Question

After a bit of searching, I’ve found that it’s possible to instantiate a WTForms object in web.py using the following code:

form = my_form(**web.input())

web.input() returns a “dictionary-like” web.storage object, but without the double asterisks WTForms will raise an exception:

TypeError: formdata should be a multidict-type wrapper that supports the 'getlist' method

From the Python documentation I understand that the two asterisks are used to unpack a dictionary of named arguments. That said, I’m still a bit confused about exactly what is going on. What makes the web.storage object returned by web.input() “dictionary-like” enough that it can be unpacked by ** but not “dictionary-like” enough that it can be passed as-is to the WTForms constructor?

I know that this is an extremely basic question, but any advice to help a novice programmer would be greatly appreciated!

Answer 1

From WTForms source:

def process(self, formdata=None, obj=None, **kwargs):
    if formdata is not None and not hasattr(formdata, 'getlist'):
        if hasattr(formdata, 'getall'):
            formdata = WebobInputWrapper(formdata)
        else:
            raise TypeError("formdata should be a multidict-type wrapper that supports the 'getlist' method")

    for name, field, in iteritems(self._fields):
        if obj is not None and hasattr(obj, name):
            field.process(formdata, getattr(obj, name))
        elif name in kwargs:
            field.process(formdata, kwargs[name])
        else:
            field.process(formdata)

You currently pass arguments as **kwargs, but if you want formdata to work, then it should be multidict-type that is not like a regular dict or webpy’s web.storage, because both of them don’t have getlist method. multidict is a dict that stores multiple values for the same key.

You may try to pass web.input() as obj argument, like this: form = my_form(None, web.input()) I think it should work.