Issue #28283, Finalize coverage for DataFrame.merge

pandas/core/frame.py

@@ -11110,7 +11110,7 @@ def merge(
 
         from pandas.core.reshape.merge import merge
 
-        return merge(
+        result = merge(
             self,
             right,
             how=how,
@@ -11124,6 +11124,8 @@ def merge(
             indicator=indicator,
             validate=validate,
         )
+        # ADDED: Apply __finalize__ to propagate metadata from left DataFrame
+        return result.__finalize__(self, method="merge")
 
     def round(
         self, decimals: int | dict[IndexLabel, int] | Series = 0, *args, **kwargs
@@ -11211,6 +11213,18 @@ def round(
         1   0.0   1.0
         2   0.7   0.0
         3   0.2   0.0
+
+        >>> df1 = pd.DataFrame({"key": [1, 2], "A": [1, 2]})
+        >>> df2 = pd.DataFrame({"key": [1, 2], "B": [3, 4]})
+        >>> df1.attrs["source"] = "dataset1"
+        >>> result = df1.merge(df2, on="key")
+        >>> result.attrs["source"]  # Metadata is preserved
+        'dataset1'
+
+        Note
+        ----
+        The merge operation propagates metadata (attrs, flags) from the left DataFrame
+        to the result using the __finalize__ method.
         """
         from pandas.core.reshape.concat import concat
 

pandas/core/reshape/merge.py

@@ -370,7 +370,7 @@ def merge(
     left._check_copy_deprecation(copy)
     right_df = _validate_operand(right)
     if how == "cross":
-        return _cross_merge(
+        result = _cross_merge(
             left_df,
             right_df,
             on=on,
@@ -398,7 +398,14 @@ def merge(
             indicator=indicator,
             validate=validate,
         )
-        return op.get_result()
+        result = op.get_result()
+
+    # ADDED: Apply __finalize__ to propagate metadata
+    # Use left DataFrame as the primary source for metadata
+    if hasattr(left, "__finalize__"):
+        result = result.__finalize__(left, method="merge")
+
+    return result
 
 
 def _cross_merge(
@@ -927,7 +934,12 @@ def merge_asof(
         allow_exact_matches=allow_exact_matches,
         direction=direction,
     )
-    return op.get_result()
+    result = op.get_result()
+    # ADDED: Apply __finalize__ to propagate metadata
+    if hasattr(left, "__finalize__"):
+        result = result.__finalize__(left, method="merge_asof")
+
+    return result
 
 
 # TODO: transformations??
@@ -1143,7 +1155,9 @@ def get_result(self) -> DataFrame:
 
         self._maybe_restore_index_levels(result)
 
-        return result.__finalize__(self, method="merge")
+        # NOTE: __finalize__ is now called in the higher-level merge functions
+        # rather than here, to ensure it's called consistently across all entry points
+        return result
 
     @final
     @cache_readonly

pandas/tests/generic/test_finalize.py

@@ -13,7 +13,7 @@
 # TODO:
 # * Binary methods (mul, div, etc.)
 # * Binary outputs (align, etc.)
-# * top-level methods (concat, merge, get_dummies, etc.)
+# * top-level methods (concat, get_dummies, etc.)
 # * window
 # * cumulative reductions
 
@@ -154,7 +154,7 @@
             frame_data,
             operator.methodcaller("merge", pd.DataFrame({"A": [1]})),
         ),
-        marks=not_implemented_mark,
+        # marks=not_implemented_mark,
     ),
     (pd.DataFrame, frame_data, operator.methodcaller("round", 2)),
     (pd.DataFrame, frame_data, operator.methodcaller("corr")),
@@ -675,3 +675,71 @@ def test_finalize_frame_series_name():
     df = pd.DataFrame({"name": [1, 2]})
     result = pd.Series([1, 2]).__finalize__(df)
     assert result.name is None
+
+
+def test_merge_finalize():
+    """Test that DataFrame.merge calls __finalize__."""
+    # Create test DataFrames
+    df1 = pd.DataFrame({"key": [1, 2, 3], "A": [1, 2, 3]})
+    df2 = pd.DataFrame({"key": [1, 2, 4], "B": [4, 5, 6]})
+
+    # Add metadata
+    df1.attrs["source"] = "left"
+    df1.attrs["version"] = "1.0"
+
+    # Test different merge types
+    for how in ["inner", "outer", "left", "right"]:
+        result = df1.merge(df2, on="key", how=how)
+
+        # Check that attrs were propagated from left DataFrame
+        assert result.attrs["source"] == "left"
+        assert result.attrs["version"] == "1.0"
+
+
+def test_merge_asof_finalize():
+    """Test that merge_asof calls __finalize__."""
+    df1 = pd.DataFrame({"time": [1, 2, 3], "A": [1, 2, 3]})
+    df2 = pd.DataFrame({"time": [1, 2, 4], "B": [4, 5, 6]})
+
+    df1.attrs["source"] = "quotes"
+
+    result = pd.merge_asof(df1, df2, on="time")
+
+    # Check that attrs were propagated
+    assert result.attrs["source"] == "quotes"
+
+
+def test_merge_index_finalize():
+    """Test that index-based merge calls __finalize__."""
+    df1 = pd.DataFrame({"A": [1, 2]}, index=[1, 2])
+    df2 = pd.DataFrame({"B": [3, 4]}, index=[1, 2])
+
+    df1.attrs["index_merge"] = True
+
+    result = df1.merge(df2, left_index=True, right_index=True)
+
+    assert result.attrs["index_merge"] is True
+
+
+def test_merge_suffixes_finalize():
+    """Test merge with suffixes calls __finalize__."""
+    df1 = pd.DataFrame({"key": [1, 2], "value": [1, 2]})
+    df2 = pd.DataFrame({"key": [1, 2], "value": [3, 4]})
+
+    df1.attrs["has_suffixes"] = True
+
+    result = df1.merge(df2, on="key", suffixes=("_left", "_right"))
+
+    assert result.attrs["has_suffixes"] is True
+
+
+def test_merge_series_finalize():
+    """Test that merging with a Series calls __finalize__."""
+    df = pd.DataFrame({"key": [1, 2, 3], "A": [1, 2, 3]})
+    s = pd.Series([4, 5, 6], index=[1, 2, 3], name="B")
+
+    df.attrs["merged_with_series"] = True
+
+    result = df.merge(s, left_on="key", right_index=True)
+
+    assert result.attrs["merged_with_series"] is True