percentileofscore groupby expanding

I'm trying to calculate the following:

1. Compute the percentile rank of a value relative to a list of valuesin a different series.
2. By group.
3. And in an expanding dataframe window.

I've tried the following without success.

df['pct']=df.groupby(['Category']).expanding(1).apply(lambda x: stats.percentileofscore(df['values'],1)).reset_index(0,drop=True)