git a/pandas/core/base.py b/pandas/core/base.py
index ce91c232bb92d..9951e9eacb1c6 100644
--- a/pandas/core/base.py
+++ b/pandas/core/base.py
@@ -786,8 +786,7 @@ def base(self):
         return self.values.base
 
     @property
-    def array(self):
-        # type: () -> ExtensionArray
+    def array(self) -> ExtensionArray:
         """
         The ExtensionArray of the data backing this Series or Index.
 
@@ -962,8 +961,7 @@ def to_numpy(self, dtype=None, copy=False):
         return result
 
     @property
-    def _ndarray_values(self):
-        # type: () -> np.ndarray
+    def _ndarray_values(self) -> np.ndarray:
         """
         The data as an ndarray, possibly losing information.
 
diff --git a/pandas/core/common.py b/pandas/core/common.py
index 77b7b94e7a1f7..43c9aea0e0a0e 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -93,8 +93,7 @@ def maybe_box_datetimelike(value):
 values_from_object = lib.values_from_object
 
 
-def is_bool_indexer(key):
-    # type: (Any) -> bool
+def is_bool_indexer(key: Any) -> bool:
     """
     Check whether `key` is a valid boolean indexer.
 
diff --git a/pandas/core/dtypes/base.py b/pandas/core/dtypes/base.py
index 8269f8c88ffd3..7b06e20420a98 100644
--- a/pandas/core/dtypes/base.py
+++ b/pandas/core/dtypes/base.py
@@ -1,5 +1,5 @@
 """Extend pandas with custom array types"""
-from typing import List, Optional, Type
+from typing import List, Optional
 
 import numpy as np
 
@@ -65,8 +65,7 @@ def __ne__(self, other):
         return not self.__eq__(other)
 
     @property
-    def names(self):
-        # type: () -> Optional[List[str]]
+    def names(self) -> Optional[List[str]]:
         """Ordered list of field names, or None if there are no fields.
 
         This is for compatibility with NumPy arrays, and may be removed in the
@@ -116,8 +115,7 @@ def is_dtype(cls, dtype):
             return False
 
     @property
-    def _is_numeric(self):
-        # type: () -> bool
+    def _is_numeric(self) -> bool:
         """
         Whether columns with this dtype should be considered numeric.
 
@@ -128,8 +126,7 @@ def _is_numeric(self):
         return False
 
     @property
-    def _is_boolean(self):
-        # type: () -> bool
+    def _is_boolean(self) -> bool:
         """
         Whether this dtype should be considered boolean.
 
@@ -212,8 +209,7 @@ def __str__(self):
         return self.name
 
     @property
-    def type(self):
-        # type: () -> Type
+    def type(self) -> bool:
         """
         The scalar type for the array, e.g. ``int``
 
@@ -242,8 +238,7 @@ def kind(self):
         return 'O'
 
     @property
-    def name(self):
-        # type: () -> str
+    def name(self) -> str:
         """
         A string identifying the data type.
 
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index ec881c730c365..855846d722ef9 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -19,7 +19,7 @@
 import sys
 import warnings
 from textwrap import dedent
-from typing import List, Union
+from typing import List, TypeVar, Union
 
 import numpy as np
 import numpy.ma as ma
@@ -284,6 +284,10 @@
     Index(['value'], dtype='object')
 """
 
+
+DF = TypeVar('DF', bound='DataFrame')
+
+
 # -----------------------------------------------------------------------
 # DataFrame class
 
@@ -6243,11 +6247,9 @@ def diff(self, periods=1, axis=0):
     # Function application
 
     def _gotitem(self,
-                 key,           # type: Union[str, List[str]]
-                 ndim,          # type: int
-                 subset=None    # type: Union[Series, DataFrame, None]
-                 ):
-        # type: (...) -> Union[Series, DataFrame]
+                 key: Union[str, List[str]],
+                 ndim: int,
+                 subset: Union[Series, DF, None] = None) -> Union[Series, DF]:
         """
         Sub-classes to define. Return a sliced object.
 
diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index 96eafbfae2cdb..c2fad0af19b8f 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -1040,8 +1040,7 @@ def _bool_agg(self, val_test, skipna):
         Shared func to call any / all Cython GroupBy implementations.
         """
 
-        def objs_to_bool(vals):
-            # type: (np.ndarray) -> (np.ndarray, Type)
+        def objs_to_bool(vals: np.ndarray) -> types.Tuple[np.ndarray, Type]:
             if is_object_dtype(vals):
                 vals = np.array([bool(x) for x in vals])
             else:
@@ -1049,8 +1048,7 @@ def objs_to_bool(vals):
 
             return vals.view(np.uint8), np.bool
 
-        def result_to_bool(result, inference):
-            # type: (np.ndarray, Type) -> np.ndarray
+        def result_to_bool(result: np.ndarray, inference: Type) -> np.ndarray:
             return result.astype(inference, copy=False)
 
         return self._get_cythonized_result('group_any_all', self.grouper,
@@ -1738,8 +1736,8 @@ def quantile(self, q=0.5, interpolation='linear'):
         b    3.0
         """
 
-        def pre_processor(vals):
-            # type: (np.ndarray) -> (np.ndarray, Optional[Type])
+        def pre_processor(vals: np.ndarray) -> \
+                types.Tuple[np.ndarray, Optional[Type]]:
             if is_object_dtype(vals):
                 raise TypeError("'quantile' cannot be performed against "
                                 "'object' dtypes!")
@@ -1753,8 +1751,8 @@ def pre_processor(vals):
 
             return vals, inference
 
-        def post_processor(vals, inference):
-            # type: (np.ndarray, Optional[Type]) -> np.ndarray
+        def post_processor(vals: np.ndarray, inference: Optional[Type]) -> \
+                np.ndarray:
             if inference:
                 # Check for edge case
                 if not (is_integer_dtype(inference) and
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index a3eed90ab5d80..ea475e335ce0d 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -3632,8 +3632,7 @@ def values(self):
         return self._data.view(np.ndarray)
 
     @property
-    def _values(self):
-        # type: () -> Union[ExtensionArray, Index, np.ndarray]
+    def _values(self) -> Union[ExtensionArray, Index, np.ndarray]:
         # TODO(EA): remove index types as they become extension arrays
         """
         The best array representation.
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 830f234b85757..4575081f52b71 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -56,7 +56,7 @@ class DatetimeIndexOpsMixin(ExtensionOpsMixin):
     """
     common ops mixin to support a unified interface datetimelike Index
     """
-    _data = None  # type: DatetimeLikeArrayMixin
+    _data: DatetimeLikeArrayMixin = None
 
     # DatetimeLikeArrayMixin assumes subclasses are mutable, so these are
     # properties there.  They can be made into cache_readonly for Index
@@ -129,8 +129,7 @@ def _ndarray_values(self):
     # Abstract data attributes
 
     @property
-    def values(self):
-        # type: () -> np.ndarray
+    def values(self) -> np.ndarray:
         # Note: PeriodArray overrides this to return an ndarray of objects.
         return self._data._data
 
diff --git a/pandas/core/indexes/period.py b/pandas/core/indexes/period.py
index a4bd7f9017eb4..ae18f63fe3ba4 100644
--- a/pandas/core/indexes/period.py
+++ b/pandas/core/indexes/period.py
@@ -170,7 +170,7 @@ class PeriodIndex(DatetimeIndexOpsMixin, Int64Index, PeriodDelegateMixin):
     _is_numeric_dtype = False
     _infer_as_myclass = True
 
-    _data = None  # type: PeriodArray
+    _data: PeriodArray = None
 
     _engine_type = libindex.PeriodEngine
 
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index a61bc30a126e6..c79cf90a9c447 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -3,7 +3,7 @@
 import functools
 import inspect
 import re
-from typing import Any, List
+from typing import Any, List, Optional, TypeVar
 import warnings
 
 import numpy as np
@@ -1646,6 +1646,9 @@ def _get_unstack_items(self, unstacker, new_columns):
         return new_placement, new_values, mask
 
 
+EB = TypeVar('EB', bound='ExtensionBlock')
+
+
 class ExtensionBlock(NonConsolidatableMixIn, Block):
     """Block for holding extension types.
 
