Remove row if does not contain the same value in column pandas

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Hello I have a dataframe such as

COL1 COL2 
A    seq1
B    seq1
A    seq2
A    seq3
A    seq4
A    seq5
B    seq9
B    seq4

And I would like to only keep row where A and B within COL1 have the same COL2 value.

I should then get;

COL1 COL2 
A    seq1
B    seq1
A    seq4
B    seq4

Does someone have an idea please ?

pandas dataframe python-3.x python
3 Answers

You can group by COL2 (unintuitive, isn't it?), and get the number of unique items of COL1 for each group:

filtered_df = df[df.groupby('COL2')['COL1'].transform('nunique') == 2]

Output:

>>> filtered_df
  COL1  COL2
0    A  seq1
1    B  seq1
4    A  seq4
7    B  seq4
Admin1

If you COL1 has only 2 values, you could also filter df for values in "COL1", merge on COL2; then melt:

out = (pd.melt(df[df['COL1']=='A'].merge(df[df['COL1']=='B'], on='COL2'), 
               id_vars=['COL2'], value_name='COL1')
       .drop(columns='variable')[['COL1', 'COL2']])

Output:

  COL1  COL2
0    A  seq1
1    A  seq4
2    B  seq1
3    B  seq4
Admin1

You can also use pd.crosstab:

seq = pd.crosstab(df['COL1'], df['COL2']).all().loc[lambda x: x].index
out = df.loc[df['COL2'].isin(seq)]
print(out)

# Output
  COL1  COL2
0    A  seq1
1    B  seq1
4    A  seq4
7    B  seq4

Intermediate result:

>>> pd.crosstab(df['COL1'], df['COL2'])
COL2  seq1  seq2  seq3  seq4  seq5  seq9
COL1                                    
A        1     1     1     1     1     0
B        1     0     0     1     0     1
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