size aggregation does not seem to work properly when used with ungrouped dataframes. When replacing size with count the examples run through. However, this change null semantics, counting nulls (size) in contrast to skipping nulls (count). The examples below are for Pandas version 0.20.1.
When used with .groupby(...), size aggregation works as expected:
>>> pd.DataFrame({'g': [0, 0, 1], 'v': [1, 2, None]}).groupby('g').agg({'v': 'size'})
v
g
0 2
1 1
Without .groupby, it raises an recursion error:
>>> pd.DataFrame({'g': [0, 0, 1], 'v': [1, 2, None]}).agg({'v': 'size'})
---------------------------------------------------------------------------
RecursionError Traceback (most recent call last)
<ipython-input-15-5fc5c38cdb5f> in <module>()
----> 1 pd.DataFrame({'g': [0, 0, 1], 'v': [1, 2, 3]}).agg({'v': 'size'})
/Volumes/Home/venvs/py3-data-science/lib/python3.5/site-packages/pandas/core/frame.py in aggregate(self, func, axis, *args, **kwargs)
4250 pass
4251 if result is None:
-> 4252 return self.apply(func, axis=axis, args=args, **kwargs)
4253 return result
4254
/Volumes/Home/venvs/py3-data-science/lib/python3.5/site-packages/pandas/core/frame.py in apply(self, func, axis, broadcast, raw, reduce, args, **kwds)
4322 # dispatch to agg
4323 if axis == 0 and isinstance(func, (list, dict)):
-> 4324 return self.aggregate(func, axis=axis, *args, **kwds)
4325
4326 if len(self.columns) == 0 and len(self.index) == 0:
... last 2 frames repeated, from the frame below ...
/Volumes/Home/venvs/py3-data-science/lib/python3.5/site-packages/pandas/core/frame.py in aggregate(self, func, axis, *args, **kwargs)
4250 pass
4251 if result is None:
-> 4252 return self.apply(func, axis=axis, args=args, **kwargs)
4253 return result
4254
RecursionError: maximum recursion depth exceeded
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