@@ -1828,10 +1831,9 @@ def interpolate(self, method='pad', axis=0, inplace=False, limit=None,
             placement=self.mgr_locs)
 
     def shift(self,
-              periods,                  # type: int
-              axis=0,                   # type: libinternals.BlockPlacement
-              fill_value=None):         # type: Any
-        # type: (...) -> List[ExtensionBlock]
+              periods: int,
+              axis: libinternals.BlockPlacement = 0,
+              fill_value: Optional[Any] = None) -> List[EB]:
         """
         Shift the block by `periods`.
 
diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index b9d478f3f14eb..06dd0596a6183 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -1863,8 +1863,8 @@ def _shape_compat(x):
     return stacked, placement
 
 
-def _interleaved_dtype(blocks):
-    # type: (List[Block]) -> Optional[Union[np.dtype, ExtensionDtype]]
+def _interleaved_dtype(blocks: List[Block]) \
+        -> Optional[Union[np.dtype, ExtensionDtype]]:
     """Find the common dtype for `blocks`.
 
     Parameters

From 46cc9090da5231c3032049fce408c54c8a938ac2 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Wed, 20 Mar 2019 09:22:19 -0500
Subject: [PATCH 02/13] #25790 Added TypeVar to hint in core/indexes/base.py

---
 pandas/core/indexes/base.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index ea475e335ce0d..6ec94b995d284 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -1,7 +1,7 @@
 from datetime import datetime, timedelta
 import operator
 from textwrap import dedent
-from typing import Union
+from typing import TypeVar, Union
 import warnings
 
 import numpy as np
@@ -164,6 +164,9 @@ def _new_Index(cls, d):
     return cls.__new__(cls, **d)
 
 
+INDEXTYPE = TypeVar('INDEXTYPE', bound='Index')
+
+
 class Index(IndexOpsMixin, PandasObject):
     """
     Immutable ndarray implementing an ordered, sliceable set. The basic object
@@ -3632,7 +3635,7 @@ def values(self):
         return self._data.view(np.ndarray)
 
     @property
-    def _values(self) -> Union[ExtensionArray, Index, np.ndarray]:
+    def _values(self) -> Union[ExtensionArray, INDEXTYPE, np.ndarray]:
         # TODO(EA): remove index types as they become extension arrays
         """
         The best array representation.

From 7acb89aff3d409d97f740aa30f3f97c3c7021ada Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Wed, 20 Mar 2019 10:12:15 -0500
Subject: [PATCH 03/13] Fixed reference to types.Tuple that should have been to
 typing.Tuple

---
 pandas/core/groupby/groupby.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index c2fad0af19b8f..25bbdd81d0a89 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -12,7 +12,7 @@ class providing the base-class of operations.
 import datetime
 from functools import partial, wraps
 import types
-from typing import Optional, Type
+from typing import Optional, Type, Tuple
 import warnings
 
 import numpy as np
@@ -1040,7 +1040,7 @@ def _bool_agg(self, val_test, skipna):
         Shared func to call any / all Cython GroupBy implementations.
         """
 
-        def objs_to_bool(vals: np.ndarray) -> types.Tuple[np.ndarray, Type]:
+        def objs_to_bool(vals: np.ndarray) -> Tuple[np.ndarray, Type]:
             if is_object_dtype(vals):
                 vals = np.array([bool(x) for x in vals])
             else:
@@ -1737,7 +1737,7 @@ def quantile(self, q=0.5, interpolation='linear'):
         """
 
         def pre_processor(vals: np.ndarray) -> \
-                types.Tuple[np.ndarray, Optional[Type]]:
+                Tuple[np.ndarray, Optional[Type]]:
             if is_object_dtype(vals):
                 raise TypeError("'quantile' cannot be performed against "
                                 "'object' dtypes!")

From d9c53abbc44c97cfc76b247be6fbcce1db906ec9 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Wed, 20 Mar 2019 10:43:12 -0500
Subject: [PATCH 04/13] Fixed import sort

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

diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index 25bbdd81d0a89..c5a1a78093b2b 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -12,7 +12,7 @@ class providing the base-class of operations.
 import datetime
 from functools import partial, wraps
 import types
-from typing import Optional, Type, Tuple
+from typing import Optional, Tuple, Type
 import warnings
 
 import numpy as np

From e73fe18eee729d2cac0aa7c9ad5aa17d945373d1 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Wed, 20 Mar 2019 12:19:16 -0500
Subject: [PATCH 05/13] #25802 Updated type hints to python3 syntax (array
 files)

---
 pandas/core/arrays/array_.py       | 10 ++---
 pandas/core/arrays/base.py         | 69 +++++++++++++++---------------
 pandas/core/arrays/datetimelike.py | 34 ++++++++-------
 pandas/core/arrays/datetimes.py    |  5 +--
 pandas/core/arrays/integer.py      |  6 +--
 pandas/core/arrays/period.py       | 32 +++++++-------
 pandas/core/arrays/sparse.py       | 25 +++++++----
 7 files changed, 95 insertions(+), 86 deletions(-)

diff --git a/pandas/core/arrays/array_.py b/pandas/core/arrays/array_.py
index 254ab876af1ac..ff58f848701d5 100644
--- a/pandas/core/arrays/array_.py
+++ b/pandas/core/arrays/array_.py
@@ -4,6 +4,7 @@
 
 from pandas._libs import lib, tslibs
 
+from pandas.core.arrays.base import ExtensionArray
 from pandas.core.dtypes.common import (
     is_datetime64_ns_dtype, is_extension_array_dtype, is_timedelta64_ns_dtype)
 from pandas.core.dtypes.dtypes import ExtensionDtype, registry
@@ -11,11 +12,10 @@
 from pandas import compat
 
 
-def array(data,         # type: Sequence[object]
-          dtype=None,   # type: Optional[Union[str, np.dtype, ExtensionDtype]]
-          copy=True,    # type: bool
-          ):
-    # type: (...) -> ExtensionArray
+def array(data: Sequence[object],
+          dtype: Optional[Union[str, np.dtype, ExtensionDtype]] = None,
+          copy: bool = True,
+          ) -> ExtensionArray:
     """
     Create an array.
 
diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index f7d427ce26e6a..53656c4d3ba77 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -214,8 +214,7 @@ def __getitem__(self, item):
         """
         raise AbstractMethodError(self)
 
-    def __setitem__(self, key, value):
-        # type: (Union[int, np.ndarray], Any) -> None
+    def __setitem__(self, key: Union[int, np.ndarray], value: Any) -> None:
         """
         Set one or more values inplace.
 
