TypeError: unsupported operand type(s) for *: 'NoneType' and 'decimal.Decimal'

Following is the error:

File "F:\Coding Related\Karmachari-BackEnd\Karmachari_App\mainapp\models.py", line 97, in calculate_net_salary
    gross_pay = self.hours_worked * self.basic_pay_rate
                
TypeError: unsupported operand type(s) for *: 'NoneType' and 'decimal.Decimal'
These are the lasted error message in terminal.

models.py

class Payroll(models.Model):
    user = models.ForeignKey(User,on_delete=models.CASCADE)
    basic_pay_rate = models.DecimalField(max_digits=8, default=10000, decimal_places=2)
    overtime = models.DecimalField(max_digits=8,null=True, decimal_places=2)
    hours_worked = models.FloatField(null=True, blank=True)
    deductions = models.DecimalField(max_digits=8,null=True, decimal_places=2)
    net_pay = models.DecimalField(max_digits=8,null=True, blank=True, decimal_places=2)

    def calculate_net_salary(self):
        gross_pay = self.hours_worked * self.basic_pay_rate
        net_pay = gross_pay + self.overtime - self.deductions
        self.net_pay.save()

    # def save(self, *args, **kwargs):
    #     self.net_salary = self.calculate_net_salary()
    #     super(Payroll, self).save(*args, **kwargs)
    
    def __str__(self):
        return self.user.username

views.py

def payroll(request):
    payrolls = Payroll.objects.all()
    for payroll in payrolls:
        payroll.calculate_net_salary()
    user_object = User.objects.get(username=request.user.username)
    profile = Profile.objects.get(user=user_object)
    context={
        'profile':profile,
        'navbar':'salary',
        'payroll': payroll,
    }
    return render(request,'Salary_Sheet.html',context)

I want to access payroll of each employee and calculate net salary and provide them in pdf.
This is my first project so I dont know the concept clearly and want a beginner friendly explanation or solution.

The error is very clear: You are trying to multiply a None and a Decimal.

I'm guessing self.hours_worked is None.

A dirty way to solve this issue would be to simply use zero when this value is None:

gross_pay = (self.hours_worked or 0) * self.basic_pay_rate

But I'd recommend not doing this as this leads to unnecessary code pollution.

Better way to do this is not to use null=True but, instead, set default=0 in your numeric model fields. Eg:

hours_worked = models.FloatField(default=0, blank=True)

You'll also need to reapply your migrations.

The error message is telling you that you are trying to multiply None with a Decimal value in your calculate_net_salary() method. This means that the self.hours_worked attribute is None when you are trying to calculate the gross_pay. You can add a check to see if self.hours_worked is not None before calculating gross_pay.

Better way to add default=0, like so:

hours_worked = models.FloatField(default=0, blank=True)