Say I have the following df:

```
years = []
months = []
ys = [2003,2003,2004,2005]

for y in ys:
    for i in range(1,4):
        years.append(y)
        months.append(i)

df= pd.DataFrame({"year":years,'month':months})
df

    year    month
0   2003    1
1   2003    2
2   2003    3
3   2003    1
4   2003    2
5   2003    3
6   2004    1
7   2004    2
8   2004    3
9   2005    1
10  2005    2
11  2005    3

```

* note that 2003 repeats itself.

I wish to group by year and month and transform back to the df so the result would be:

```
    year    month   count
0   2003    1        1
1   2003    2        2
2   2003    3        3
3   2003    1        1
4   2003    2        2
5   2003    3        3
6   2004    1        4
7   2004    2        5
8   2004    3        6
9   2005    1        7
10  2005    2        8 
11  2005    3        9

```

I tired `df['count'] = df.groupby(['year','month']).transform('count')` but I get 'Wrong number of items passed 0, placement implies 1'