@@ -262,8 +261,7 @@ def __setitem__(self, key, value):
             type(self), '__setitem__')
         )
 
-    def __len__(self):
-        # type: () -> int
+    def __len__(self) -> int:
         """
         Length of this array
 
@@ -287,32 +285,28 @@ def __iter__(self):
     # Required attributes
     # ------------------------------------------------------------------------
     @property
-    def dtype(self):
-        # type: () -> ExtensionDtype
+    def dtype(self) -> ExtensionDtype:
         """
         An instance of 'ExtensionDtype'.
         """
         raise AbstractMethodError(self)
 
     @property
-    def shape(self):
-        # type: () -> Tuple[int, ...]
+    def shape(self) -> Tuple[int, ...]:
         """
         Return a tuple of the array dimensions.
         """
         return (len(self),)
 
     @property
-    def ndim(self):
-        # type: () -> int
+    def ndim(self) -> int:
         """
         Extension Arrays are only allowed to be 1-dimensional.
         """
         return 1
 
     @property
-    def nbytes(self):
-        # type: () -> int
+    def nbytes(self) -> int:
         """
         The number of bytes needed to store this object in memory.
         """
@@ -343,8 +337,7 @@ def astype(self, dtype, copy=True):
         """
         return np.array(self, dtype=dtype, copy=copy)
 
-    def isna(self):
-        # type: () -> Union[ExtensionArray, np.ndarray]
+    def isna(self) -> Union['ExtensionArray', np.ndarray]:
         """
         A 1-D array indicating if each value is missing.
 
@@ -366,8 +359,7 @@ def isna(self):
         """
         raise AbstractMethodError(self)
 
-    def _values_for_argsort(self):
-        # type: () -> np.ndarray
+    def _values_for_argsort(self) -> np.ndarray:
         """
         Return values for sorting.
 
@@ -482,8 +474,11 @@ def dropna(self):
         """
         return self[~self.isna()]
 
-    def shift(self, periods=1, fill_value=None):
-        # type: (int, object) -> ExtensionArray
+    def shift(
+            self,
+            periods: int = 1,
+            fill_value: object = None,
+    ) -> 'ExtensionArray':
         """
         Shift values by desired number.
 
@@ -598,8 +593,7 @@ def searchsorted(self, value, side="left", sorter=None):
         arr = self.astype(object)
         return arr.searchsorted(value, side=side, sorter=sorter)
 
-    def _values_for_factorize(self):
-        # type: () -> Tuple[np.ndarray, Any]
+    def _values_for_factorize(self) -> Tuple[np.ndarray, Any]:
         """
         Return an array and missing value suitable for factorization.
 
@@ -623,8 +617,10 @@ def _values_for_factorize(self):
         """
         return self.astype(object), np.nan
 
-    def factorize(self, na_sentinel=-1):
-        # type: (int) -> Tuple[np.ndarray, ExtensionArray]
+    def factorize(
+            self,
+            na_sentinel: int = -1,
+    ) -> Tuple[np.ndarray, 'ExtensionArray']:
         """
         Encode the extension array as an enumerated type.
 
@@ -726,8 +722,12 @@ def repeat(self, repeats, axis=None):
     # Indexing methods
     # ------------------------------------------------------------------------
 
-    def take(self, indices, allow_fill=False, fill_value=None):
-        # type: (Sequence[int], bool, Optional[Any]) -> ExtensionArray
+    def take(
+            self,
+            indices: Sequence[int],
+            allow_fill: bool = False,
+            fill_value: Optional[Any] = None
+    ) -> 'ExtensionArray':
         """
         Take elements from an array.
 
@@ -816,8 +816,7 @@ def take(self, indices, allow_fill=False, fill_value=None):
         # pandas.api.extensions.take
         raise AbstractMethodError(self)
 
-    def copy(self, deep=False):
-        # type: (bool) -> ExtensionArray
+    def copy(self, deep: bool = False) -> 'ExtensionArray':
         """
         Return a copy of the array.
 
@@ -853,8 +852,10 @@ def __repr__(self):
                                length=len(self),
                                dtype=self.dtype)
 
-    def _formatter(self, boxed=False):
-        # type: (bool) -> Callable[[Any], Optional[str]]
+    def _formatter(
+            self,
+            boxed: bool = False,
+    ) -> Callable[[Any], Optional[str]]:
         """Formatting function for scalar values.
 
         This is used in the default '__repr__'. The returned formatting
@@ -881,8 +882,7 @@ def _formatter(self, boxed=False):
             return str
         return repr
 
-    def _formatting_values(self):
-        # type: () -> np.ndarray
+    def _formatting_values(self) -> np.ndarray:
         # At the moment, this has to be an array since we use result.dtype
         """
         An array of values to be printed in, e.g. the Series repr
@@ -898,8 +898,10 @@ def _formatting_values(self):
     # ------------------------------------------------------------------------
 
     @classmethod
-    def _concat_same_type(cls, to_concat):
-        # type: (Sequence[ExtensionArray]) -> ExtensionArray
+    def _concat_same_type(
+            cls,
+            to_concat: Sequence['ExtensionArray']
+    ) -> 'ExtensionArray':
         """
         Concatenate multiple array
 
@@ -921,8 +923,7 @@ def _concat_same_type(cls, to_concat):
     _can_hold_na = True
 
     @property
-    def _ndarray_values(self):
-        # type: () -> np.ndarray
+    def _ndarray_values(self) -> np.ndarray:
         """
         Internal pandas method for lossy conversion to a NumPy ndarray.
 
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 27d7d4f888550..9e6602daa92bb 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -58,8 +58,7 @@ def _get_attributes_dict(self):
         return {k: getattr(self, k, None) for k in self._attributes}
 
     @property
-    def _scalar_type(self):
-        # type: () -> Union[type, Tuple[type]]
+    def _scalar_type(self) -> Union[type, Tuple[type]]:
         """The scalar associated with this datelike
 
         * PeriodArray : Period
@@ -68,8 +67,10 @@ def _scalar_type(self):
         """
         raise AbstractMethodError(self)
 
-    def _scalar_from_string(self, value):
-        # type: (str) -> Union[Period, Timestamp, Timedelta, NaTType]
+    def _scalar_from_string(
+            self,
+            value: str,
+    ) -> Union[Period, Timestamp, Timedelta, NaTType]:
         """
         Construct a scalar type from a string.
 
@@ -89,8 +90,10 @@ def _scalar_from_string(self, value):
         """
         raise AbstractMethodError(self)
 
-    def _unbox_scalar(self, value):
-        # type: (Union[Period, Timestamp, Timedelta, NaTType]) -> int
+    def _unbox_scalar(
+            self,
+            value: Union[Period, Timestamp, Timedelta, NaTType],
+    ) -> int:
         """
         Unbox the integer value of a scalar `value`.
 
@@ -109,8 +112,10 @@ def _unbox_scalar(self, value):
         """
         raise AbstractMethodError(self)
 
-    def _check_compatible_with(self, other):
-        # type: (Union[Period, Timestamp, Timedelta, NaTType]) -> None
+    def _check_compatible_with(
+            self,
+            other: Union[Period, Timestamp, Timedelta, NaTType],
+    ) -> None:
         """
         Verify that `self` and `other` are compatible.
 
@@ -350,8 +355,7 @@ def __iter__(self):
         return (self._box_func(v) for v in self.asi8)
 
     @property
-    def asi8(self):
-        # type: () -> np.ndarray
+    def asi8(self) -> np.ndarray:
         """
         Integer representation of the values.
 
