fix(#61434): Improve error message when merging with incompatible DataFrame types

Duly330PK · Duly330PK · commit 122bfd98b741 · 2025-10-24T09:19:29.000+02:00
- Add _get_merge_error_message() to provide helpful error messages
- Special handling for polars.DataFrame with conversion suggestion
- Generic helpful message for other incompatible types
- Add comprehensive regression tests
- All validation tests pass
diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
@@ -3029,10 +3029,36 @@ def _validate_operand(obj: DataFrame | Series) -> DataFrame:
         return obj.to_frame()
     else:
         raise TypeError(
-            f"Can only merge Series or DataFrame objects, a {type(obj)} was passed"
+            _get_merge_error_message(obj)
         )
 
 
+def _get_merge_error_message(obj: object) -> str:
+    """Generate a helpful error message for invalid merge arguments."""
+    obj_type_name = type(obj).__name__
+    obj_module_name = type(obj).__module__
+    
+    # Special handling for known DataFrame-like libraries
+    if obj_module_name == "polars.dataframe.frame" and obj_type_name == "DataFrame":
+        return (
+            "Can only merge Series or DataFrame objects, received "
+            "polars.DataFrame. Please convert the polars DataFrame to a "
+            "pandas DataFrame using `.to_pandas()` or pass it to "
+            "pd.DataFrame()."
+        )
+    elif "polars" in obj_module_name.lower():
+        return (
+            f"Can only merge Series or DataFrame objects, received "
+            f"{obj_module_name}.{obj_type_name} (a polars object). "
+            "Please convert to a pandas DataFrame using `.to_pandas()`."
+        )
+    
+    return (
+        f"Can only merge Series or DataFrame objects, received "
+        f"{obj_module_name}.{obj_type_name}. Expected a pandas Series or DataFrame."
+    )
+
+
 def _items_overlap_with_suffix(
     left: Index, right: Index, suffixes: Suffixes
 ) -> tuple[Index, Index]:
diff --git a/test_issue_61434_repro.py b/test_issue_61434_repro.py
@@ -0,0 +1,129 @@
+"""
+Pandas Issue #61434 - Reproduction Test
+
+Issue: When attempting to merge a pandas DataFrame with a polars DataFrame,
+the error message is unhelpful. 
+
+Current behavior: Generic error about missing attributes or type errors
+Expected behavior: Clear message saying "other must be pandas.DataFrame, 
+                   received: polars.DataFrame"
+
+Snippet from issue #61434:
+https://github.com/pandas-dev/pandas/issues/61434
+"""
+
+import pandas as pd
+
+# Try to import polars for testing
+try:
+    import polars as pl
+    POLARS_AVAILABLE = True
+except ImportError:
+    POLARS_AVAILABLE = False
+    print("Warning: polars not installed. Install with: pip install polars")
+
+
+def test_merge_with_polars():
+    """
+    Reproduce the issue: Merging pandas DataFrame with polars DataFrame.
+    
+    Before fix: Generic/confusing error message
+    After fix: Clear message about type mismatch
+    """
+    if not POLARS_AVAILABLE:
+        print("Skipping test - polars not available")
+        return False
+    
+    print("=" * 70)
+    print("Test: Merging pandas DataFrame with polars DataFrame")
+    print("=" * 70)
+    
+    # Create pandas DataFrame
+    pdf = pd.DataFrame({
+        'key': ['a', 'b', 'c'],
+        'value_x': [1, 2, 3]
+    })
+    
+    # Create polars DataFrame
+    plf = pl.DataFrame({
+        'key': ['a', 'b', 'c'],
+        'value_y': [10, 20, 30]
+    })
+    
+    print(f"\nPandas DataFrame type: {type(pdf)}")
+    print(f"Polars DataFrame type: {type(plf)}")
+    print("\nAttempting merge...")
+    
+    try:
+        result = pd.merge(pdf, plf, on='key')
+        print(f"✗ Unexpected: merge succeeded with result type {type(result)}")
+        return False
+    except TypeError as e:
+        error_msg = str(e)
+        print(f"\nError caught: {type(e).__name__}")
+        print(f"Error message: {error_msg}")
+        
+        # Check if error message is helpful
+        if "polars" in error_msg.lower() and "pandas" in error_msg.lower():
+            print("\n✓ GOOD: Error message mentions both polars and pandas")
+            print("✓ GOOD: User knows what went wrong")
+            return True
+        elif "must be" in error_msg.lower() or "expected" in error_msg.lower():
+            print("\n✓ GOOD: Error message explains what's expected")
+            return True
+        else:
+            print(f"\n✗ BAD: Error message is not helpful enough")
+            print(f"   Expected something like:")
+            print(f"   'other must be pandas.DataFrame, received: polars.DataFrame'")
+            print(f"   But got: {error_msg}")
+            return False
+    except Exception as e:
+        print(f"\n✗ Unexpected error type: {type(e).__name__}")
+        print(f"   {e}")
+        return False
+
+
+def test_merge_pandas_baseline():
+    """
+    Baseline test: merge two pandas DataFrames should work.
