I'm using Django 1.8.

The documentation on writing validators has an example of a function-based validator. It also says the following on using a class:

You can also use a class with a __call__() method for more complex or configurable validators. RegexValidator, for example, uses this technique. If a class-based validator is used in the validators model field option, you should make sure it is serializable by the migration framework by adding deconstruct() and __eq__() methods.

• What are the pros/cons of class-based against function-based validators?
• What is __call__() used for, and how is it used?
• What is deconstruct() used for, and how is it used?
• What is __eq__() used for, and how is it used?

An example would be helpful. A full answer may also be worth submitting to be in the official documentation.

Thanks!

A big advantage of validator functions is that they are really simple. They simply take one value as an argument, check that it's valid, and and raise ValidationError if it's not. You don't need to worry about deconstruct and __eq__ methods to make migrations work.

The validate_even example in the docs is much simpler than a validator class would be.

def validate_even(value):if value % 2 != 0:raise ValidationError('%s is not an even number' % value)

If you need to check divisibility by other numbers as well, then it would be worth creating a validator class ValidateDivisibleBy. Then you could use ValidateDivisibleBy(2), ValidateDivisibleBy(3) and so on. But a lot of the time, a validator function is good enough.