BaseModelFormSet.clean() recursively triggers is_valid() which may cause problems when is_valid() was overriden

[Christophe Henry]

The current implementation of ​`BaseModelFormSet.clean()` calls ​`BaseModelFormSet.validate_unique()` which itself calls ​`self.is_valid()` though ​`self.deleted_forms`.

This recursive call of is_valid() may mess up the validation if is_valid() was overriden and self.data is altered during the process. For instance:

`

from django.forms import BaseModelFormSet
from django.forms.formsets import TOTAL_FORM_COUNT


class ExampleFormSet(BaseModelFormSet):
    def __init__(self, extra_data_provider, **kwargs):
        self.extra_data_provider = extra_data_provider
        super().__init__(**kwargs)

    def is_valid(self):
        if not self.is_bound:
            return False

        next_idx = self.data[self.management_form.add_prefix(TOTAL_FORM_COUNT)]

        old_data = self.data
        self.data = self.data.copy()

        self.data[self.management_form.add_prefix(TOTAL_FORM_COUNT)] = f"{next_idx+1}"
        self.management_form.full_clean()

        # Retreiving form data from somewhere else (this is an example)
        next_form_data = self.extra_data_provider.retreive()
        self.data.update(next_form_data)
        self.forms.append(
            self._construct_form(next_idx, **self.get_form_kwargs(next_idx))
        )

        result = super().is_valid()

        self._validate_empty_form = False

        if not result:
            # Reverting our changes
            self.data = old_data
            self.management_form.full_clean()
            self.forms.pop()

        return result

Let's assume super().is_valid() is False because self.clean() failed. Then self.forms.append() will be called twice, but the cleaning in the if not result block ` will perform only once.

In my opinion, this is a bug that produces unexpected behavior and is difficult to debug.

This could be solved ​by not calling `self.is_valid()` in `BaseFormSet.deleted_forms` and instead perform only a partial validation here. I enclosed a patch to this ticket to show my solution.

[Natalia Bidart]

Hello Christophe Henry! Thank you for taking the time to create this ticket.

I think I understand your explanation but, from my point of view, the method is_valid() should not be changed and instead, custom validation logic should be done in clean() or similar methods. If your app absolutely needs to override is_valid(), it should do so in a way that the method remains idempotent. Django provides no guarantees that is_valid() is going to be called only once, so your app should not depend on that.

Given the above, I'll close the ticket accordingly, but if you disagree, you can consider starting a new conversation on the ​Django Forum, where you'll reach a wider audience and likely get extra feedback. If you do so, please make sure to provide a complete and correct example, because the code provided in the description is incomplete and can not be used to reproduce the issue you are describing.