SelectMultiple widget value_from_datadict enhancement suggestion

[Marcin Nowak]

Dear all.

I would just say that implementation of SelectMultiple.value_from_datadict is not based on duck typing, and has some disadvantages.

Source: https://github.com/django/django/blob/master/django/forms/widgets.py#L606

    def value_from_datadict(self, data, files, name):

        if isinstance(data, MultiValueDict):

            return data.getlist(name)

        return data.get(name)

First, always checking the instance type has worse efficiency than try-except block, because most hits may be returned without any additional checking.

Second, as an author of library which also works with Django, I'm providing multivalue-like dict, but own implemented - it is not an instance of MultiValueDict and never be, but it has similar interface (i.e. `get()` and `getlist()` methods) 

Duck typing will work for both cases. The implementation might look like:

    def value_from_datadict(self, data, files, name):

        try:        

            return data.getlist(name)

        except AttributeError:        

            return data.get(name)

For most cases a MultiValueDict / QueryDict are passed to forms, so the function will immediately return a proper value, and will also work for objects that "looks like MultiValueDict". On other cases it assumes that the `data` object is dict-like.

What do you think?

Kind Regards,
Marcin

[Tim Graham]

I've seen a few patches recently to move things toward duck-typing so this seems welcome.