I get this error unsupported operand type(s) for -: 'method' and 'int'

Question

I want to subtract available_quantity to total_quantity . but i get this error how can i solve it

class Order(models.Model):

 product = models.ForeignKey(Product, on_delete=models.CASCADE)
 category = models.ForeignKey(Category, on_delete=models.CASCADE)
 brand = models.ForeignKey(Brand, on_delete=models.CASCADE)
 distribute_price = models.IntegerField()
 mrp_price = models.IntegerField(null=True)
 created_at = models.DateTimeField(auto_now_add=True)
 user_give_quantity = models.IntegerField(null=True)
 user = models.ForeignKey(UserProfile, on_delete=models.CASCADE)

 def __str__(self):
    return self.product.item_name

 def aaa_qqq(self):
    return self.mrp_price * self.user_give_quantity    

 def available_quantity(self):
    return self.product.quantity - self.user_give_quantity if all([self.product, 
           self.product.quantity, self.user, self.user_give_quantity]) else 0


 def total_quantity(self):
    return self.available_quantity - self.user_give_quantity

Answer 1

The reason that this happens is because self.available_quantity is a method, not a property. You thus should call the method:

def total_quantity(self):
    return self.available_quantity() - self.user_give_quantity

Note that if user_give_quantity is NULL/None, then this will raise an error as well. You thus should check this, for example with:

def total_quantity(self):
    if self.user_give_quantity is not None:
        return self.available_quantity() - self.user_give_quantity

Then total_quantity() will return None if the self.user_give_quantity is None.

You might want to clean up the available_quantity as well:

def available_quantity(self):
    if self.product and self.product.quantity and self.user and self.user_give_quantity is not None:
        return self.product.quantity - self.user_give_quantity
    return 0

It is furthermore not entirely clear to me how available_quantity will work here: if there are multiple Orders that each need some quantity, then you normally need to add up the quantities of all orders, here you only focus on the quantity of a specific order.

You can also use the @property decorator [python-doc] to define these as properties, then you can work with self.available_quantity:

class Order(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    brand = models.ForeignKey(Brand, on_delete=models.CASCADE)
    distribute_price = models.IntegerField()
    mrp_price = models.IntegerField(null=True)
    created_at = models.DateTimeField(auto_now_add=True)
    user_give_quantity = models.IntegerField(null=True)
    user = models.ForeignKey(UserProfile, on_delete=models.CASCADE)

    def __str__(self):
        return self.product.item_name

    @property
    def aaa_qqq(self):
        return self.mrp_price * self.user_give_quantity    

    @property
    def available_quantity(self):
        if self.product and self.product.quantity and self.user and self.user_give_quantity is not None:
            return self.product.quantity - self.user_give_quantity
        return 0

    @property
    def total_quantity(self):
        if self.user_give_quantity is not None:
            return self.available_quantity - self.user_give_quantity