Support callable minimum and maximum integer values and string lengths

[Lucidiot]

The Django docs for the validators mention that for MaxValueValidator, MinValueValidator, MaxLengthValidator and MinLengthValidator, the limit may be a callable. So in theory, it should be possible to do this:

from django.db import models
from django.core.validators import MaxValueValidator, MinValueValidator, MaxLengthValidator, MinLengthValidator

def small_funny_number():
    return 42

def large_funny_number():
    return 1337

class Thingy(models.Model):
    number = models.IntegerField(validators=[
        MinValueValidator(small_funny_number),
        MaxValueValidator(large_funny_number),
    ])
    text = models.TextField(validators=[
        MinLengthValidator(small_funny_number),
        MaxLengthValidator(large_funny_number),
    ])

Note that you cannot use lambdas here, as the callable must have a name for Django's makemigrations to work.

Now, put that in a serializer and into a POST API view:

from rest_framework import serializers, generics

class ThingySerializer(serializers.ModelSerializer):
    class Meta:
        model = Thingy
        fields = ['number', 'text']

class ThingyCreate(generics.CreateAPIView):
    queryset = Thingy.objects.all()
    serializer_class = ThingySerializer

If you make some API requests with invalid values, you will get the following error messages:

{
  "number": ["Ensure this value is greater than or equal to <function small_funny_number at 0x7f0bdae20670>."],
  "text": ["Ensure this value has at least 42 characters (it has 1)."]
}

{
  "number": ["Ensure this value is less than or equal to <function large_funny_number at 0x7f6b15a830a0>."],
  "text": ["Ensure this value has at most 1337 characters (it has 9999)."]
}

When you declare the serializer fields yourself, the messages are slightly different. The docs for CharField and IntegerField do not mention support for callable min/max values and min/max lengths, but you can still use them.

class ThingySerializer(serializers.ModelSerializer):
    number = serializers.IntegerField(min_value=small_funny_number, max_value=large_funny_number)
    text = serializers.CharField(min_length=small_funny_number, max_length=large_funny_number)

    class Meta:
        model = Thingy
        fields = ['number', 'text']

You will then get these messages:

{
  "number": ["Ensure this value is greater than or equal to <function small_funny_number at 0x7f0bdae20670>."],
  "text": ["Ensure this value has at least <function small_funny_number at 0x7f0bdae20670> characters."]
}

{
  "number": ["Ensure this value is less than or equal to <function large_funny_number at 0x7f6b15a830a0>."],
  "text": ["Ensure this value has at most <function large_funny_number at 0x7f6b15a830a0> characters."]
}

This time, the CharField is also affected, and does not display the number of characters that the user sent as (it has X). I guess that the ModelSerializer's automatic serializer field creation generates the two fields differently, using Django's MaxLengthValidator instead of max_length= for a CharField, but using min_value= and max_value= for an IntegerField.

So I suppose there would multiple changes here:

• Handle callable min/max values and lengths in the error messages when translating the min/max_value and min/max_length kwargs into validators. Maybe just leave that up to Django's validators, since they already generate validation errors with similar messages?
• Document that callables can be used for those attributes
• Consistently use either the min/max kwargs or just validators=[] in the ModelSerializer's automatic field generation

Thoughts?