BUG: Can't store callables using __setitem__

doc/source/whatsnew/v0.18.2.txt

@@ -481,7 +481,7 @@ Bug Fixes
 - Bug in ``.tz_convert`` on a tz-aware ``DateTimeIndex`` that relied on index being sorted for correct results (:issue:`13306`)
 - Bug in ``pd.read_hdf()`` where attempting to load an HDF file with a single dataset, that had one or more categorical columns, failed unless the key argument was set to the name of the dataset. (:issue:`13231`)
 - Bug in ``.rolling()`` that allowed a negative integer window in contruction of the ``Rolling()`` object, but would later fail on aggregation (:issue:`13383`)
-
+- Bug in ``__setitem__`` causing a callable rhs to be applied rather than stored (:issue:`13299`)
 
 - Bug in various index types, which did not propagate the name of passed index (:issue:`12309`)
 - Bug in ``DatetimeIndex``, which did not honour the ``copy=True`` (:issue:`13205`)

pandas/core/frame.py

@@ -2395,7 +2395,7 @@ def _setitem_frame(self, key, value):
 
         self._check_inplace_setting(value)
         self._check_setitem_copy()
-        self.where(-key, value, inplace=True)
+        self.where(-key, value, inplace=True, apply_other=False)
 
     def _ensure_valid_index(self, value):
         """

pandas/core/generic.py

@@ -4455,6 +4455,9 @@ def _align_series(self, other, join='outer', axis=None, level=None,
         raise_on_error : boolean, default True
             Whether to raise on invalid data types (e.g. trying to where on
             strings)
+        apply_other : boolean, default True
+            If False, other will be stored directly rather than applied to
+            self, even if callable
 
         Returns
         -------
@@ -4463,10 +4466,12 @@ def _align_series(self, other, join='outer', axis=None, level=None,
 
     @Appender(_shared_docs['where'] % dict(_shared_doc_kwargs, cond="True"))
     def where(self, cond, other=np.nan, inplace=False, axis=None, level=None,
-              try_cast=False, raise_on_error=True):
+              try_cast=False, raise_on_error=True, apply_other=True):
 
         cond = com._apply_if_callable(cond, self)
-        other = com._apply_if_callable(other, self)
+
+        if apply_other:
+            other = com._apply_if_callable(other, self)
 
         if isinstance(cond, NDFrame):
             cond, _ = cond.align(self, join='right', broadcast_axis=1)

pandas/core/series.py

@@ -738,7 +738,7 @@ def setitem(key, value):
             if is_bool_indexer(key):
                 key = check_bool_indexer(self.index, key)
                 try:
-                    self.where(~key, value, inplace=True)
+                    self.where(~key, value, inplace=True, apply_other=False)
                     return
                 except InvalidIndexError:
                     pass

pandas/tests/frame/test_indexing.py

@@ -207,6 +207,16 @@ def test_setitem_callable(self):
         exp = pd.DataFrame({'A': [11, 12, 13, 14], 'B': [5, 6, 7, 8]})
         tm.assert_frame_equal(df, exp)
 
+    def test_setitem_other_callable(self):
+        # GH 13299
+        inc = lambda x: x + 1
+
+        df = pd.DataFrame([[-1, 1], [1, -1]])
+        df[df > 0] = inc
+
+        expected = pd.DataFrame([[-1, inc], [inc, -1]])
+        tm.assert_frame_equal(df, expected)
+
     def test_getitem_boolean(self):
         # boolean indexing
         d = self.tsframe.index[10]

pandas/tests/series/test_indexing.py

@@ -441,6 +441,16 @@ def test_setitem_callable(self):
         s[lambda x: 'A'] = -1
         tm.assert_series_equal(s, pd.Series([-1, 2, 3, 4], index=list('ABCD')))
 
+    def test_setitem_other_callable(self):
+        # GH 13299
+        inc = lambda x: x + 1
+
+        s = pd.Series([1, 2, -1, 4])
+        s[s < 0] = inc
+
+        expected = pd.Series([1, 2, inc, 4])
+        tm.assert_series_equal(s, expected)
+
     def test_slice(self):
         numSlice = self.series[10:20]
         numSliceEnd = self.series[-10:]