Sumprod-if formula in Python

I have a dataframe like this (a1,a2 and a3 are string)

A	B	C
a1	0.1	500
a2	0.2	300
a3	0.11	200
a2	0.13	700
a1	0.3	300

I trying to build a new dataframe as a sumprod of B and C based on A values. The output should look something like:

A_values	sumprodif
a1	140
a2	151
a3	22

My initial solution was to use an if formula, create a list and edit the new dataframe from there. However, my if formula does not seem to be working:

if df['A']=="a1":
    sumprodif_a1 = (df['B']*df['C']).sum()  
return sumproduif_a1

Which returns the following error

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

I would love to use this as a chance to learn a better way to do this kind of operation.

Thank you!

Solution

You can use groupby.sum on the multiplied values:

out = df['B'].mul(df['C']).groupby(df['A']).sum().reset_index(name='sumprodif')

# or
# out = (df.assign(sumprodif=df['B'].mul(df['C']))
#          .groupby('A', as_index=False)['sumprodif'].sum()
#        )

Output:

    A  sumprodif
0  a1      140.0
1  a2      151.0
2  a3       22.0