def _validate_freq(self):
from pandas.tseries.frequencies import to_offset<EOL>try:<EOL><INDENT>return to_offset(self.window)<EOL><DEDENT>except (TypeError, ValueError):<EOL><INDENT>raise ValueError("<STR_LIT>"<EOL>"<STR_LIT>"<EOL>"<STR_LIT:index>".format(self.window))<EOL><DEDENT>
Validate & return window frequency.