Fix bug in validators documentation

[FlorisHoogenboom]

Function validators seem to have to return their value (which will become part of validated_data) as part of their signature.

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[Ariki]

I believe it's a mistake. There is no much difference between function based and class based validators. These are just two different ways to provide a callable, and the returned values are not used.

In contrast to this, .validate_<field_name> serializer methods must return the validated value.

[auvipy]

are you suggesting to revert this?

[FlorisHoogenboom]

Are you sure about that @Ariki. I think you are right for the validators that come with Django but it seems behaviour of dango-rest is a bit different here, but I might be off.

If I just run a test and trace the stack I eventually end up here:

django-rest-framework/rest_framework/serializers.py

Line 487 in 48a21aa

validated_value = validate_method(validated_value)

I don't have time to create a simple example to reproduce right now, but I remember I initiated this change because I was running into unexplainable null values suddenly when adding validators which seems to agree with the snippet above.

[Ariki]

The code line above is responsible for running validation methods of a serializer which indeed should return a value. The class-based and function-based validators are run in run_validators method which mostly inherits its behavior from the Field class. This method does not use values returned from the validators it calls.

django-rest-framework/rest_framework/fields.py

Lines 560 to 563 in 48a21aa

	if getattr(validator, 'requires_context', False):
	validator(value, self)
	else:
	validator(value)

So yes, I suggest to revert this.

[FlorisHoogenboom]

Ok I see what you are mean. You are right indeed, but I must say I find that difference in behaviour of those two than rather confusing to be honest. Any design choice that's behind that?

[auvipy]

opened a revert PR