@@ -402,8 +406,7 @@ def shape(self):
         return (len(self),)
 
     @property
-    def size(self):
-        # type: () -> int
+    def size(self) -> int:
         """The number of elements in this array."""
         return np.prod(self.shape)
 
@@ -461,10 +464,9 @@ def __getitem__(self, key):
 
     def __setitem__(
             self,
-            key,    # type: Union[int, Sequence[int], Sequence[bool], slice]
-            value,  # type: Union[NaTType, Any, Sequence[Any]]
-    ):
-        # type: (...) -> None
+            key: Union[int, Sequence[int], Sequence[bool], slice],
+            value: Union[NaTType, Any, Sequence[Any]]
+    ) -> None:
         # I'm fudging the types a bit here. "Any" above really depends
         # on type(self). For PeriodArray, it's Period (or stuff coercible
         # to a period in from_sequence). For DatetimeArray, it's Timestamp...
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 33e6674389e7c..cb84a862bc936 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -275,7 +275,7 @@ class DatetimeArray(dtl.DatetimeLikeArrayMixin,
     # Constructors
 
     _attributes = ["freq", "tz"]
-    _dtype = None  # type: Union[np.dtype, DatetimeTZDtype]
+    _dtype: Union[np.dtype, DatetimeTZDtype] = None
     _freq = None
 
     def __init__(self, values, dtype=_NS_DTYPE, freq=None, copy=False):
@@ -514,8 +514,7 @@ def _box_func(self):
         return lambda x: Timestamp(x, freq=self.freq, tz=self.tz)
 
     @property
-    def dtype(self):
-        # type: () -> Union[np.dtype, DatetimeTZDtype]
+    def dtype(self) -> Union[np.dtype, DatetimeTZDtype]:
         """
         The dtype for the DatetimeArray.
 
diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index 0144d04c6e197..f192ff22aedf6 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -452,8 +452,7 @@ def astype(self, dtype, copy=True):
         return astype_nansafe(data, dtype, copy=None)
 
     @property
-    def _ndarray_values(self):
-        # type: () -> np.ndarray
+    def _ndarray_values(self) -> np.ndarray:
         """Internal pandas method for lossy conversion to a NumPy ndarray.
 
         This method is not part of the pandas interface.
@@ -509,8 +508,7 @@ def value_counts(self, dropna=True):
 
         return Series(array, index=index)
 
-    def _values_for_argsort(self):
-        # type: () -> np.ndarray
+    def _values_for_argsort(self) -> np.ndarray:
         """Return values for sorting.
 
         Returns
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 55d12d444fb45..ccc28ddec0714 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -183,8 +183,12 @@ def _simple_new(cls, values, freq=None, **kwargs):
         return cls(values, freq=freq, **kwargs)
 
     @classmethod
-    def _from_sequence(cls, scalars, dtype=None, copy=False):
-        # type: (Sequence[Optional[Period]], PeriodDtype, bool) -> PeriodArray
+    def _from_sequence(
+            cls,
+            scalars: Sequence[Optional[Period]],
+            dtype: PeriodDtype = None,
+            copy: bool = False,
+    ) -> 'PeriodArray':
         if dtype:
             freq = dtype.freq
         else:
@@ -246,8 +250,7 @@ def _generate_range(cls, start, end, periods, freq, fields):
     # -----------------------------------------------------------------
     # DatetimeLike Interface
 
-    def _unbox_scalar(self, value):
-        # type: (Union[Period, NaTType]) -> int
+    def _unbox_scalar(self, value: Union[Period, NaTType]) -> int:
         if value is NaT:
             return value.value
         elif isinstance(value, self._scalar_type):
@@ -258,8 +261,7 @@ def _unbox_scalar(self, value):
             raise ValueError("'value' should be a Period. Got '{val}' instead."
                              .format(val=value))
 
-    def _scalar_from_string(self, value):
-        # type: (str) -> Period
+    def _scalar_from_string(self, value: str) -> Period:
         return Period(value, freq=self.freq)
 
     def _check_compatible_with(self, other):
@@ -540,14 +542,9 @@ def _sub_period(self, other):
     @Appender(dtl.DatetimeLikeArrayMixin._addsub_int_array.__doc__)
     def _addsub_int_array(
             self,
-            other,      # type: Union[ExtensionArray, np.ndarray[int]]
-            op          # type: Callable[Any, Any]
-    ):
-        # type: (...) -> PeriodArray
-
-        # TODO: ABCIndexClass is a valid type for other but had to be excluded
-        # due to length of Py2 compatability comment; add back in once migrated
-        # to Py3 syntax
+            other: Union[ExtensionArray, np.ndarray, ABCIndexClass],
+            op: Callable[[Any], Any]
+    ) -> 'PeriodArray':
         assert op in [operator.add, operator.sub]
         if op is operator.sub:
             other = -other
@@ -716,8 +713,11 @@ def _raise_on_incompatible(left, right):
 # -------------------------------------------------------------------
 # Constructor Helpers
 
-def period_array(data, freq=None, copy=False):
-    # type: (Sequence[Optional[Period]], Optional[Tick], bool) -> PeriodArray
+def period_array(
+        data: Sequence[Optional[Period]],
+        freq: Optional[Tick] = None,
+        copy: bool = False,
+) -> PeriodArray:
     """
     Construct a new PeriodArray from a sequence of Period scalars.
 
diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index 8a4422120b7a3..b05b5b450e4b1 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -79,8 +79,11 @@ class SparseDtype(ExtensionDtype):
     # hash(nan) is (sometimes?) 0.
     _metadata = ('_dtype', '_fill_value', '_is_na_fill_value')
 
-    def __init__(self, dtype=np.float64, fill_value=None):
-        # type: (Union[str, np.dtype, 'ExtensionDtype', type], Any) -> None
+    def __init__(
+            self,
+            dtype: Union[str, np.dtype, 'ExtensionDtype', type] = np.float64,
+            fill_value: Any = None
+    ) -> None:
         from pandas.core.dtypes.missing import na_value_for_dtype
         from pandas.core.dtypes.common import (
             pandas_dtype, is_string_dtype, is_scalar
@@ -372,8 +375,7 @@ def _subtype_with_str(self):
 _sparray_doc_kwargs = dict(klass='SparseArray')
 
 
-def _get_fill(arr):
-    # type: (SparseArray) -> np.ndarray
+def _get_fill(arr: 'SparseArray') -> np.ndarray:
     """
     Create a 0-dim ndarray containing the fill value
 
@@ -397,8 +399,11 @@ def _get_fill(arr):
         return np.asarray(arr.fill_value)
 
 
-def _sparse_array_op(left, right, op, name):
-    # type: (SparseArray, SparseArray, Callable, str) -> Any
+def _sparse_array_op(
+        left: 'SparseArray',
+        right: 'SparseArray',
+        op: Callable, name: str
+) -> Any:
     """
     Perform a binary operation between two arrays.
 
