def ds_format(ds, input_format, output_format):
return datetime.strptime(ds, input_format).strftime(output_format)<EOL>
Takes an input string and outputs another string
as specified in the output format

:param ds: input string which contains a date
:type ds: str
:param input_format: input string format. E.g. %Y-%m-%d
:type input_format: str
:param output_format: output string format  E.g. %Y-%m-%d
:type output_format: str

>>> ds_format('2015-01-01', "%Y-%m-%d", "%m-%d-%y")
'01-01-15'
>>> ds_format('1/5/2015', "%m/%d/%Y",  "%Y-%m-%d")
'2015-01-05'