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

You can also use GroupBy + transform with size:

df['Occur'] = df.groupby('Col1')['Col1'].transform('size')

print(df)

    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