pandas filter rows based on atmost matching criteria

I have a dataframe like as shown below

df=pd.DataFrame({'subjects':['A','A','D','B','B','C'],
                'B':['12','12','13','14','14','16'],
                'C':[21,23,24,25,26,27]
                })
df['r_no'] = df.groupby(['subjects','B']).cumcount()+1

Now, I would like to only select rows that had only r_no = 1 (and not r_no > 1).

I tried the below

df[df['subjects'].value_counts() == 1]
df.iloc[df['subjects'].value_counts() == 1:,]
df.ix[df['subjects'].value_counts() == 1:,]
df[(df['r_no'] == 1) & (df['r_no'] < 2)]

None of them worked.

I expect my output to be like as shown below.

You can see that subjects = A and subjects = B is excluded because they also had rows where r_no > 1. Basically, I want to select subjects that had only one record for them (r_no) in dataframe

>Solution :

IIUC, what you want to do it just to keep the groups of size 1:

df[df.groupby(['subjects','B'])['C'].transform(len).le(1)]

Or maybe even just keeping the subjects with unique rows:

df[~df['subjects'].duplicated(keep=False)]

output:

  subjects   B   C
2        D  13  24
5        C  16  27