WizardView.get_context_data() does not compatible to TemplateView.get_context_data()

[denisw]

Summary

WizardView defines get_context_data() with the following signature:

def get_context_data(self, form, **kwargs)

However, in its superclass TemplateView (or, specifically, ContextMixin) it has the following signature:

def get_context_data(self, **kwargs)

This means that the override in WizardView is incompatible because it breaks the Liskov substitution principle: form is required by WizardView, but not by TemplateView.

Why it matters

• Code designed to work with any TemplateView (such as a mixin that overrides get_context_data() to add an additional entry) may not work with WizardView.

• When the user overrides get_context_data() in a WizardView, type checkers like Ty complain about the override being incompatible, no matter whether you define it with form (incompatible with ContextMixin) or without (incompatible with WizardView).

• As a consequence of the above, it is basically impossible to write a type stub for WizardView.get_context_data(). This is a blocker for adding typings to Typeshed or directly to Formtools.

Proposed solution

Change the signature of WizardView.get_context_data() to:

def  get_context_data(self, *, form=None, **kwargs)

That is, make form an optional keyword-only parameter. This is compatible with the plain **kwargs defined in ContextMixin because it allows form not to be passed, and because both versions don't accept any positional arguments.

For convenience, a get_context_data_for_step(self, form, **kwargs) method could be provided, which would called by get_context_data() either with the received form or self.get_form() (same logic as currently in render()):

def get_context_data(self, *, form=None, **kwargs):
    form = form or self.get_form()  # moved here from render()
    context = super().get_context_data(form=form, **kwargs)
    context.update(self.get_context_data_for_step(form, **kwargs))
    ...

def get_context_data_for_step(self, *, form, **kwargs):
    return {}  # default implementation