BUG: applymap on empty DataFrame returns Series (pandas-dev#8222)

Author: mroeschke

doc/source/whatsnew/v0.20.0.txt

@@ -284,11 +284,10 @@ Bug Fixes
 - Bug in ``DataFrame(..).apply(to_numeric)`` when values are of type decimal.Decimal. (:issue:`14827`)
 - Bug in ``describe()`` when passing a numpy array which does not contain the median to the ``percentiles`` keyword argument (:issue:`14908`)
 - Bug in ``DataFrame.sort_values()`` when sorting by multiple columns where one column is of type ``int64`` and contains ``NaT`` (:issue:`14922`)
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+- Bug in ``applymap()`` returning a ``Series`` with an empty ``DataFrame`` (:issue:`8222`)
 
 
 - Bug in ``pd.read_msgpack()`` in which ``Series`` categoricals were being improperly processed (:issue:`14901`)
-- Bug in ``Series.ffill()`` with mixed dtypes containing tz-aware datetimes. (:issue:`14956`)
 
 
 
@@ -301,20 +300,5 @@ Bug Fixes
 
 - Bug in ``Series.unique()`` in which unsigned 64-bit integers were causing overflow (:issue:`14721`)
 - Bug in ``pd.unique()`` in which unsigned 64-bit integers were causing overflow (:issue:`14915`)
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 - Require at least 0.23 version of cython to avoid problems with character encodings (:issue:`14699`)
 - Bug in converting object elements of array-like objects to unsigned 64-bit integers (:issue:`4471`)
-- Bug in ``pd.pivot_table()`` where no error was raised when values argument was not in the columns (:issue:`14938`)

pandas/core/frame.py

@@ -4288,6 +4288,8 @@ def applymap(self, func):
 
         # if we have a dtype == 'M8[ns]', provide boxed values
         def infer(x):
+            if x.empty:
+                return lib.map_infer(x, func)
             return lib.map_infer(x.asobject, func)
 
         return self.apply(infer)

pandas/tests/frame/test_apply.py

@@ -405,6 +405,15 @@ def test_applymap(self):
         for f in ['datetime', 'timedelta']:
             self.assertEqual(result.loc[0, f], str(df.loc[0, f]))
 
+        # GH 8222
+        empty_frames = [pd.DataFrame(),
+                        pd.DataFrame(columns=list('ABC')),
+                        pd.DataFrame(index=list('ABC'))]
+        for frame in empty_frames:
+            for func in [round, lambda x: x]:
+                result = frame.applymap(func)
+                tm.assert_frame_equal(result, frame)
+
     def test_applymap_box(self):
         # ufunc will not be boxed. Same test cases as the test_map_box
         df = pd.DataFrame({'a': [pd.Timestamp('2011-01-01'),