Backport PR #47949 on branch 1.4.x (TST: Add test for loc not updating cache correctly) (#47949) (#47954)

Author: phofl

doc/source/whatsnew/v1.4.4.rst

@@ -15,6 +15,7 @@ including other versions of pandas.
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed regression in :func:`concat` materializing :class:`Index` during sorting even if :class:`Index` was already sorted (:issue:`47501`)
+- Fixed regression in :meth:`DataFrame.loc` not updating the cache correctly after values were set (:issue:`47867`)
 - Fixed regression in setting ``None`` or non-string value into a ``string``-dtype Series using a mask (:issue:`47628`)
 -
 

pandas/tests/frame/indexing/test_indexing.py

@@ -1300,6 +1300,29 @@ def test_loc_expand_empty_frame_keep_midx_names(self):
         )
         tm.assert_frame_equal(df, expected)
 
+    def test_loc_internals_not_updated_correctly(self):
+        # GH#47867 all steps are necessary to reproduce the initial bug
+        df = DataFrame(
+            {"bool_col": True, "a": 1, "b": 2.5},
+            index=MultiIndex.from_arrays([[1, 2], [1, 2]], names=["idx1", "idx2"]),
+        )
+        idx = [(1, 1)]
+
+        df["c"] = 3
+        df.loc[idx, "c"] = 0
+
+        df.loc[idx, "c"]
+        df.loc[idx, ["a", "b"]]
+
+        df.loc[idx, "c"] = 15
+        result = df.loc[idx, "c"]
+        expected = df = Series(
+            15,
+            index=MultiIndex.from_arrays([[1], [1]], names=["idx1", "idx2"]),
+            name="c",
+        )
+        tm.assert_series_equal(result, expected)
+
 
 class TestDataFrameIndexingUInt64:
     def test_setitem(self, uint64_frame):