Count instances of strings in multiple columns python

Question

I have the following simple data frame

import pandas as pd
df = pd.DataFrame({'column_a': ['a', 'b', 'c', 'd', 'e'],
                   'column_b': ['b', 'x', 'y', 'c', 'z']})


      column_a column_b
0        a        b
1        b        x
2        c        y
3        d        c
4        e        z

I'm looking to display the strings which occur in both columns:

result = ("b", "c")

Thanks

Answer 1

intersection

This generalizes over any number of columns.

set.intersection(*map(set, map(df.get, df)))

{'b', 'c'}

Answer 2

Use python's set object:

in_a = set(df.column_a)
in_b = set(df.column_b)
in_both = in_a.intersection(in_b)