Pandas Dataframe: how to add column with number of occurrences in other column

Question

I have to following df:

Col1    Col2
test    Something
test2   Something
test3   Something
test    Something
test2   Something
test5   Something

Answer 1

groupby on 'col1' and then apply transform on Col2 to return a Series with its index aligned to the original df so you can add it as a column:

In [3]:
df['Occur'] = df.groupby('Col1')['Col2'].transform(pd.Series.value_counts)
df

Out[3]:
    Col1       Col2 Occur
0   test  Something     2
1  test2  Something     2
2  test3  Something     1
3   test  Something     2
4  test2  Something     2
5  test5  Something     1