@@ -671,8 +676,12 @@ def __init__(self, data, sparse_index=None, index=None, fill_value=None,
         self._dtype = SparseDtype(sparse_values.dtype, fill_value)
 
     @classmethod
-    def _simple_new(cls, sparse_array, sparse_index, dtype):
-        # type: (np.ndarray, SparseIndex, SparseDtype) -> 'SparseArray'
+    def _simple_new(
+            cls,
+            sparse_array: np.ndarray,
+            sparse_index: SparseIndex,
+            dtype: SparseDtype
+    ) -> 'SparseArray':
         new = cls([])
         new._sparse_index = sparse_index
         new._sparse_values = sparse_array

From 545d38d9b8f82ecc39a926f7aec418f3d1759b5a Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Thu, 21 Mar 2019 09:09:01 -0500
Subject: [PATCH 06/13] Removed instance variable type hint for 3.5
 compatibility

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

diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index cb84a862bc936..a718cb1cdba65 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -275,7 +275,7 @@ class DatetimeArray(dtl.DatetimeLikeArrayMixin,
     # Constructors
 
     _attributes = ["freq", "tz"]
-    _dtype: Union[np.dtype, DatetimeTZDtype] = None
+    _dtype = None  # type: Union[np.dtype, DatetimeTZDtype]
     _freq = None
 
     def __init__(self, values, dtype=_NS_DTYPE, freq=None, copy=False):

From abc09faf7d349d2e5b977d374909e6359fde06d5 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Thu, 21 Mar 2019 21:02:09 -0500
Subject: [PATCH 07/13] Cleaned up array/ type hints

---
 pandas/core/arrays/array_.py | 2 +-
 pandas/core/arrays/base.py   | 2 +-
 pandas/core/arrays/sparse.py | 3 ++-
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/pandas/core/arrays/array_.py b/pandas/core/arrays/array_.py
index ff58f848701d5..33cddb6bae615 100644
--- a/pandas/core/arrays/array_.py
+++ b/pandas/core/arrays/array_.py
@@ -13,7 +13,7 @@
 
 
 def array(data: Sequence[object],
-          dtype: Optional[Union[str, np.dtype, ExtensionDtype]] = None,
+          dtype: Optional[str, np.dtype, ExtensionDtype] = None,
           copy: bool = True,
           ) -> ExtensionArray:
     """
diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 53656c4d3ba77..127cc43406c28 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -726,7 +726,7 @@ def take(
             self,
             indices: Sequence[int],
             allow_fill: bool = False,
-            fill_value: Optional[Any] = None
+            fill_value: Any = None
     ) -> 'ExtensionArray':
         """
         Take elements from an array.
diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index b05b5b450e4b1..f478a6e58367d 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -402,7 +402,8 @@ def _get_fill(arr: 'SparseArray') -> np.ndarray:
 def _sparse_array_op(
         left: 'SparseArray',
         right: 'SparseArray',
-        op: Callable, name: str
+        op: Callable,
+        name: str
 ) -> Any:
     """
     Perform a binary operation between two arrays.

From c37ef88d40da422131cd528b7c316e0107b345d3 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Thu, 21 Mar 2019 22:01:20 -0500
Subject: [PATCH 08/13] #25790 Updating type hints to Python3 syntax in
 pandas/core/array

---
 pandas/core/arrays/array_.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/pandas/core/arrays/array_.py b/pandas/core/arrays/array_.py
index 33cddb6bae615..9ab703c863f31 100644
--- a/pandas/core/arrays/array_.py
+++ b/pandas/core/arrays/array_.py
@@ -1,10 +1,9 @@
-from typing import Optional, Sequence, Union
+from typing import Optional, Sequence, TYPE_CHECKING, Union
 
 import numpy as np
 
 from pandas._libs import lib, tslibs
 
-from pandas.core.arrays.base import ExtensionArray
 from pandas.core.dtypes.common import (
     is_datetime64_ns_dtype, is_extension_array_dtype, is_timedelta64_ns_dtype)
 from pandas.core.dtypes.dtypes import ExtensionDtype, registry
@@ -12,10 +11,14 @@
 from pandas import compat
 
 
+if TYPE_CHECKING:
+    from pandas.core.arrays.base import ExtensionArray
+
+
 def array(data: Sequence[object],
-          dtype: Optional[str, np.dtype, ExtensionDtype] = None,
+          dtype: Optional[Union[str, np.dtype, ExtensionDtype]] = None,
           copy: bool = True,
-          ) -> ExtensionArray:
+          ) -> 'ExtensionArray':
     """
     Create an array.
 

From ea9c2b3396458f80f2c590c01b430a0833893226 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Thu, 21 Mar 2019 22:45:38 -0500
Subject: [PATCH 09/13] #25790 Updating type hints to Python3 syntax in
 pandas/core/array

---
 pandas/core/arrays/array_.py       | 15 ++++---
 pandas/core/arrays/base.py         | 69 +++++++++++++++---------------
 pandas/core/arrays/datetimelike.py | 34 ++++++++-------
 pandas/core/arrays/datetimes.py    |  3 +-
 pandas/core/arrays/integer.py      |  6 +--
 pandas/core/arrays/period.py       | 32 +++++++-------
 pandas/core/arrays/sparse.py       | 26 +++++++----
 7 files changed, 99 insertions(+), 86 deletions(-)

diff --git a/pandas/core/arrays/array_.py b/pandas/core/arrays/array_.py
index 254ab876af1ac..9ab703c863f31 100644
--- a/pandas/core/arrays/array_.py
+++ b/pandas/core/arrays/array_.py
@@ -1,4 +1,4 @@
-from typing import Optional, Sequence, Union
+from typing import Optional, Sequence, TYPE_CHECKING, Union
 
 import numpy as np
 
@@ -11,11 +11,14 @@
 from pandas import compat
 
 
-def array(data,         # type: Sequence[object]
-          dtype=None,   # type: Optional[Union[str, np.dtype, ExtensionDtype]]
-          copy=True,    # type: bool
-          ):
-    # type: (...) -> ExtensionArray
+if TYPE_CHECKING:
+    from pandas.core.arrays.base import ExtensionArray
+
+
+def array(data: Sequence[object],
+          dtype: Optional[Union[str, np.dtype, ExtensionDtype]] = None,
+          copy: bool = True,
+          ) -> 'ExtensionArray':
     """
     Create an array.
 
diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index f7d427ce26e6a..127cc43406c28 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -214,8 +214,7 @@ def __getitem__(self, item):
         """
         raise AbstractMethodError(self)
 
-    def __setitem__(self, key, value):
-        # type: (Union[int, np.ndarray], Any) -> None
+    def __setitem__(self, key: Union[int, np.ndarray], value: Any) -> None:
         """
         Set one or more values inplace.
 
@@ -262,8 +261,7 @@ def __setitem__(self, key, value):
             type(self), '__setitem__')
         )
 
-    def __len__(self):
-        # type: () -> int
+    def __len__(self) -> int:
         """
         Length of this array
 
@@ -287,32 +285,28 @@ def __iter__(self):
     # Required attributes
     # ------------------------------------------------------------------------
     @property
-    def dtype(self):
-        # type: () -> ExtensionDtype
+    def dtype(self) -> ExtensionDtype:
         """
         An instance of 'ExtensionDtype'.
         """
         raise AbstractMethodError(self)
 
     @property
-    def shape(self):
-        # type: () -> Tuple[int, ...]
+    def shape(self) -> Tuple[int, ...]:
         """
         Return a tuple of the array dimensions.
         """
         return (len(self),)
 
     @property
-    def ndim(self):
-        # type: () -> int
+    def ndim(self) -> int:
         """
         Extension Arrays are only allowed to be 1-dimensional.
         """
         return 1
 
     @property
-    def nbytes(self):
-        # type: () -> int
+    def nbytes(self) -> int:
         """
         The number of bytes needed to store this object in memory.
         """
@@ -343,8 +337,7 @@ def astype(self, dtype, copy=True):
         """
         return np.array(self, dtype=dtype, copy=copy)
 
-    def isna(self):
-        # type: () -> Union[ExtensionArray, np.ndarray]
+    def isna(self) -> Union['ExtensionArray', np.ndarray]:
         """
         A 1-D array indicating if each value is missing.
 
