Broadcasting TimeSeries

[Davide Dalmasso]

Hello,

I opened my Python Shell and I wrote the following:

>>> import numpy as np
>>> import pandas as pd

then I made a function

>>> def afunc(aframe):
    return aframe - aframe.mean(axis=1)

and I defined a DataFrame with time as index

>>> A = pd.DataFrame(np.random.randn(5,3),columns=['a','b','c'],index=pd.date_range(start='1984-12-20',periods=5))
>>> A
                   a         b         c
1984-12-20 -0.257916 -0.137923 -0.669796
1984-12-21 -1.632874 -1.850365  1.571715
1984-12-22  1.185828 -0.149839 -1.565930
1984-12-23 -0.757311  0.034627  0.794608
1984-12-24  0.548785 -1.126786 -0.438457

now, if I call the function the following error is generated:

>>> afunc(A)
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    afunc(A)
  File "<pyshell#3>", line 2, in afunc
    return aframe - aframe.mean(axis=1)
  File "C:\Python33\lib\site-packages\pandas\core\frame.py", line 217, in f
    return self._combine_series(other, na_op, fill_value, axis, level)
  File "C:\Python33\lib\site-packages\pandas\core\frame.py", line 3601, in _combine_series
    return self._combine_series_infer(other, func, fill_value)
  File "C:\Python33\lib\site-packages\pandas\core\frame.py", line 3619, in _combine_series_infer
    FutureWarning)
  File "C:\Python33\lib\idlelib\PyShell.py", line 60, in idle_showwarning
    file.write(warnings.formatwarning(message, category, filename,
AttributeError: 'NoneType' object has no attribute 'write'

then I recall the same function (without any change) and the function works!

>>> afunc(A)
                   a         b         c
1984-12-20  0.097295  0.217289 -0.314584
1984-12-21 -0.995699 -1.213190  2.208890
1984-12-22  1.362475  0.026808 -1.389283
1984-12-23 -0.781285  0.010652  0.770633
1984-12-24  0.887604 -0.787967 -0.099637

Why does this happen? Why I must call the function two times before it finally works?
How can I overcome this problem?

Many thanks in advance

Davide