Keep rows of dataframe if multiple conditions met

Question

Given the following dataframe:

df = pd.DataFrame({'A': ["EQ", "CB", "CB", "FF", "EQ", "EQ", "CB", "CB"],
                   'B': ["ANT", "ANT", "DQ", "DQ", "BQ", "VGQ", "GHB", "VGQ"]})

How can I keep the rows of column B if it meets the condition of exist for both EQ and CB. For example, I would want to keep ANT because it exists for both EQ and CB, while DQ would be deleted. So the expected output for the df would be:

out = pd.DataFrame({'A': ["EQ", "CB", "EQ", "CB"],
                       'B': ["ANT", "ANT", "VGQ", "VGQ"]})

Thanks!

Answer 1

Here is a solution not using groupby() if you want code that may be easier to think about:

equities = df.B[df.A == 'EQ']
bonds = df.B[df.A == 'CB']
both = equities[equities.isin(bonds)]

That gives you:

0    ANT
5    VGQ

Which makes the last part easy:

df[df.B.isin(both)]
Out: 
    A    B
0  EQ  ANT
1  CB  ANT
5  EQ  VGQ
7  CB  VGQ

This is 3x faster on small data sets than groupby().filter().