DataFrame.join can not have datetime as column names

[turkeytest]

Issue:
For DataFrameA.join( DataFrameB , on = 'somecol' ), DataFrameB can not have datetime objects as column labels

Example:

from datetime import datetime
from pandas import DataFrame

str_dates = [ '20120209' , '20120222' ]
dt_dates = [ datetime(2012,2,9) , datetime(2012,2,22)]

A = DataFrame(str_dates , index = range(2) , columns = ['aa'] )

B = DataFrame([[1,2],[3,4]] , index = str_dates , columns = str_dates)
C = DataFrame([[1,2],[3,4]] , index = str_dates , columns = dt_dates )

works = A.join( B , on = 'aa' ) # works -- extra column labels are string
fails = A.join( C , on = 'aa' ) # fails -- extra column labels are datetime

[adamklein]

The problem is not that the columns cannot be datetime, but rather that strings and datetimes cannot be compared (and thus the resulting columns cannot be ordered). So, if you have on the other hand

A = DataFrame(str_dates , index = range(2) , columns = [datetime(2012,1,1)] )

it should all work. eg,

In [16]: A = DataFrame(str_dates , index = range(2) , columns = [datetime(2012,1,1)] )

In [17]: A
Out[17]: 
  2012-01-01
0   20120209
1   20120222

In [18]: A.join(C, on=datetime(2012,1,1))
Out[18]: 
  2012-01-01  2012-02-09  2012-02-22
0   20120209           1           2
1   20120222           3           4

Not sure what can be done about this one.

[wesm]

This is a bug in Index.union. If two indexes are monotonic but their elements are incomparable, self._inner_indexer will fail. Write a unit test with a union of these two indexes:

ipdb> self
Index([aa], dtype=object)
ipdb> other
Index([2012-02-09 00:00:00, 2012-02-22 00:00:00], dtype=object)

ipdb> self
Index([aa], dtype=object)
ipdb> other
Index([2012-02-09 00:00:00, 2012-02-22 00:00:00], dtype=object)

then add a workaround in case of TypeError from the Cython method

[adamklein]

Wes, please have a look. Falls back on slower non-monotonic methodology in intersect on TypeError (union works fine - I think you meant intersection?).

[wesm]

Yeah intersection, sorry. Let me look at the PR