如何根据 pandas dataframe 中的多列按元素分组并将每组的元素数量保存在另一列中？

Question

I have a dataframe of the next form:

+--------+--------+--------+
|  Col1  |  Col2  |  Col3  |
+--------+--------+--------+
|  1     |   2    |    1   |
+--------+--------+--------+
|  1     |   2    |    1   |
+--------+--------+--------+
|  1     |   3    |    1   |
+--------+--------+--------+
|  2     |   4    |    1   |
+--------+--------+--------+ 

I want to get the next dataframe:

+--------+--------+--------+--------+
|  Col1  |  Col2  |  Col3  |  Count |
+--------+--------+--------+--------+
|  1     |   2    |    1   |   2    |
+--------+--------+--------+--------+
|  1     |   2    |    1   |   2    |
+--------+--------+--------+--------+
|  1     |   3    |    1   |   1    |
+--------+--------+--------+--------+
|  2     |   4    |    1   |   1    |
+--------+--------+--------+--------+

How can I get that dataframe? I am trying with the size() method after grouping by the dataframe, but that result is not what I would like. I would like to get another column in which the number of occurrences of the full row appears.

Answer 1

Use GroupBy.transform to get an equal length vector back, and assing it as new column:

df['Count'] = df.groupby(df.columns.tolist())['Col1'].transform('size')

Output

   Col1  Col2  Col3  Count
0     1     2     1      2
1     1     2     1      2
2     1     3     1      1
3     2     4     1      1