Cumsum as a new column in an existing Pandas data

I have a pandas dataframe defined as:

A   B   SUM_C      
1   1   10     
1   2   20

I would like to do a cumulative sum of SUM_C and add it as a new column to the same dataframe. In other words, my end goal is to have a dataframe that looks like below:

A   B   SUM_C   CUMSUM_C       
1   1   10      10     
1   2   20      30

Using cumsum in pandas on group() shows the possibility of generating a new dataframe where column name SUM_C is replaced with cumulative sum. However, my ask is to add the cumulative sum as a new column to the existing dataframe.

Thank you

Answers:

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Method 1

Just apply cumsum on the pandas.Series df['SUM_C'] and assign it to a new column:

df['CUMSUM_C'] = df['SUM_C'].cumsum()

Result:

df
Out[34]: 
   A  B  SUM_C  CUMSUM_C
0  1  1     10       10
1  1  2     20       30

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