How to check in which column is certain value in pandas.DataFrame?

Question

I have DataFrame in Python like below:

df = pd.DataFrame({"col1"  : ["a", "b", "c"], "col2" : ["a", "d", "e"], "col3" : ["r", "t" , "g"]})

And I would like to check in which columns is value "a" (of course in "col1" and "col2"). How can I check it?

Answer 1

(df=='a').any()

col1     True
col2     True
col3    False

Answer 2

If need columns names in list compare all values by DataFrame.eq with DataFrame.any for check if at least one True (match) per columns, last filter columns names:

c = df.columns[df.eq('a').any()].tolist()
print (c)
['col1', 'col2']

If need filter columns to new DataFrame use DataFrame.loc :

df1 = df.loc[:, df.eq('a').any()]
print (df1)
  col1 col2
0    a    a
1    b    d
2    c    e