ENH include the first incompatible key in error message when merging

doc/source/whatsnew/v2.1.0.rst

@@ -32,7 +32,8 @@ Other enhancements
 - :meth:`MultiIndex.sortlevel` and :meth:`Index.sortlevel` gained a new keyword ``na_position`` (:issue:`51612`)
 - Improve error message when setting :class:`DataFrame` with wrong number of columns through :meth:`DataFrame.isetitem` (:issue:`51701`)
 - Let :meth:`DataFrame.to_feather` accept a non-default :class:`Index` and non-string column names (:issue:`51787`)
-- :class:`api.extensions.ExtensionArray` now has a :meth:`~api.extensions.ExtensionArray.map` method (:issue:`51809`).
+- :class:`api.extensions.ExtensionArray` now has a :meth:`~api.extensions.ExtensionArray.map` method (:issue:`51809`)
+- Improve error message when having incompatible columns using :meth:`DataFrame.merge` (:issue:`51861`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_210.notable_bug_fixes:

pandas/core/reshape/merge.py

@@ -1297,8 +1297,8 @@ def _maybe_coerce_merge_keys(self) -> None:
                 continue
 
             msg = (
-                f"You are trying to merge on {lk.dtype} and "
-                f"{rk.dtype} columns. If you wish to proceed you should use pd.concat"
+                f"You are trying to merge on {lk.dtype} and {rk.dtype} columns "
+                f"for key '{name}'. If you wish to proceed you should use pd.concat"
             )
 
             # if we are numeric, then allow differing

pandas/tests/reshape/merge/test_join.py

@@ -141,10 +141,9 @@ def test_join_on(self, target_source):
 
         # overlap
         source_copy = source.copy()
-        source_copy["A"] = 0
         msg = (
-            "You are trying to merge on float64 and object columns. If "
-            "you wish to proceed you should use pd.concat"
+            "You are trying to merge on float64 and object columns for key 'A'. "
+            "If you wish to proceed you should use pd.concat"
         )
         with pytest.raises(ValueError, match=msg):
             target.join(source_copy, on="A")

pandas/tests/reshape/merge/test_merge.py

@@ -1630,24 +1630,50 @@ def test_merge_incompat_dtypes_error(self, df1_vals, df2_vals):
         df2 = DataFrame({"A": df2_vals})
 
         msg = (
-            f"You are trying to merge on {df1['A'].dtype} and "
-            f"{df2['A'].dtype} columns. If you wish to proceed "
-            "you should use pd.concat"
+            f"You are trying to merge on {df1['A'].dtype} and {df2['A'].dtype} "
+            "columns for key 'A'. If you wish to proceed you should use pd.concat"
         )
         msg = re.escape(msg)
         with pytest.raises(ValueError, match=msg):
             merge(df1, df2, on=["A"])
 
         # Check that error still raised when swapping order of dataframes
         msg = (
-            f"You are trying to merge on {df2['A'].dtype} and "
-            f"{df1['A'].dtype} columns. If you wish to proceed "
-            "you should use pd.concat"
+            f"You are trying to merge on {df2['A'].dtype} and {df1['A'].dtype} "
+            "columns for key 'A'. If you wish to proceed you should use pd.concat"
         )
         msg = re.escape(msg)
         with pytest.raises(ValueError, match=msg):
             merge(df2, df1, on=["A"])
 
+        # Check that error still raised when merging on multiple columns
+        # The error message should mention the first incompatible column
+        if len(df1_vals) == len(df2_vals):
+            # Column A in df1 and df2 is of compatible (the same) dtype
+            # Columns B and C in df1 and df2 are of incompatible dtypes
+            df3 = DataFrame({"A": df2_vals, "B": df1_vals, "C": df1_vals})
+            df4 = DataFrame({"A": df2_vals, "B": df2_vals, "C": df2_vals})
+
+            # Check that error raised correctly when merging all columns A, B, and C
+            # The error message should mention key 'B'
+            msg = (
+                f"You are trying to merge on {df3['B'].dtype} and {df4['B'].dtype} "
+                "columns for key 'B'. If you wish to proceed you should use pd.concat"
+            )
+            msg = re.escape(msg)
+            with pytest.raises(ValueError, match=msg):
+                merge(df3, df4)
+
+            # Check that error raised correctly when merging columns A and C
+            # The error message should mention key 'C'
+            msg = (
+                f"You are trying to merge on {df3['C'].dtype} and {df4['C'].dtype} "
+                "columns for key 'C'. If you wish to proceed you should use pd.concat"
+            )
+            msg = re.escape(msg)
+            with pytest.raises(ValueError, match=msg):
+                merge(df3, df4, on=["A", "C"])
+
     @pytest.mark.parametrize(
         "expected_data, how",
         [