BUG: Inconsistent return type for downsampling on resample of empty DataFrame (#15093)

closes #14692

Author: discort

doc/source/whatsnew/v0.21.0.txt

@@ -113,6 +113,7 @@ Plotting
 
 Groupby/Resample/Rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
+- Bug in ``DataFrame.resample().size()`` where an empty DataFrame did not return a Series (:issue:`14962`)
 
 
 

pandas/core/resample.py

@@ -17,6 +17,7 @@
 from pandas.core.indexes.period import PeriodIndex, period_range
 import pandas.core.common as com
 import pandas.core.algorithms as algos
+from pandas.core.dtypes.generic import ABCDataFrame
 
 import pandas.compat as compat
 from pandas.compat.numpy import function as nv
@@ -549,6 +550,15 @@ def var(self, ddof=1, *args, **kwargs):
         nv.validate_resampler_func('var', args, kwargs)
         return self._downsample('var', ddof=ddof)
 
+    @Appender(GroupBy.size.__doc__)
+    def size(self):
+        # It's a special case as higher level does return
+        # a copy of 0-len objects. GH14962
+        result = self._downsample('size')
+        if not len(self.ax) and isinstance(self._selected_obj, ABCDataFrame):
+            result = pd.Series([], index=result.index, dtype='int64')
+        return result
+
 
 Resampler._deprecated_valids += dir(Resampler)
 
@@ -563,8 +573,7 @@ def f(self, _method=method, *args, **kwargs):
     setattr(Resampler, method, f)
 
 # groupby & aggregate methods
-for method in ['count', 'size']:
-
+for method in ['count']:
     def f(self, _method=method):
         return self._downsample(_method)
     f.__doc__ = getattr(GroupBy, method).__doc__

pandas/tests/test_resample.py

@@ -783,15 +783,19 @@ def test_resample_empty_dataframe(self):
 
         for freq in ['M', 'D', 'H']:
             # count retains dimensions too
-            methods = downsample_methods + ['count']
+            methods = downsample_methods + upsample_methods
             for method in methods:
                 result = getattr(f.resample(freq), method)()
+                if method != 'size':
+                    expected = f.copy()
+                else:
+                    # GH14962
+                    expected = Series([])
 
-                expected = f.copy()
                 expected.index = f.index._shallow_copy(freq=freq)
                 assert_index_equal(result.index, expected.index)
                 assert result.index.freq == expected.index.freq
-                assert_frame_equal(result, expected, check_dtype=False)
+                assert_almost_equal(result, expected, check_dtype=False)
 
             # test size for GH13212 (currently stays as df)