Unique values of two columns for pandas dataframe

You need groupby + size + Series.reset_index:

df = df.groupby(['Col1', 'Col2']).size().reset_index(name='Freq')
print (df)
   Col1  Col2  Freq
0     1     1     1
1     1     2     3
2     3     4     2

You could try

df.groupby(['Col1', 'Col2']).size()

for a different visual output in comparison to jez's answer, you can extend that solution with

pd.DataFrame(df.groupby(['Col1', 'Col2']).size().rename('Freq'))

gives

           Freq
Col1 Col2      
1    1        1
     2        3
3    4        2