from __future__ import division
from __future__ import print_function
from __future__ import absolute_import

import pandas as pd

def df_prepend_datetime(df,date_columns,date_format,date_name = "_datetime"):
    to_convert = df[date_columns].astype(str).apply("-".join, axis=1)
    return pd.concat([pd.DataFrame(pd.to_datetime(to_convert, format = "-".join(date_format), errors = 'coerce'),columns = [date_name]),df],sort = False,axis=1)