Misleading error message when validator cannot be serialized

[Peter Inglesby]

I have a validator factory that looks like this:

def validate_max_words(max_words):
    def validator(value):
        num_words = len(value.split())
        if num_words > max_words:
            raise ValidationError(f'Field is too long: {num_words} words / {max_words} limit')
    return validator

and I would like to use it in a field definition like this:

description = models.TextField(validators=[validate_max_words(300)])

However, when running a migration to create the model in question, I get the following error:

ValueError: Could not find function validator in myapp.validators.
Please note that due to Python 2 limitations, you cannot serialize unbound method functions (e.g. a method declared and used in the same class body). Please move the function into the main module body to use migrations.
For more information, see https://docs.djangoproject.com/en/1.11/topics/migrations/#serializing-values

The error message is misleading, in that it assumes that because the validator cannot be serialized, it must be an unbound method. However, this is not the case -- the validator is instead a function that's returned by another function.

[Tim Graham]

That error message is gone in master because Python 2 support is removed. The new error is something like "ValueError: Could not find function validator in app.models." I'm not sure if it's possible to serialize this case, if the error message could be improved, or if we can close this ticket. What do you think?

[Peter Inglesby]

Poking about at this some more, I think it is possible to serialize the validator in my example, because the validate_max_words(300) returns a function object that has a __closure__ attribute which contains bindings to the environment in which the function object was created.

I'm not sure though how reliable this would be for less simple examples, and given how easy it is to work around, I think the new error message is good enough.