Merge pull request pandas-dev#11025 from sinhrks/groupby_count_doc

DOC: Add GroupBy.count

Author: jreback

pandas/core/groupby.py

@@ -719,6 +719,12 @@ def irow(self, i):
                       FutureWarning, stacklevel=2)
         return self.nth(i)
 
+    def count(self):
+        """ Compute count of group, excluding missing values """
+
+        # defined here for API doc
+        raise NotImplementedError
+
     def mean(self):
         """
         Compute mean of groups, excluding missing values
@@ -2674,6 +2680,7 @@ def value_counts(self, normalize=False, sort=True, ascending=False,
         return Series(out, index=mi)
 
     def count(self):
+        """ Compute count of group, excluding missing values """
         ids, _, ngroups = self.grouper.group_info
         val = self.obj.get_values()
 
@@ -3458,6 +3465,7 @@ def _apply_to_column_groupbys(self, func):
             keys=self._selected_obj.columns, axis=1)
 
     def count(self):
+        """ Compute count of group, excluding missing values """
         from functools import partial
         from pandas.lib import count_level_2d
         from pandas.core.common import _isnull_ndarraylike as isnull