get_error_detail should not format without error.params

[WolfgangFellger]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I @michael-k has included a failing test as a pull request. (Thanks!)

get_error_detail should not attempt to format the message in DjangoValidationErrors if params aren't used. This is also in line with how Django behaves.

If f-strings are used to generate the error message, this means duplicate formatting, and fails if a % occurs in the message.

Steps to reproduce

Most basic example I could muster (this is actually supposed to be ModelValidator with the validator on a Django model)

from rest_framework import serializers
from django.core.exceptions import ValidationError


def validate(value):
    raise ValidationError(f'{value} is invalid')

class MySerializer(serializers.Serializer):
    field = serializers.CharField(validators=[validate])


s = MySerializer(data={'field': 'f%'})  
if not s.is_valid():  # throws TypeError: not enough arguments for format string
    print(s.errors)

Expected behavior

Should return the error message "f% is not valid"

Actual behavior

Throws a TypeError in

django-rest-framework/rest_framework/fields.py

Line 242 in 1a0a8dd

ErrorDetail(error.message % (error.params or ()),

[michael-k]

FYI: I'm working on a PR to solve this. Including a failing test. Done

Btw., this is the PR that introduced the formatting: #5785 (released with v3.9.0)