Allow Forms to set a default prefix attribute

[Keryn Knight]

Example of what I'd like to be able to do:

from django import forms
class MyForm(forms.Form):
    field = ChoiceField(...)
    prefix = 'myform-is-awesome'

>>> form_itself = MyForm(data=request.GET, files=None)
>>> form_itself.prefix
'myform-is-awesome'

However this isn't currently workable, because ​self.prefix is always bound based on the argument given in the arguments, the default of which is None

By hoisting the prefix to be a class attribute, and then doing

if prefix is not None: 
    self.prefix = prefix

it seems like forms which ship with apps could effectively namespace themselves such that N overlapping form fields could be POSTed at once and resolved to the correct form.

However, that's only a suggested use-case that hints at why it might be useful to have; the reason I specifically needed this was for validating whether a GET request was likely my filtering form, before instantiating and validating the data:

filtered_form = any(x.startswith(MyForm.prefix) for x in request.GET.keys())

so that I can selectively reset session-saved filters only when a new filterset is done (before then saving validated submitted data back into the session).

My workaround ended up looking like:

class MyForm(forms.Form):
    prefix = 'woo'
    def __init__(self, *a, *kw):
        super(MyForm, self).__init__(*a, **kw)
        self.prefix = self.__class__.prefix

which seems a bit ... rubbish.

[Paweł Marczewski]

​https://github.com/django/django/pull/4707

[Tim Graham <timograham@…>]

In 4df7e848:

Fixed #24788 -- Allowed Forms to specify a prefix at the class level.