How to increment a row count in groupby in DataFrame

by

The groupby is the right idea, but the right method is cumcount:

>>> product_df['month_num'] = product_df.groupby('product_desc').cumcount()
>>> product_df

  product_desc activity_month  prod_count    pct_ch  month_num
0    product_a     2014-01-01          53       NaN          0
3    product_a     2014-02-01          52 -0.018868          1
6    product_a     2014-03-01          50 -0.038462          2
1    product_b     2014-01-01          44       NaN          0
4    product_b     2014-02-01          43 -0.022727          1
7    product_b     2014-03-01          41 -0.046512          2
2    product_c     2014-01-01          36       NaN          0
5    product_c     2014-02-01          35 -0.027778          1
8    product_c     2014-03-01          34 -0.028571          2

If your really want it to start with 1 then just do this instead:

>>> product_df['month_num'] = product_df.groupby('product_desc').cumcount() + 1

  product_desc activity_month  prod_count    pct_ch  month_num
0    product_a     2014-01-01          53       NaN          1
3    product_a     2014-02-01          52 -0.018868          2
6    product_a     2014-03-01          50 -0.038462          3
1    product_b     2014-01-01          44       NaN          1
4    product_b     2014-02-01          43 -0.022727          2
7    product_b     2014-03-01          41 -0.046512          3
2    product_c     2014-01-01          36       NaN          1
5    product_c     2014-02-01          35 -0.027778          2
8    product_c     2014-03-01          34 -0.028571          3

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