Commits on Aug 14, 2017

1. BUG: Fix behavior of argmax and argmin with inf (pandas-dev#16449) …

   Closes pandas-dev#13595
   
   The implementations of `nanargmin` and `nanargmax` in `nanops` were
   forcing the `_get_values` utility function to always mask out infinite
   values. For example, in `nanargmax`,
   
       >>> nanops._get_values(np.array([1, np.nan, np.inf]), True,
       isfinite=True, fill_value_typ='-inf')
   
       (array([  1., -inf, -inf]),
        array([False,  True,  True], dtype=bool),
        dtype('float64'),
        numpy.float64)
   
   The first element of the result tuple (the masked version of the
   values array) is used for actually finding the max or min argument. As
   a result, infinite values could never be correctly recognized as the
   maximum or minimum values in an array.
   
   This also affects the behavior of `Series.idxmax` with string data (or
   the `object` dtype generally). Previously, `nanargmax` would always
   attempt to coerce its input to float, even when there were no missing
   values. Now, it will not, and so will work correctly in the particular
   case of a `Series` of strings with no missing values. However, because
   it's difficult to ensure that `nanargmin` and `nanargmax` will behave
   consistently for arbitrary `Series` of `object` with and without
   missing values, these functions are now explicitly disallowed for
   `object`.

   

   DGrady authored and Daniel Grady committed Aug 14, 2017
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