ENH: Add support for ea dtypes in merge

asv_bench/benchmarks/join_merge.py

@@ -273,6 +273,38 @@ def time_merge_dataframes_cross(self, sort):
         merge(self.left.loc[:2000], self.right.loc[:2000], how="cross", sort=sort)
 
 
+class MergeEA:
+
+    params = [
+        "Int64",
+        "Int32",
+        "Int16",
+        "UInt64",
+        "UInt32",
+        "UInt16",
+        "Float64",
+        "Float32",
+    ]
+    param_names = ["dtype"]
+
+    def setup(self, dtype):
+        N = 10_000
+        indices = np.arange(1, N)
+        key = np.tile(indices[:8000], 10)
+        self.left = DataFrame(
+            {"key": Series(key, dtype=dtype), "value": np.random.randn(80000)}
+        )
+        self.right = DataFrame(
+            {
+                "key": Series(indices[2000:], dtype=dtype),
+                "value2": np.random.randn(7999),
+            }
+        )
+
+    def time_merge(self, dtype):
+        merge(self.left, self.right)
+
+
 class I8Merge:
 
     params = ["inner", "outer", "left", "right"]

doc/source/whatsnew/v2.0.0.rst

@@ -57,6 +57,7 @@ Other enhancements
 - :func:`assert_frame_equal` now shows the first element where the DataFrames differ, analogously to ``pytest``'s output (:issue:`47910`)
 - Added new argument ``use_nullable_dtypes`` to :func:`read_csv` and :func:`read_excel` to enable automatic conversion to nullable dtypes (:issue:`36712`)
 - Added ``index`` parameter to :meth:`DataFrame.to_dict` (:issue:`46398`)
+- Added support for extension array dtypes in :func:`merge` (:issue:`44240`)
 - Added metadata propagation for binary operators on :class:`DataFrame` (:issue:`28283`)
 - :class:`.CategoricalConversionWarning`, :class:`.InvalidComparison`, :class:`.InvalidVersion`, :class:`.LossySetitemError`, and :class:`.NoBufferPresent` are now exposed in ``pandas.errors`` (:issue:`27656`)
 - Fix ``test`` optional_extra by adding missing test package ``pytest-asyncio`` (:issue:`48361`)

pandas/_libs/hashtable.pyi

@@ -1,4 +1,5 @@
 from typing import (
+    Any,
     Hashable,
     Literal,
 )
@@ -13,6 +14,7 @@ def unique_label_indices(
 
 class Factorizer:
     count: int
+    uniques: Any
     def __init__(self, size_hint: int) -> None: ...
     def get_count(self) -> int: ...
     def factorize(
@@ -21,6 +23,7 @@ class Factorizer:
         sort: bool = ...,
         na_sentinel=...,
         na_value=...,
+        mask=...,
     ) -> npt.NDArray[np.intp]: ...
 
 class ObjectFactorizer(Factorizer):

pandas/core/reshape/merge.py

@@ -42,6 +42,7 @@
 )
 from pandas.util._exceptions import find_stack_level
 
+from pandas.core.dtypes.base import ExtensionDtype
 from pandas.core.dtypes.cast import find_common_type
 from pandas.core.dtypes.common import (
     ensure_float64,
@@ -79,7 +80,10 @@
     Series,
 )
 import pandas.core.algorithms as algos
-from pandas.core.arrays import ExtensionArray
+from pandas.core.arrays import (
+    BaseMaskedArray,
+    ExtensionArray,
+)
 from pandas.core.arrays._mixins import NDArrayBackedExtensionArray
 import pandas.core.common as com
 from pandas.core.construction import extract_array
@@ -92,6 +96,24 @@
     from pandas.core import groupby
     from pandas.core.arrays import DatetimeArray
 
+_factorizers = {
+    np.int64: libhashtable.Int64Factorizer,
+    np.longlong: libhashtable.Int64Factorizer,
+    np.int32: libhashtable.Int32Factorizer,
+    np.int16: libhashtable.Int16Factorizer,
+    np.int8: libhashtable.Int8Factorizer,
+    np.uint64: libhashtable.UInt64Factorizer,
+    np.uint32: libhashtable.UInt32Factorizer,
+    np.uint16: libhashtable.UInt16Factorizer,
+    np.uint8: libhashtable.UInt8Factorizer,
+    np.bool_: libhashtable.UInt8Factorizer,
+    np.float64: libhashtable.Float64Factorizer,
+    np.float32: libhashtable.Float32Factorizer,
+    np.complex64: libhashtable.Complex64Factorizer,
+    np.complex128: libhashtable.Complex128Factorizer,
+    np.object_: libhashtable.ObjectFactorizer,
+}
+
 
