Python Pandas count function on condition and subset

i have a dataframe like this

F_Class  Product    Packages
Apple    Apple_A          1           
Apple    Apple_A          2           
Apple    Apple_A          1           
Apple    Apple_B          2           
Bananas  Banana_A      n.a.           
Bananas  Banana_A      n.a.           

I want to build the following count function to count the items in my dataframe like shown below.

• The Function should count by the Subset ['F_Class','Product']
• If df['Packages'] == 2 then increase by +2 else increase by +1

The result should look like this:

F_Class  Product    Packages    Counter
Apple    Apple_A          1           1
Apple    Apple_A          2           3
Apple    Apple_A          1           4
Apple    Apple_B          2           2
Bananas  Banana_A      n.a.           1
Bananas  Banana_A      n.a.           2

Answer

If need sum by Packages numbers use DataFrameGroupBy.cumsum with replace missing values to 1:

df['Packages'] = pd.to_numeric(df['Packages'], errors='coerce')

df['Counter'] = (df.assign(Packages = df['Packages'].fillna(1).astype(int))
                   .groupby(['F_Class','Product'])['Packages'].cumsum())
print (df)
   F_Class   Product  Packages  Counter
0    Apple   Apple_A       1.0        1
1    Apple   Apple_A       2.0        3
2    Apple   Apple_A       1.0        4
3    Apple   Apple_B       2.0        2
4  Bananas  Banana_A       NaN        1
5  Bananas  Banana_A       NaN        2

Detail:

print (df.assign(Packages = df['Packages'].fillna(1).astype(int)))
   F_Class   Product  Packages
0    Apple   Apple_A         1
1    Apple   Apple_A         2
2    Apple   Apple_A         1
3    Apple   Apple_B         2
4  Bananas  Banana_A         1
5  Bananas  Banana_A         1