concat() ignores ignore_index=True
Unless I misunderstood what the documentation says, this is not the intended behavior:

df1 = pandas.DataFrame({'a':np.arange(5),'b':np.arange(5)},index=3+np.arange(5))
df2 = pandas.DataFrame({'c':np.arange(5),'d':np.arange(5)})
pandas.concat([df1,df2],axis=1,ignore_index=False)
    a   b   c   d
0 NaN NaN   0   0
1 NaN NaN   1   1
2 NaN NaN   2   2
3   0   0   3   3
4   1   1   4   4
5   2   2 NaN NaN
6   3   3 NaN NaN
7   4   4 NaN NaN

The following function does a work-around for this bug by explicitly re-setting the index:

def myConcat(df1,df2):
    if df1.shape[0] != df2.shape[0]:
        return False
    else:
        df2.index = df1.index
        return pandas.concat((df1,df2),axis=1)

Pandas version: commit de6cce5dde22870e279d495c25013a5696bc31df