 @Substitution("\nleft : DataFrame or named Series")
 @Appender(_merge_doc, indents=0)
@@ -2335,25 +2357,37 @@ def _factorize_keys(
         rk = ensure_int64(rk.codes)
 
     elif isinstance(lk, ExtensionArray) and is_dtype_equal(lk.dtype, rk.dtype):
-        lk, _ = lk._values_for_factorize()
-
-        # error: Item "ndarray" of "Union[Any, ndarray]" has no attribute
-        # "_values_for_factorize"
-        rk, _ = rk._values_for_factorize()  # type: ignore[union-attr]
-
-    klass: type[libhashtable.Factorizer] | type[libhashtable.Int64Factorizer]
-    if is_integer_dtype(lk.dtype) and is_integer_dtype(rk.dtype):
-        # GH#23917 TODO: needs tests for case where lk is integer-dtype
-        #  and rk is datetime-dtype
-        klass = libhashtable.Int64Factorizer
-        lk = ensure_int64(np.asarray(lk))
-        rk = ensure_int64(np.asarray(rk))
-
-    elif needs_i8_conversion(lk.dtype) and is_dtype_equal(lk.dtype, rk.dtype):
-        # GH#23917 TODO: Needs tests for non-matching dtypes
-        klass = libhashtable.Int64Factorizer
-        lk = ensure_int64(np.asarray(lk, dtype=np.int64))
-        rk = ensure_int64(np.asarray(rk, dtype=np.int64))
+        if not isinstance(lk, BaseMaskedArray):
+            lk, _ = lk._values_for_factorize()
+
+            # error: Item "ndarray" of "Union[Any, ndarray]" has no attribute
+            # "_values_for_factorize"
+            rk, _ = rk._values_for_factorize()  # type: ignore[union-attr]
+
+    klass: type[libhashtable.Factorizer]
+    if is_numeric_dtype(lk.dtype):
+        if not is_dtype_equal(lk, rk):
+            dtype = find_common_type([lk.dtype, rk.dtype])
+            if isinstance(dtype, ExtensionDtype):
+                cls = dtype.construct_array_type()
+                if not isinstance(lk, ExtensionArray):
+                    lk = cls._from_sequence(lk, dtype=dtype, copy=False)
+                else:
+                    lk = lk.astype(dtype)
+
+                if not isinstance(rk, ExtensionArray):
+                    rk = cls._from_sequence(rk, dtype=dtype, copy=False)
+                else:
+                    rk = rk.astype(dtype)
+            else:
+                lk = lk.astype(dtype)
+                rk = rk.astype(dtype)
+        if isinstance(lk, BaseMaskedArray):
+            #  Invalid index type "type" for "Dict[Type[object], Type[Factorizer]]";
+            #  expected type "Type[object]"
+            klass = _factorizers[lk.dtype.type]  # type: ignore[index]
+        else:
+            klass = _factorizers[lk.dtype.type]
 
     else:
         klass = libhashtable.ObjectFactorizer
@@ -2362,8 +2396,16 @@ def _factorize_keys(
 
     rizer = klass(max(len(lk), len(rk)))
 
-    llab = rizer.factorize(lk)
-    rlab = rizer.factorize(rk)
+    if isinstance(lk, BaseMaskedArray):
+        assert isinstance(rk, BaseMaskedArray)
+        llab = rizer.factorize(lk._data, mask=lk._mask)
+        rlab = rizer.factorize(rk._data, mask=rk._mask)
+    else:
+        # Argument 1 to "factorize" of "ObjectFactorizer" has incompatible type
+        # "Union[ndarray[Any, dtype[signedinteger[_64Bit]]],
+        # ndarray[Any, dtype[object_]]]"; expected "ndarray[Any, dtype[object_]]"
+        llab = rizer.factorize(lk)  # type: ignore[arg-type]
+        rlab = rizer.factorize(rk)  # type: ignore[arg-type]
     assert llab.dtype == np.dtype(np.intp), llab.dtype
     assert rlab.dtype == np.dtype(np.intp), rlab.dtype
 

pandas/tests/reshape/merge/test_merge.py

@@ -2714,3 +2714,27 @@ def test_merge_different_index_names():
     result = merge(left, right, left_on="c", right_on="d")
     expected = DataFrame({"a_x": [1], "a_y": 1})
     tm.assert_frame_equal(result, expected)
+
+
+def test_merge_ea(any_numeric_ea_dtype, join_type):
+    # GH#44240
+    left = DataFrame({"a": [1, 2, 3], "b": 1}, dtype=any_numeric_ea_dtype)
+    right = DataFrame({"a": [1, 2, 3], "c": 2}, dtype=any_numeric_ea_dtype)
+    result = left.merge(right, how=join_type)
+    expected = DataFrame({"a": [1, 2, 3], "b": 1, "c": 2}, dtype=any_numeric_ea_dtype)
+    tm.assert_frame_equal(result, expected)
+
+
+def test_merge_ea_and_non_ea(any_numeric_ea_dtype, join_type):
+    # GH#44240
+    left = DataFrame({"a": [1, 2, 3], "b": 1}, dtype=any_numeric_ea_dtype)
+    right = DataFrame({"a": [1, 2, 3], "c": 2}, dtype=any_numeric_ea_dtype.lower())
+    result = left.merge(right, how=join_type)
+    expected = DataFrame(
+        {
+            "a": Series([1, 2, 3], dtype=any_numeric_ea_dtype),
+            "b": Series([1, 1, 1], dtype=any_numeric_ea_dtype),
+            "c": Series([2, 2, 2], dtype=any_numeric_ea_dtype.lower()),
+        }
+    )
+    tm.assert_frame_equal(result, expected)