DataFrame.asof(where, subset=None)
[source]
The last row without any NaN is taken (or the last row without NaN considering only the subset of columns in the case of a DataFrame)
New in version 0.19.0: For DataFrame
If there is no good value, NaN is returned for a Series a Series of NaN values for a DataFrame
Parameters: 
subset : string or list of strings, default None if not None use these columns for NaN propagation 

Returns: 
where is scalar

See also
Dates are assumed to be sorted Raises if this is not the case
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Licensed under the 3clause BSD License.
http://pandas.pydata.org/pandasdocs/version/0.23.4/generated/pandas.DataFrame.asof.html