Python / Pandas getting division by zero

I’m trying to divide but get a "ZeroDivisionError: division by zero" despite I check for 0 values before the calculation:

df['var'] = None
df.loc[(df['use'] == 1) & (df['cont_var'] != 0) , 'var'] = (df['par1'] + df['par2'] + df['par3'] + df['par4']) / df['cont_var']

If I check all unique values for that condition, there is nothing wrong:

df[(df['use'] == 1) & (df['cont_var'] != 0)]['cont_var'].unique() 

I get: array([4, 2, 1], dtype=object)

I’m using Python 3.9.12 in Jupyter Notebook.

>Solution :

You can filter column cont_var used for division, also is used sum for simplify solution:

data = {
    'use': [1,1,1,1,1,0,0,0],
    'cont_var': [1,0,0,1,1,1,1,1],
    'par1':range(8),
    'par2':range(1,9),
    'par3':range(5,13),
    'par4':range(6,14),
}
df = pd.DataFrame(data)

mask = (df['use'] == 1) & (df['cont_var'] != 0)
df['var'] = df[['par1','par2','par3','par4']].sum(axis=1) / df.loc[mask, 'cont_var']

print(df)
   use  cont_var  par1  par2  par3  par4   var
0    1         1     0     1     5     6  12.0
1    1         0     1     2     6     7   NaN
2    1         0     2     3     7     8   NaN
3    1         1     3     4     8     9  24.0
4    1         1     4     5     9    10  28.0
5    0         1     5     6    10    11   NaN
6    0         1     6     7    11    12   NaN
7    0         1     7     8    12    13   NaN