Cumsum Reset based on a condition in Pandas

Question

My question is very similar to Cumsum within group and reset on condition in pandas and Pandas: cumsum per category based on additional condition but they don\'t quite get m

Answer 1

Use groupby.cumsum:

df['Cumulative'] = df.groupby('TransactionId')['Delta'].cumsum()

print (df)

  TransactionId  Delta  Cumulative
0       14         2       2
1       14         3       5
2       14         1       6
3       14         2       8
4       15         4       4 
5       15         2       6 
6       15         3       9