Python pandas: Add a column to my dataframe that counts a variable

Question

I have a dataframe 'gt' like this:

org     group
org1      1
org2      1
org3      2
org4      3
org5      3
org6      3

and I would like to add column 'count' to gt dataframe to counts number member of the groups, expected results like this:

org     group   count
org1      1       2
org2      1       2
org3      2       1
org4      3       3
org5      3       3
org6      3       3

I know how to do it per one item of the group, but do not know how to make the count repeated for all of the group items, here is the code I have used:

gtcounts = gt.groupby('group').count()

Can anybody help?

question from:https://stackoverflow.com/questions/29791785/python-pandas-add-a-column-to-my-dataframe-that-counts-a-variable

Answer 1

Call transform this will return a Series aligned with the original df:

In [223]:

df['count'] = df.groupby('group')['group'].transform('count')
df
Out[223]:
    org  group  count
0  org1      1      2
1  org2      1      2
2  org3      2      1
3  org4      3      3
4  org5      3      3
5  org6      3      3