BUG: Frame.iat item_cache invalidation bug

doc/source/whatsnew/v1.5.0.rst

@@ -268,6 +268,7 @@ Indexing
 - Bug in :meth:`Series.__setitem__` where setting :attr:`NA` into a numeric-dtpye :class:`Series` would incorrectly upcast to object-dtype rather than treating the value as ``np.nan`` (:issue:`44199`)
 - Bug in :meth:`DataFrame.mask` with ``inplace=True`` and ``ExtensionDtype`` columns incorrectly raising (:issue:`45577`)
 - Bug in getting a column from a DataFrame with an object-dtype row index with datetime-like values: the resulting Series now preserves the exact object-dtype Index from the parent DataFrame (:issue:`42950`)
+- Bug in :meth:`DataFrame.iat` setting values leading to not propagating correctly in subsequent lookups (:issue:`45684`)
 - Bug in indexing on a :class:`DatetimeIndex` with a ``np.str_`` key incorrectly raising (:issue:`45580`)
 - Bug in :meth:`CategoricalIndex.get_indexer` when index contains ``NaN`` values, resulting in elements that are in target but not present in the index to be mapped to the index of the NaN element, instead of -1 (:issue:`45361`)
 -

pandas/core/frame.py

@@ -3878,15 +3878,15 @@ def _set_value(
         try:
             if takeable:
                 series = self._ixs(col, axis=1)
-                series._set_value(index, value, takeable=True)
-                return
-
-            series = self._get_item_cache(col)
-            loc = self.index.get_loc(index)
+                loc = index
+            else:
+                series = self._get_item_cache(col)
+                loc = self.index.get_loc(index)
 
-            # series._set_value will do validation that may raise TypeError
+            # setitem_inplace will do validation that may raise TypeError
             #  or ValueError
-            series._set_value(loc, value, takeable=True)
+            series._mgr.setitem_inplace(loc, value)
+
         except (KeyError, TypeError, ValueError):
             # set using a non-recursive method & reset the cache
             if takeable:

pandas/tests/indexing/test_iat.py

@@ -29,3 +29,20 @@ def test_iat_getitem_series_with_period_index():
     expected = ser[index[0]]
     result = ser.iat[0]
     assert expected == result
+
+
+def test_iat_setitem_item_cache_cleared():
+    # GH#45684
+    data = {"x": np.arange(8, dtype=np.int64), "y": np.int64(0)}
+    df = DataFrame(data).copy()
+    ser = df["y"]
+
+    # previously this iat setting would split the block and fail to clear
+    #  the item_cache.
+    df.iat[7, 0] = 9999
+
+    df.iat[7, 1] = 1234
+
+    assert df.iat[7, 1] == 1234
+    assert ser.iloc[-1] == 1234
+    assert df.iloc[-1, -1] == 1234