How can a form within a formset know it's index within the formset?

Question

I have a basic Form subclass with a formset generated such as:

MyFormset = formset_factory(
    MyForm,
    extra=5,
    max_num=5,
)

I would like to be able to access the index of the form from within the form.save() and form.__init__ methods.

Answer 1

You can subclass BaseFormSet to pass the index to the form:

from django.forms.formsets import BaseFormSet

class BaseMyFormSet(BaseFormSet):
    def add_fields(self, form, index):
        """A hook for adding extra fields on to each form instance."""
        super(BaseMyFormSet, self).add_fields(form, index)
        # here call a custom method or perform any other required action
        form.set_index(index)

MyFormset = formset_factory(
    MyForm,
    formset=BaseMyFormSet,
    extra=5,
    max_num=5,
)

This method will be called for each created form in the formset, you may perform whatever operation you need.

To get the index on save, I see two possibilities:

1) You use an hidden field that you set in the set_index method above

class MyForm(forms.Form):
    formset_index = forms.IntegerField(widget=forms.HiddenInput())

    def set_index(self, index):
        self.fields['formset_index'].initial = index

2) You can use enumerate

# definition
class MyForm(forms.Form):
    def save(self, index):
    ...

# save
for index, form in enumerate(my_formset):
   form.save(index)