@@ -366,8 +359,7 @@ def isna(self):
         """
         raise AbstractMethodError(self)
 
-    def _values_for_argsort(self):
-        # type: () -> np.ndarray
+    def _values_for_argsort(self) -> np.ndarray:
         """
         Return values for sorting.
 
@@ -482,8 +474,11 @@ def dropna(self):
         """
         return self[~self.isna()]
 
-    def shift(self, periods=1, fill_value=None):
-        # type: (int, object) -> ExtensionArray
+    def shift(
+            self,
+            periods: int = 1,
+            fill_value: object = None,
+    ) -> 'ExtensionArray':
         """
         Shift values by desired number.
 
@@ -598,8 +593,7 @@ def searchsorted(self, value, side="left", sorter=None):
         arr = self.astype(object)
         return arr.searchsorted(value, side=side, sorter=sorter)
 
-    def _values_for_factorize(self):
-        # type: () -> Tuple[np.ndarray, Any]
+    def _values_for_factorize(self) -> Tuple[np.ndarray, Any]:
         """
         Return an array and missing value suitable for factorization.
 
@@ -623,8 +617,10 @@ def _values_for_factorize(self):
         """
         return self.astype(object), np.nan
 
-    def factorize(self, na_sentinel=-1):
-        # type: (int) -> Tuple[np.ndarray, ExtensionArray]
+    def factorize(
+            self,
+            na_sentinel: int = -1,
+    ) -> Tuple[np.ndarray, 'ExtensionArray']:
         """
         Encode the extension array as an enumerated type.
 
@@ -726,8 +722,12 @@ def repeat(self, repeats, axis=None):
     # Indexing methods
     # ------------------------------------------------------------------------
 
-    def take(self, indices, allow_fill=False, fill_value=None):
-        # type: (Sequence[int], bool, Optional[Any]) -> ExtensionArray
+    def take(
+            self,
+            indices: Sequence[int],
+            allow_fill: bool = False,
+            fill_value: Any = None
+    ) -> 'ExtensionArray':
         """
         Take elements from an array.
 
@@ -816,8 +816,7 @@ def take(self, indices, allow_fill=False, fill_value=None):
         # pandas.api.extensions.take
         raise AbstractMethodError(self)
 
-    def copy(self, deep=False):
-        # type: (bool) -> ExtensionArray
+    def copy(self, deep: bool = False) -> 'ExtensionArray':
         """
         Return a copy of the array.
 
@@ -853,8 +852,10 @@ def __repr__(self):
                                length=len(self),
                                dtype=self.dtype)
 
-    def _formatter(self, boxed=False):
-        # type: (bool) -> Callable[[Any], Optional[str]]
+    def _formatter(
+            self,
+            boxed: bool = False,
+    ) -> Callable[[Any], Optional[str]]:
         """Formatting function for scalar values.
 
         This is used in the default '__repr__'. The returned formatting
@@ -881,8 +882,7 @@ def _formatter(self, boxed=False):
             return str
         return repr
 
-    def _formatting_values(self):
-        # type: () -> np.ndarray
+    def _formatting_values(self) -> np.ndarray:
         # At the moment, this has to be an array since we use result.dtype
         """
         An array of values to be printed in, e.g. the Series repr
@@ -898,8 +898,10 @@ def _formatting_values(self):
     # ------------------------------------------------------------------------
 
     @classmethod
-    def _concat_same_type(cls, to_concat):
-        # type: (Sequence[ExtensionArray]) -> ExtensionArray
+    def _concat_same_type(
+            cls,
+            to_concat: Sequence['ExtensionArray']
+    ) -> 'ExtensionArray':
         """
         Concatenate multiple array
 
@@ -921,8 +923,7 @@ def _concat_same_type(cls, to_concat):
     _can_hold_na = True
 
     @property
-    def _ndarray_values(self):
-        # type: () -> np.ndarray
+    def _ndarray_values(self) -> np.ndarray:
         """
         Internal pandas method for lossy conversion to a NumPy ndarray.
 
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 27d7d4f888550..9e6602daa92bb 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -58,8 +58,7 @@ def _get_attributes_dict(self):
         return {k: getattr(self, k, None) for k in self._attributes}
 
     @property
-    def _scalar_type(self):
-        # type: () -> Union[type, Tuple[type]]
+    def _scalar_type(self) -> Union[type, Tuple[type]]:
         """The scalar associated with this datelike
 
         * PeriodArray : Period
@@ -68,8 +67,10 @@ def _scalar_type(self):
         """
         raise AbstractMethodError(self)
 
-    def _scalar_from_string(self, value):
-        # type: (str) -> Union[Period, Timestamp, Timedelta, NaTType]
+    def _scalar_from_string(
+            self,
+            value: str,
+    ) -> Union[Period, Timestamp, Timedelta, NaTType]:
         """
         Construct a scalar type from a string.
 
@@ -89,8 +90,10 @@ def _scalar_from_string(self, value):
         """
         raise AbstractMethodError(self)
 
-    def _unbox_scalar(self, value):
-        # type: (Union[Period, Timestamp, Timedelta, NaTType]) -> int
+    def _unbox_scalar(
+            self,
+            value: Union[Period, Timestamp, Timedelta, NaTType],
+    ) -> int:
         """
         Unbox the integer value of a scalar `value`.
 
@@ -109,8 +112,10 @@ def _unbox_scalar(self, value):
         """
         raise AbstractMethodError(self)
 
-    def _check_compatible_with(self, other):
-        # type: (Union[Period, Timestamp, Timedelta, NaTType]) -> None
+    def _check_compatible_with(
+            self,
+            other: Union[Period, Timestamp, Timedelta, NaTType],
+    ) -> None:
         """
         Verify that `self` and `other` are compatible.
 
@@ -350,8 +355,7 @@ def __iter__(self):
         return (self._box_func(v) for v in self.asi8)
 
     @property
-    def asi8(self):
-        # type: () -> np.ndarray
+    def asi8(self) -> np.ndarray:
         """
         Integer representation of the values.
 
@@ -402,8 +406,7 @@ def shape(self):
         return (len(self),)
 
     @property
-    def size(self):
-        # type: () -> int
+    def size(self) -> int:
         """The number of elements in this array."""
         return np.prod(self.shape)
 
@@ -461,10 +464,9 @@ def __getitem__(self, key):
 
     def __setitem__(
             self,
-            key,    # type: Union[int, Sequence[int], Sequence[bool], slice]
-            value,  # type: Union[NaTType, Any, Sequence[Any]]
-    ):
-        # type: (...) -> None
+            key: Union[int, Sequence[int], Sequence[bool], slice],
+            value: Union[NaTType, Any, Sequence[Any]]
+    ) -> None:
         # I'm fudging the types a bit here. "Any" above really depends
         # on type(self). For PeriodArray, it's Period (or stuff coercible
         # to a period in from_sequence). For DatetimeArray, it's Timestamp...
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 33e6674389e7c..a718cb1cdba65 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -514,8 +514,7 @@ def _box_func(self):
         return lambda x: Timestamp(x, freq=self.freq, tz=self.tz)
 
     @property
-    def dtype(self):
-        # type: () -> Union[np.dtype, DatetimeTZDtype]
+    def dtype(self) -> Union[np.dtype, DatetimeTZDtype]:
         """
         The dtype for the DatetimeArray.
 
diff --git a/pandas/core/arrays/integer.py b/pandas/core/arrays/integer.py
index 0144d04c6e197..f192ff22aedf6 100644
--- a/pandas/core/arrays/integer.py
+++ b/pandas/core/arrays/integer.py
@@ -452,8 +452,7 @@ def astype(self, dtype, copy=True):
         return astype_nansafe(data, dtype, copy=None)
 
     @property
-    def _ndarray_values(self):
-        # type: () -> np.ndarray
+    def _ndarray_values(self) -> np.ndarray:
         """Internal pandas method for lossy conversion to a NumPy ndarray.
 
