<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def rank(self, method='average', ascending=True, na_option='keep', pct=False, axis=0):
 """ Provide the rank of values within each group. Parameters method : {'average', 'min', 'max', 'first', 'dense'}, default 'average' * average: average rank of group * min: lowest rank in group * max: highest rank in group * first: ranks assigned in order they appear in the array * dense: like 'min', but rank always increases by 1 between groups ascending : boolean, default True False for ranks by high (1) to low (N) na_option : {'keep', 'top', 'bottom'}, default 'keep' * keep: leave NA values where they are * top: smallest rank if ascending * bottom: smallest rank if descending pct : boolean, default False Compute percentage rank of data within each group axis : int, default 0 The axis of the object over which to compute the rank. Returns ------- DataFrame with ranking of values within each group """

        if na_option not in {'keep', 'top', 'bottom'}:
            msg = "na_option must be one of 'keep', 'top', or 'bottom'"
            raise ValueError(msg)
        return self._cython_transform('rank', numeric_only=False,
                                      ties_method=method, ascending=ascending,
                                      na_option=na_option, pct=pct, axis=axis)