Reduce boilerplate with custom ChoiceField

[ffigiel]

The readme suggests this usage:

type = models.CharField(max_length=1,
                            choices=PersonType.choices,
                            validators=[PersonType.validator])

However, we could provide default behavior with a custom CharField:

• choices accepts DjangoChoices subclass
• max_length is inferred from max(len(l) for l in GivenChoices.labels)
• validators is [GivenChoices.validator] by default

(While most of these are a matter of preference, automatically picking the max_length value would surely be an useful feature)

Then we can simply do this

type = ChoiceField(choices=PersonType.choices)

[ffigiel]

Of course this is a case for Choices with string values - numeric values would need to be handled differently

[sergei-maertens]

I think the documentation needs updating, since Django applies a ChoicesValidator by itself as soon as you use the choices option. Leaving that off would already reduce some boilerplate.

You correctly mention the case where the field is not a CharField - this would require some 'magic' introspection to figure out the 'best fitting' field type.

The same kind of magic where the max_length is guessed is also only relevant for CharField, and doesn't cover the case where you add an extra choice with a longer max_length. It would be caught in a migration though, but it's again guessing/magic, and that is against the Zen of Python (https://www.python.org/dev/peps/pep-0020/): In the face of ambiguity, refuse the temptation to guess.

I don't feel that this is a good direction to take.

[ffigiel]

I didn't know about that validator, thanks!

If you added a new choice, ChoiceField.max_length would be updated and require a migration, which makes sense to me - adding new choices wouldn't require you to manually count characters and update max_length.

Anyway, this field would be simple enough to develop by self in case someone needs it.

[merwok]

I think the documentation needs updating, since Django applies a ChoicesValidator by itself as soon as you use the choices option. Leaving that off would already reduce some boilerplate.

This is still a valid doc change IMHO.

[sergei-maertens]

@merwok thanks for the comment, I've cleaned up the docs a bit in the referenced PR.