         This method is not part of the pandas interface.
@@ -509,8 +508,7 @@ def value_counts(self, dropna=True):
 
         return Series(array, index=index)
 
-    def _values_for_argsort(self):
-        # type: () -> np.ndarray
+    def _values_for_argsort(self) -> np.ndarray:
         """Return values for sorting.
 
         Returns
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 55d12d444fb45..ccc28ddec0714 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -183,8 +183,12 @@ def _simple_new(cls, values, freq=None, **kwargs):
         return cls(values, freq=freq, **kwargs)
 
     @classmethod
-    def _from_sequence(cls, scalars, dtype=None, copy=False):
-        # type: (Sequence[Optional[Period]], PeriodDtype, bool) -> PeriodArray
+    def _from_sequence(
+            cls,
+            scalars: Sequence[Optional[Period]],
+            dtype: PeriodDtype = None,
+            copy: bool = False,
+    ) -> 'PeriodArray':
         if dtype:
             freq = dtype.freq
         else:
@@ -246,8 +250,7 @@ def _generate_range(cls, start, end, periods, freq, fields):
     # -----------------------------------------------------------------
     # DatetimeLike Interface
 
-    def _unbox_scalar(self, value):
-        # type: (Union[Period, NaTType]) -> int
+    def _unbox_scalar(self, value: Union[Period, NaTType]) -> int:
         if value is NaT:
             return value.value
         elif isinstance(value, self._scalar_type):
@@ -258,8 +261,7 @@ def _unbox_scalar(self, value):
             raise ValueError("'value' should be a Period. Got '{val}' instead."
                              .format(val=value))
 
-    def _scalar_from_string(self, value):
-        # type: (str) -> Period
+    def _scalar_from_string(self, value: str) -> Period:
         return Period(value, freq=self.freq)
 
     def _check_compatible_with(self, other):
@@ -540,14 +542,9 @@ def _sub_period(self, other):
     @Appender(dtl.DatetimeLikeArrayMixin._addsub_int_array.__doc__)
     def _addsub_int_array(
             self,
-            other,      # type: Union[ExtensionArray, np.ndarray[int]]
-            op          # type: Callable[Any, Any]
-    ):
-        # type: (...) -> PeriodArray
-
-        # TODO: ABCIndexClass is a valid type for other but had to be excluded
-        # due to length of Py2 compatability comment; add back in once migrated
-        # to Py3 syntax
+            other: Union[ExtensionArray, np.ndarray, ABCIndexClass],
+            op: Callable[[Any], Any]
+    ) -> 'PeriodArray':
         assert op in [operator.add, operator.sub]
         if op is operator.sub:
             other = -other
@@ -716,8 +713,11 @@ def _raise_on_incompatible(left, right):
 # -------------------------------------------------------------------
 # Constructor Helpers
 
-def period_array(data, freq=None, copy=False):
-    # type: (Sequence[Optional[Period]], Optional[Tick], bool) -> PeriodArray
+def period_array(
+        data: Sequence[Optional[Period]],
+        freq: Optional[Tick] = None,
+        copy: bool = False,
+) -> PeriodArray:
     """
     Construct a new PeriodArray from a sequence of Period scalars.
 
diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index 8a4422120b7a3..f478a6e58367d 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -79,8 +79,11 @@ class SparseDtype(ExtensionDtype):
     # hash(nan) is (sometimes?) 0.
     _metadata = ('_dtype', '_fill_value', '_is_na_fill_value')
 
-    def __init__(self, dtype=np.float64, fill_value=None):
-        # type: (Union[str, np.dtype, 'ExtensionDtype', type], Any) -> None
+    def __init__(
+            self,
+            dtype: Union[str, np.dtype, 'ExtensionDtype', type] = np.float64,
+            fill_value: Any = None
+    ) -> None:
         from pandas.core.dtypes.missing import na_value_for_dtype
         from pandas.core.dtypes.common import (
             pandas_dtype, is_string_dtype, is_scalar
@@ -372,8 +375,7 @@ def _subtype_with_str(self):
 _sparray_doc_kwargs = dict(klass='SparseArray')
 
 
-def _get_fill(arr):
-    # type: (SparseArray) -> np.ndarray
+def _get_fill(arr: 'SparseArray') -> np.ndarray:
     """
     Create a 0-dim ndarray containing the fill value
 
@@ -397,8 +399,12 @@ def _get_fill(arr):
         return np.asarray(arr.fill_value)
 
 
-def _sparse_array_op(left, right, op, name):
-    # type: (SparseArray, SparseArray, Callable, str) -> Any
+def _sparse_array_op(
+        left: 'SparseArray',
+        right: 'SparseArray',
+        op: Callable,
+        name: str
+) -> Any:
     """
     Perform a binary operation between two arrays.
 
@@ -671,8 +677,12 @@ def __init__(self, data, sparse_index=None, index=None, fill_value=None,
         self._dtype = SparseDtype(sparse_values.dtype, fill_value)
 
     @classmethod
-    def _simple_new(cls, sparse_array, sparse_index, dtype):
-        # type: (np.ndarray, SparseIndex, SparseDtype) -> 'SparseArray'
+    def _simple_new(
+            cls,
+            sparse_array: np.ndarray,
+            sparse_index: SparseIndex,
+            dtype: SparseDtype
+    ) -> 'SparseArray':
         new = cls([])
         new._sparse_index = sparse_index
         new._sparse_values = sparse_array

From 20b63e4e21da31d99ce43fca31a640573650c233 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Sat, 23 Mar 2019 22:15:39 -0500
Subject: [PATCH 10/13] Remove conditional import

---
 pandas/core/arrays/array_.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/pandas/core/arrays/array_.py b/pandas/core/arrays/array_.py
index 9ab703c863f31..2404e1a71f5ad 100644
--- a/pandas/core/arrays/array_.py
+++ b/pandas/core/arrays/array_.py
@@ -1,4 +1,4 @@
-from typing import Optional, Sequence, TYPE_CHECKING, Union
+from typing import Optional, Sequence, Union
 
 import numpy as np
 
@@ -7,18 +7,15 @@
 from pandas.core.dtypes.common import (
     is_datetime64_ns_dtype, is_extension_array_dtype, is_timedelta64_ns_dtype)
 from pandas.core.dtypes.dtypes import ExtensionDtype, registry
+from pandas.core.dtypes.generic import ABCExtensionArray
 
 from pandas import compat
 
 
-if TYPE_CHECKING:
-    from pandas.core.arrays.base import ExtensionArray
-
-
 def array(data: Sequence[object],
           dtype: Optional[Union[str, np.dtype, ExtensionDtype]] = None,
           copy: bool = True,
-          ) -> 'ExtensionArray':
+          ) -> ABCExtensionArray:
     """
     Create an array.
 