+    """
+    print("\n" + "=" * 70)
+    print("Test: Merging two pandas DataFrames (baseline)")
+    print("=" * 70)
+    
+    df1 = pd.DataFrame({
+        'key': ['a', 'b', 'c'],
+        'value_x': [1, 2, 3]
+    })
+    
+    df2 = pd.DataFrame({
+        'key': ['a', 'b', 'c'],
+        'value_y': [10, 20, 30]
+    })
+    
+    try:
+        result = pd.merge(df1, df2, on='key')
+        print(f"✓ Merge succeeded")
+        print(f"  Result shape: {result.shape}")
+        print(f"  Result columns: {list(result.columns)}")
+        return True
+    except Exception as e:
+        print(f"✗ Baseline test failed: {e}")
+        return False
+
+
+if __name__ == "__main__":
+    print("\n" + "=" * 70)
+    print("PANDAS ISSUE #61434 - REPRODUCTION TEST")
+    print("=" * 70)
+    print()
+    
+    baseline_ok = test_merge_pandas_baseline()
+    polars_test_ok = test_merge_with_polars()
+    
+    print("\n" + "=" * 70)
+    print("SUMMARY")
+    print("=" * 70)
+    print(f"Baseline (pandas merge): {'✓ PASS' if baseline_ok else '✗ FAIL'}")
+    print(f"Polars test (error msg): {'✓ GOOD' if polars_test_ok else '✗ NEEDS FIX'}")
+    print()
diff --git a/test_issue_61434_tests.py b/test_issue_61434_tests.py
@@ -0,0 +1,141 @@
+"""
+Regression tests for issue #61434: Improved error message for incompatible merge types
+
+Tests that:
+1. Merging with polars.DataFrame raises TypeError with helpful message
+2. Merging with other incompatible types also gets helpful messages
+3. Normal pandas merges still work correctly
+"""
+
+import pytest
+import pandas as pd
+from pandas import DataFrame, Series
+import pandas._testing as tm
+
+
+class TestMergeIncompatibleTypes:
+    """Test merge error messages with incompatible DataFrame types."""
+    
+    def test_merge_with_polars_dataframe(self):
+        """
+        Test that merging with polars.DataFrame raises helpful TypeError.
+        
+        Regression test for issue #61434.
+        """
+        pytest.importorskip("polars")
+        import polars as pl
+        
+        pdf = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_x": [1, 2, 3]
+        })
+        
+        plf = pl.DataFrame({
+            "key": ["a", "b", "c"],
+            "value_y": [10, 20, 30]
+        })
+        
+        with pytest.raises(TypeError, match=".*polars.*pandas.*"):
+            pd.merge(pdf, plf, on="key")
+    
+    def test_merge_polars_to_pandas_conversion(self):
+        """
+        Test that converting polars to pandas works.
+        
+        Shows the workaround mentioned in error message.
+        """
+        pytest.importorskip("polars")
+        import polars as pl
+        
+        pdf = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_x": [1, 2, 3]
+        })
+        
+        plf = pl.DataFrame({
+            "key": ["a", "b", "c"],
+            "value_y": [10, 20, 30]
+        })
+        
+        # Convert polars to pandas - this should work
+        plf_pd = plf.to_pandas()
+        result = pd.merge(pdf, plf_pd, on="key")
+        
+        expected = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_x": [1, 2, 3],
+            "value_y": [10, 20, 30]
+        })
+        
+        tm.assert_frame_equal(result, expected)
+    
+    def test_merge_with_dict(self):
+        """Test that merging with dict raises TypeError with helpful message."""
+        df = DataFrame({"key": ["a", "b"], "value": [1, 2]})
+        
+        dict_obj = {"key": ["a", "b"], "value": [3, 4]}
+        
+        with pytest.raises(TypeError, match=".*dict.*"):
+            pd.merge(df, dict_obj, on="key")
+    
+    def test_merge_with_list(self):
+        """Test that merging with list raises TypeError with helpful message."""
+        df = DataFrame({"key": ["a", "b"], "value": [1, 2]})
+        
+        list_obj = [["a", 1], ["b", 2]]
+        
+        msg = "Can only merge Series or DataFrame objects"
+        
+        with pytest.raises(TypeError, match=msg):
+            pd.merge(df, list_obj, on="key")
+    
+    def test_merge_pandas_baseline(self):
+        """
+        Test that normal pandas merge still works.
+        
+        Baseline test to ensure fix doesn't break existing functionality.
+        """
+        df1 = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_x": [1, 2, 3]
+        })
+        
+        df2 = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_y": [10, 20, 30]
+        })
+        
+        result = pd.merge(df1, df2, on="key")
+        
+        expected = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_x": [1, 2, 3],
+            "value_y": [10, 20, 30]
+        })
+        
+        tm.assert_frame_equal(result, expected)
+    
+    def test_merge_with_series_name(self):
+        """Test that merging with named Series works (baseline)."""
+        df = DataFrame({"key": ["a", "b", "c"], "value_x": [1, 2, 3]})
+        s = Series([10, 20, 30], name="value_y")
+        
+        result = pd.merge(df, s, left_index=True, right_index=True)
+        
+        expected = DataFrame({
+            "key": ["a", "b", "c"],
+            "value_x": [1, 2, 3],
+            "value_y": [10, 20, 30]
+        })
+        
+        tm.assert_frame_equal(result, expected)
+    
+    def test_merge_with_unnamed_series(self):
+        """Test that merging with unnamed Series raises helpful error."""
+        df = DataFrame({"key": ["a", "b", "c"], "value": [1, 2, 3]})
+        s = Series([10, 20, 30])  # No name
+        
+        msg = "Cannot merge a Series without a name"
+        
+        with pytest.raises(ValueError, match=msg):
+            pd.merge(df, s, left_index=True, right_index=True)
diff --git a/test_validation_61434.py b/test_validation_61434.py
