Allow the formset class to contribute kwargs to form instances

[tevans]

When we use a formset, each form is instantiated by the method BaseFormSet._construct_forms(), which is called from BaseFormSet.__init__()

_construct_forms() allows for additional kwargs to be passed through to the individual form constructor, but BaseFormSet.__init__() does not provide any mechanism for kwargs to be passed through to the _construct_forms() call.

Also, we may not be able to supply a static unchanging list of kwargs that should be supplied to each form constructor - the kwargs we want to pass may depend upon the specific instance of the form being instantiated.

I propose the following changes:

Add a form_kwargs argument to BaseFormSet.__init__(). This would be stored on self, and consumed in BaseFormSet._construct_forms()

Add a method, BaseFormSet._kwargs_for_form(self, i). This method would be called for each form instantiated, passing the index of the form within the formset. The return value of this method would be merged with the form_kwargs supplied in BaseFormSet.__init__() and used to instantiate the form.

With these changes, a user could add additional constructor arguments to all forms by passing in form_kwargs to the formset constructor, and could add additional constructor arguments to specific forms by implementing _kwargs_for_form on a derived BaseFormSet class.

(I found it strange, when BaseFormSet was written, it was clearly intended to be able to pass kwargs to the forms - _construct_form() allows for it, there just is not the plumbing to do so. I worked around this by redefining - copy/pasting - the _construct_forms() method in my derived BaseFormSet class)

Patch doesn't have tests or doc changes, will add if this idea gets traction.

Cheers

Tom

[tevans]

Patch implementing features described in ticket

[tevans]

Hmm, wiki markup that turns __init__ into init is less than useful for Python discussion :/

[Łukasz Rekucki]

Replying to tevans:

Hmm, wiki markup that turns __init__ into init is less than useful for Python discussion :/

You can use formatting: {{{ __init__ }}} will show as __init__. There's both live preview and a preview button.

[Anssi Kääriäinen]

Just as an idea: how about defining a new method:

def get_single_form(self, i, defaults):
    """
    Override me to have total control over the single forms.
    """
    return self.form(**defaults)

and use that instead of self.form() in the _construct_form() method. This would be more powerful and easier to implement.

[tevans]

Replying to akaariai:

Just as an idea: how about defining a new method:

def get_single_form(self, i, defaults):
    """
    Override me to have total control over the single forms.
    """
    return self.form(**defaults)

and use that instead of self.form() in the _construct_form() method. This would be more powerful and easier to implement.

Yes, I like that. I would like to also keep the idea of being able to contribute to a form's kwargs without defining a BaseFormSet subclass, eg by passing them to BaseFormSet.__init__.

I should get some time this w/e to re-roll the patch.

[anonymous]

Agreed, a method for implementing this is a good idea, but please let's skip the redundant 'simple' prefix, just call it get_form.

[Jannis Leidel]

Oops that was me.

[Sergei Maertens]

I'll have a go at this

[Sergei Maertens <sergei@…>]

In 238e2ac3:

Fixed #18166 -- Added form_kwargs support to formsets.

By specifying form_kwargs when instantiating the formset, or overriding
the get_form_kwargs method on a formset class, you can pass extra
keyword arguments to the underlying Form instances.

Includes tests and documentation update.

[Russell Keith-Magee <russell@…>]

In fe21fb81:

Merge pull request #4757 from sergei-maertens/ticket_18166

Fixed #18166 -- Added ability to pass kwargs to the form constructor in a formset.