BUG: assert_frame_equal exception for datetime

When using assert_frame_equal on two DataFrames that are equal, besides for their dtypes, one dtype being datetime64[ns] and the other not being an extension array, the code raises an exception due to an assertion that checks whether both are extension arrays. This fix addresses issue pandas-dev#37609.

Author: ssortman

pandas/_testing.py

@@ -1456,7 +1456,19 @@ def assert_series_equal(
             check_dtype=check_dtype,
             index_values=np.asarray(left.index),
         )
-    elif needs_i8_conversion(left.dtype) or needs_i8_conversion(right.dtype):
+    elif (is_extension_array_dtype(left.dtype) and needs_i8_conversion(right.dtype)) or (
+            is_extension_array_dtype(right.dtype) and needs_i8_conversion(left.dtype)
+    ):
+        #If we have an extension array and Datetimearray / TimedeltaArray, we still
+        #want to assert_extension_array_equal even though Datetimearray / TimedeltaArray
+        #dtypes are not an instance of ExtensionArraydtype subclass
+        assert_extension_array_equal(
+            left._values,
+            right._values,
+            check_dtype=check_dtype,
+            index_values=np.asarray(left.index),
+        )
+    elif needs_i8_conversion(left.dtype) and needs_i8_conversion(right.dtype):
         # DatetimeArray or TimedeltaArray
         assert_extension_array_equal(
             left._values,

pandas/tests/util/test_assert_extension_array_equal.py

@@ -111,3 +111,8 @@ def test_assert_extension_array_equal_ignore_dtype_mismatch(right_dtype):
     left = array([1, 2, 3], dtype="Int64")
     right = array([1, 2, 3], dtype=right_dtype)
     tm.assert_extension_array_equal(left, right, check_dtype=False)
+
+def test_assert_frame_equal_datetime_dtype_mismatch():
+    df1 = DataFrame({'a': []}, dtype="datetime64[ns]")
+    df2 = DataFrame({'a': []})
+    tm.assert_frame_equal(df1, df2, check_dtype=False)