[Python] How to use a for loop in a DataFrame?

I am not able to use a for loop with a DataFrame.

My code is as follows:

for i in data["categ"].unique():
subset = data.loc[data["categ"]=="i",:]
print(subset["montant"].sum())


And here is the output:

0.0
0.0
0.0
0.0
0.0
0.0
0.0


When I get rid of the for loop and I do it manually, I get the correct values, instead of just zeros:

subset = data.loc[data["categ"]=="LOYER",:]
print(subset["montant"].sum())
-2070.0


How do I fix the for loop method?

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