BUG: clear cache on DataFrame._is_homogeneous_dtype (#34937)

Author: jbrockmendel

pandas/core/frame.py

@@ -611,7 +611,8 @@ def _is_homogeneous_type(self) -> bool:
         if self._mgr.any_extension_types:
             return len({block.dtype for block in self._mgr.blocks}) == 1
         else:
-            return not self._mgr.is_mixed_type
+            # Note: consolidates inplace
+            return not self._is_mixed_type
 
     @property
     def _can_fast_transpose(self) -> bool:

pandas/tests/frame/test_dtypes.py

@@ -233,6 +233,18 @@ def test_constructor_list_str_na(self, string_dtype):
     def test_is_homogeneous_type(self, data, expected):
         assert data._is_homogeneous_type is expected
 
+    def test_is_homogeneous_type_clears_cache(self):
+        ser = pd.Series([1, 2, 3])
+        df = ser.to_frame("A")
+        df["B"] = ser
+
+        assert len(df._mgr.blocks) == 2
+
+        a = df["B"]  # caches lookup
+        df._is_homogeneous_type  # _should_ clear cache
+        assert len(df._mgr.blocks) == 1
+        assert df["B"] is not a
+
     def test_asarray_homogenous(self):
         df = pd.DataFrame({"A": pd.Categorical([1, 2]), "B": pd.Categorical([1, 2])})
         result = np.asarray(df)