BUG: DataFrame._item_cache not cleared on on .copy()

doc/source/whatsnew/v1.1.0.rst

@@ -363,6 +363,7 @@ Indexing
 - Bug in :class:`Index` constructor where an unhelpful error message was raised for ``numpy`` scalars (:issue:`33017`)
 - Bug in :meth:`DataFrame.lookup` incorrectly raising an ``AttributeError`` when ``frame.index`` or ``frame.columns`` is not unique; this will now raise a ``ValueError`` with a helpful error message (:issue:`33041`)
 - Bug in :meth:`DataFrame.iloc.__setitem__` creating a new array instead of overwriting ``Categorical`` values in-place (:issue:`32831`)
+- Bug in :meth:`DataFrame.copy` _item_cache invalidated after copy performs consolidation (:issue:`31784`)
 
 Missing
 ^^^^^^^

pandas/core/generic.py

@@ -5680,6 +5680,7 @@ def copy(self: FrameOrSeries, deep: bool_t = True) -> FrameOrSeries:
         dtype: object
         """
         data = self._data.copy(deep=deep)
+        self._clear_item_cache()
         return self._constructor(data).__finalize__(self)
 
     def __copy__(self: FrameOrSeries, deep: bool_t = True) -> FrameOrSeries:

pandas/tests/frame/methods/test_copy.py

@@ -0,0 +1,21 @@
+from pandas import DataFrame
+import pandas._testing as tm
+
+
+class TestCopy:
+    def test_cache_on_copy(self):
+        df = DataFrame({"a": [1]})
+
+        df["x"] = [0]
+        df["a"]
+
+        df.copy()
+
+        df["a"].values[0] = -1
+
+        tm.assert_frame_equal(df, DataFrame({"a": [-1], "x": [0]}))
+
+        df["y"] = [0]
+
+        assert df["a"].values[0] == -1
+        tm.assert_frame_equal(df, DataFrame({"a": [-1], "x": [0], "y": [0]}))