From cf774d7d0f4244327c33967a0c65a9a9b3ee0d91 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Wed, 27 Mar 2019 22:57:53 -0500
Subject: [PATCH 11/13] Change forward refs to ABCs

---
 pandas/core/arrays/base.py         | 4 ++--
 pandas/core/arrays/datetimelike.py | 4 ++--
 pandas/core/arrays/period.py       | 4 ++--
 pandas/core/arrays/sparse.py       | 6 +++---
 4 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 127cc43406c28..aa186980c2646 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -17,7 +17,7 @@
 
 from pandas.core.dtypes.common import is_list_like
 from pandas.core.dtypes.dtypes import ExtensionDtype
-from pandas.core.dtypes.generic import ABCIndexClass, ABCSeries
+from pandas.core.dtypes.generic import ABCIndexClass, ABCExtensionArray, ABCSeries
 from pandas.core.dtypes.missing import isna
 
 from pandas.core import ops
@@ -337,7 +337,7 @@ def astype(self, dtype, copy=True):
         """
         return np.array(self, dtype=dtype, copy=copy)
 
-    def isna(self) -> Union['ExtensionArray', np.ndarray]:
+    def isna(self) -> Union[ABCExtensionArray, np.ndarray]:
         """
         A 1-D array indicating if each value is missing.
 
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 9e6602daa92bb..d7d14d6bdb6e3 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from datetime import datetime, timedelta
 import operator
-from typing import Any, Sequence, Tuple, Union
+from typing import Any, Sequence, Tuple, Type, Union
 import warnings
 
 import numpy as np
@@ -58,7 +58,7 @@ def _get_attributes_dict(self):
         return {k: getattr(self, k, None) for k in self._attributes}
 
     @property
-    def _scalar_type(self) -> Union[type, Tuple[type]]:
+    def _scalar_type(self) -> Union[Type, Tuple[Type]]:
         """The scalar associated with this datelike
 
         * PeriodArray : Period
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index ccc28ddec0714..b2166f6c7eea8 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -20,7 +20,7 @@
     is_list_like, is_period_dtype, pandas_dtype)
 from pandas.core.dtypes.dtypes import PeriodDtype
 from pandas.core.dtypes.generic import (
-    ABCDataFrame, ABCIndexClass, ABCPeriodIndex, ABCSeries)
+    ABCDataFrame, ABCIndexClass, ABCPeriodArray, ABCPeriodIndex, ABCSeries)
 from pandas.core.dtypes.missing import isna, notna
 
 import pandas.core.algorithms as algos
@@ -188,7 +188,7 @@ def _from_sequence(
             scalars: Sequence[Optional[Period]],
             dtype: PeriodDtype = None,
             copy: bool = False,
-    ) -> 'PeriodArray':
+    ) -> ABCPeriodArray:
         if dtype:
             freq = dtype.freq
         else:
diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index f478a6e58367d..7303573bfc13b 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -29,7 +29,7 @@
     pandas_dtype)
 from pandas.core.dtypes.dtypes import register_extension_dtype
 from pandas.core.dtypes.generic import (
-    ABCIndexClass, ABCSeries, ABCSparseSeries)
+    ABCIndexClass, ABCSeries, ABCSparseArray, ABCSparseSeries)
 from pandas.core.dtypes.missing import isna, na_value_for_dtype, notna
 
 from pandas.core.accessor import PandasDelegate, delegate_names
@@ -400,8 +400,8 @@ def _get_fill(arr: 'SparseArray') -> np.ndarray:
 
 
 def _sparse_array_op(
-        left: 'SparseArray',
-        right: 'SparseArray',
+        left: ABCSparseArray,
+        right: ABCSparseArray,
         op: Callable,
         name: str
 ) -> Any:

From 33db1eef3c3361cebe61b439ba15af4569ce6ade Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Thu, 28 Mar 2019 00:21:23 -0500
Subject: [PATCH 12/13] Change remaining forward refs to ABCs

---
 pandas/core/arrays/base.py   | 12 ++++++------
 pandas/core/arrays/period.py |  2 +-
 pandas/core/arrays/sparse.py |  4 ++--
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 387fef1443ef1..44662ce831b89 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -479,7 +479,7 @@ def shift(
             self,
             periods: int = 1,
             fill_value: object = None,
-    ) -> 'ExtensionArray':
+    ) -> ABCExtensionArray:
         """
         Shift values by desired number.
 
@@ -621,7 +621,7 @@ def _values_for_factorize(self) -> Tuple[np.ndarray, Any]:
     def factorize(
             self,
             na_sentinel: int = -1,
-    ) -> Tuple[np.ndarray, 'ExtensionArray']:
+    ) -> Tuple[np.ndarray, ABCExtensionArray]:
         """
         Encode the extension array as an enumerated type.
 
@@ -728,7 +728,7 @@ def take(
             indices: Sequence[int],
             allow_fill: bool = False,
             fill_value: Any = None
-    ) -> 'ExtensionArray':
+    ) -> ABCExtensionArray:
         """
         Take elements from an array.
 
@@ -817,7 +817,7 @@ def take(self, indices, allow_fill=False, fill_value=None):
         # pandas.api.extensions.take
         raise AbstractMethodError(self)
 
-    def copy(self, deep: bool = False) -> 'ExtensionArray':
+    def copy(self, deep: bool = False) -> ABCExtensionArray:
         """
         Return a copy of the array.
 
@@ -901,8 +901,8 @@ def _formatting_values(self) -> np.ndarray:
     @classmethod
     def _concat_same_type(
             cls,
-            to_concat: Sequence['ExtensionArray']
-    ) -> 'ExtensionArray':
+            to_concat: Sequence[ABCExtensionArray]
+    ) -> ABCExtensionArray:
         """
         Concatenate multiple array
 
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 44da1e52cdc50..70312f2e61445 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -544,7 +544,7 @@ def _addsub_int_array(
             self,
             other: Union[ExtensionArray, np.ndarray, ABCIndexClass],
             op: Callable[[Any], Any]
-    ) -> 'PeriodArray':
+    ) -> ABCPeriodArray:
         assert op in [operator.add, operator.sub]
         if op is operator.sub:
             other = -other
diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index 3da5a5f7bb62e..a4cfb38877326 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -373,7 +373,7 @@ def _subtype_with_str(self):
 _sparray_doc_kwargs = dict(klass='SparseArray')
 
 
-def _get_fill(arr: 'SparseArray') -> np.ndarray:
+def _get_fill(arr: ABCSparseArray) -> np.ndarray:
     """
     Create a 0-dim ndarray containing the fill value
 
@@ -680,7 +680,7 @@ def _simple_new(
             sparse_array: np.ndarray,
             sparse_index: SparseIndex,
             dtype: SparseDtype
-    ) -> 'SparseArray':
+    ) -> ABCSparseArray:
         new = cls([])
         new._sparse_index = sparse_index
         new._sparse_values = sparse_array

From 23aba8a5d19246b88c60c9fb0677036ad72488c3 Mon Sep 17 00:00:00 2001
From: Gregory Rome <gwrome@gmail.com>
Date: Thu, 28 Mar 2019 08:24:12 -0500
Subject: [PATCH 13/13] type to Type

---
 pandas/core/arrays/sparse.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index a4cfb38877326..287665b0165de 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -4,7 +4,7 @@
 import numbers
 import operator
 import re
-from typing import Any, Callable, Union
+from typing import Any, Callable, Type, Union
 import warnings
 
 import numpy as np
@@ -79,7 +79,7 @@ class SparseDtype(ExtensionDtype):
 
     def __init__(
             self,
-            dtype: Union[str, np.dtype, 'ExtensionDtype', type] = np.float64,
+            dtype: Union[str, np.dtype, ExtensionDtype, Type] = np.float64,
             fill_value: Any = None
     ) -> None:
         from pandas.core.dtypes.missing import na_value_for_dtype