How to prevent assigning a negative value to a numeric field of a class in Python?

To prevent assigning a negative value to a class's numeric field in Python, you can use a property with a check in the setter. Example:

class Product:
    def __init__(self, price):
        self.price = price  # calls the setter

    @property
    def price(self):
        return self._price

    @price.setter
    def price(self, value):
        if value < 0:
            raise ValueError("Price cannot be negative")
        self._price = value

# Usage
p = Product(100)
p.price = 50   # works
p.price = -10  # raises ValueError exception

This approach allows controlling the assignment and raising an error when attempting to set an invalid value.