Validators list is shared among fields of different form instances

[Claude Paroz]

When forms are created, fields are not new instances, but instead are a deepcopy of the Form class base_fields. If you modified afterwards a form field validators list, it is modified for all other form instances. Example:

from django import forms
from django.core import validators

class MyForm(forms.Form):
    myfield = forms.CharField(max_length=25)

f1 = MyForm()
f2 = MyForm()

f1.fields['myfield'].validators
>>> [<django.core.validators.MaxLengthValidator object at 0x32b53d0>]

f2.fields['myfield'].validators
>>> [<django.core.validators.MaxLengthValidator object at 0x32b53d0>]

f1.fields['myfield'].validators[0] = validators.MaxValueValidator(12)

f1.fields['myfield'].validators
>>> [<django.core.validators.MaxValueValidator object at 0x3205a90>]

f2.fields['myfield'].validators
>>> [<django.core.validators.MaxValueValidator object at 0x3205a90>]

This is a problem for me for example to properly resolve #17125.

[Claude Paroz]

Isolate validators list between form instances

[Carl Meyer]

Yes, this is a bug. Changing validators on a specific form's field instance should not change it for all instances. Patch looks good to me.

[Carl Meyer]

In [17046]:

Fixed #17127 -- Made field validators list independent per form instance. Thanks claudep for report and patch.