BUG: resolved problem with DataFrame.equals() wrongly returning True (pandas-dev#28839)

The function was returning True in case shown in added test. The cause
of the problem was sorting Blocks of DataFrame by type, and then
mgr_locs before comparison. It resulted in arranging the identical blocks
in the same way, which resulted in having the same two lists of blocks.
Changing sorting order to (mgr_locs, type) resolves the problem, while not
interrupting the other aspects of comparison.

Author: Mateusz Górski

pandas/core/internals/managers.py

@@ -1399,7 +1399,7 @@ def equals(self, other):
         # blocks (say, Categorical) which can only be distinguished by
         # the iteration order
         def canonicalize(block):
-            return (block.dtype.name, block.mgr_locs.as_array.tolist())
+            return (block.mgr_locs.as_array.tolist(), block.dtype.name)
 
         self_blocks = sorted(self.blocks, key=canonicalize)
         other_blocks = sorted(other.blocks, key=canonicalize)

pandas/tests/internals/test_internals.py

@@ -1297,3 +1297,9 @@ def test_make_block_no_pandas_array():
     result = make_block(arr.to_numpy(), slice(len(arr)), dtype=arr.dtype)
     assert result.is_integer is True
     assert result.is_extension is False
+
+
+def test_dataframe_not_equal():
+    df1 = pd.DataFrame({"a": [1, 2], "b": ["s", "d"]})
+    df2 = pd.DataFrame({"a": ["s", "d"], "b": [1, 2]})
+    assert df1.equals(df2) is False