Groupby.transform doesn't work in dask dataframe

I think you can use join:

s = AID.groupby('AID')['ANumOfF'].sum()
AID = AID.set_index('AID').drop('ANumOfF', axis=1).join(s).reset_index()
print (AID)
   AID FID  ANumOfF
0    1   X        6
1    1   Y        6
2    2   Z       36
3    2   A       36
4    2   X       36
5    2   B       36

Or faster solution with map by aggregate Series or dict:

s = AID.groupby('AID')['ANumOfF'].sum()
#a bit faster
#s = AID.groupby('AID')['ANumOfF'].sum().to_dict()
AID['ANumOfF'] = AID['AID'].map(s)
print (AID)
   AID FID  ANumOfF
0    1   X        6
1    1   Y        6
2    2   Z       36
3    2   A       36
4    2   X       36
5    2   B       36