Merge pull request #5935 from jreback/empty_frame_copy

BUG: Bug in creating an empty DataFrame, copying, then assigning (GH5932)

Author: jreback

doc/source/release.rst

@@ -89,6 +89,7 @@ Bug Fixes
   - Bug in idxmin/max with object dtypes (:issue:`5914`)
   - Bug in ``BusinessDay`` when adding n days to a date not on offset when n>5 and n%5==0 (:issue:`5890`)
   - Bug in assigning to chained series with a series via ix (:issue:`5928`)
+  - Bug in creating an empty DataFrame, copying, then assigning (:issue:`5932`)
 
 pandas 0.13.0
 -------------

pandas/core/internals.py

@@ -1961,8 +1961,11 @@ def make_empty(self, axes=None):
             ]
 
         # preserve dtype if possible
-        dtype = self.dtype if self.ndim == 1 else object
-        return self.__class__(np.array([], dtype=dtype), axes)
+        if self.ndim == 1:
+            blocks = np.array([], dtype=self.dtype)
+        else:
+            blocks = []
+        return self.__class__(blocks, axes)
 
     def __nonzero__(self):
         return True

pandas/tests/test_indexing.py

@@ -1843,6 +1843,14 @@ def f():
         df = DataFrame(Series(name='foo'))
         assert_frame_equal(df, DataFrame({ 'foo' : Series() }))
 
+        # GH 5932
+        # copy on empty with assignment fails
+        df = DataFrame(index=[0])
+        df = df.copy()
+        df['a'] = 0
+        expected = DataFrame(0,index=[0],columns=['a'])
+        assert_frame_equal(df, expected)
+
     def test_cache_updating(self):
         # GH 4939, make sure to update the cache on setitem