Assignment broken for sliced multi-indices

[mwiebusch78]

Consider the following data frame:

>>> df = pd.DataFrame(1.0, index=pd.MultiIndex.from_product([[0, 1], [0, 1]]), columns=['a'])
>>> df
       a
0 0  1.0
  1  1.0
1 0  1.0
  1  1.0

Taking a slice of this and assigning it to itself should leave the data unchanged, but look what happens:

>>> df.loc[(0,), :] = df.loc[(0,), :]
>>> df
       a
0 0  NaN
  1  NaN
1 0  1.0
  1  1.0

I'm on pandas version 0.20.1

[jreback]

a duplicate of #10440

see the examples there on how to do this. pandas doesn't know how to align your slice. it seems obvious, but is actually non-trivial.