DataFrame constructor produces NaN values when passing df as data param and an index containing values that are not present in passed in df

[alan-wong]

Whilst answering this SO question I discovered that the DataFrame constructor is ignoring the fact that I'm passing an index explicitly resulting in a df with all NaNs:

In [6]:
df = pd.DataFrame({'AAA':[4,5,6,7],'BBB':[10,20,30,40],'CCC':[100,50,-30,-50]})
df1 = pd.DataFrame(data=df,index=(['a','b','c','d']))
df1
Out[6]:
   AAA  BBB  CCC
a  NaN  NaN  NaN
b  NaN  NaN  NaN
c  NaN  NaN  NaN
d  NaN  NaN  NaN

Stepping through the code I see that it reaches here in frame.py:

            if isinstance(data, BlockManager):
            mgr = self._init_mgr(data, axes=dict(index=index, columns=columns),
                                 dtype=dtype, copy=copy)

and then ends up here in generic.py:

119         def _init_mgr(self, mgr, axes=None, dtype=None, copy=False):
120             """ passed a manager and a axes dict """
121             for a, axe in axes.items():
122                 if axe is not None:
123                     mgr = mgr.reindex_axis(
124  ->                     axe, axis=self._get_block_manager_axis(a), copy=False)

The behaviour can be reproduced simply like this:

In [46]:

data1 = pd.DataFrame({'AAA':[4,5,6,7],'BBB':[10,20,30,40],'CCC':[100,50,-30,-50]})
data1.reindex_axis(list('abcd'))
Out[46]:
   AAA  BBB  CCC
a  NaN  NaN  NaN
b  NaN  NaN  NaN
c  NaN  NaN  NaN
d  NaN  NaN  NaN

[jreback]

xref here: #9237

This is the defined behavior, to reindex the provided input to the passed index and/or columns .