ENH: Avoid copy when possible in merge

pandas/core/frame.py

@@ -9809,7 +9809,7 @@ def merge(
         right_index: bool = False,
         sort: bool = False,
         suffixes: Suffixes = ("_x", "_y"),
-        copy: bool = True,
+        copy: bool | None = None,
         indicator: str | bool = False,
         validate: str | None = None,
     ) -> DataFrame:

pandas/core/reshape/merge.py

@@ -25,6 +25,7 @@
     join as libjoin,
     lib,
 )
+from pandas._libs.lib import is_range_indexer
 from pandas._typing import (
     AnyArrayLike,
     ArrayLike,
@@ -757,7 +758,7 @@ def _reindex_and_concat(
             self.left._info_axis, self.right._info_axis, self.suffixes
         )
 
-        if left_indexer is not None:
+        if left_indexer is not None and not is_range_indexer(left_indexer, len(left)):
             # Pinning the index here (and in the right code just below) is not
             #  necessary, but makes the `.take` more performant if we have e.g.
             #  a MultiIndex for left.index.
@@ -773,7 +774,9 @@ def _reindex_and_concat(
             left = left._constructor(lmgr)
         left.index = join_index
 
-        if right_indexer is not None:
+        if right_indexer is not None and not is_range_indexer(
+            right_indexer, len(right)
+        ):
             rmgr = right._mgr.reindex_indexer(
                 join_index,
                 right_indexer,

pandas/tests/copy_view/test_functions.py

@@ -1,6 +1,5 @@
 import numpy as np
-
-import pandas.util._test_decorators as td
+import pytest
 
 from pandas import (
     DataFrame,
@@ -182,19 +181,25 @@ def test_concat_mixed_series_frame(using_copy_on_write):
     tm.assert_frame_equal(result, expected)
 
 
-@td.skip_copy_on_write_not_yet_implemented  # TODO(CoW)
-def test_merge_on_key(using_copy_on_write):
+@pytest.mark.parametrize(
+    "func",
+    [
+        lambda df1, df2, **kwargs: df1.merge(df2, **kwargs),
+        lambda df1, df2, **kwargs: merge(df1, df2, **kwargs),
+    ],
+)
+def test_merge_on_key(using_copy_on_write, func):
     df1 = DataFrame({"key": ["a", "b", "c"], "a": [1, 2, 3]})
     df2 = DataFrame({"key": ["a", "b", "c"], "b": [4, 5, 6]})
     df1_orig = df1.copy()
     df2_orig = df2.copy()
 
-    result = merge(df1, df2, on="key")
+    result = func(df1, df2, on="key")
 
     if using_copy_on_write:
         assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
         assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
-        assert not np.shares_memory(get_array(result, "key"), get_array(df1, "key"))
+        assert np.shares_memory(get_array(result, "key"), get_array(df1, "key"))
         assert not np.shares_memory(get_array(result, "key"), get_array(df2, "key"))
     else:
         assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
@@ -239,4 +244,39 @@ def test_merge_on_index(using_copy_on_write):
     tm.assert_frame_equal(df2, df2_orig)
 
 
-# TODO(CoW) add merge tests where one of left/right isn't copied
+@pytest.mark.parametrize(
+    "func, how",
+    [
+        (lambda df1, df2, **kwargs: merge(df2, df1, on="key", **kwargs), "right"),
+        (lambda df1, df2, **kwargs: merge(df1, df2, on="key", **kwargs), "left"),
+    ],
+)
+def test_merge_on_key_enlarging_one(using_copy_on_write, func, how):
+    df1 = DataFrame({"key": ["a", "b", "c"], "a": [1, 2, 3]})
+    df2 = DataFrame({"key": ["a", "b"], "b": [4, 5]})
+    df1_orig = df1.copy()
+    df2_orig = df2.copy()
+
+    result = func(df1, df2, how=how)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+        assert df2._mgr._has_no_reference(1)
+        assert df2._mgr._has_no_reference(0)
+        assert np.shares_memory(get_array(result, "key"), get_array(df1, "key")) is (
+            how == "left"
+        )
+        assert not np.shares_memory(get_array(result, "key"), get_array(df2, "key"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    if how == "left":
+        result.iloc[0, 1] = 0
+    else:
+        result.iloc[0, 2] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+    tm.assert_frame_equal(df1, df1_orig)
+    tm.assert_frame_equal(df2, df2_orig)