Tornado + WTForms = TypeError: formdata should be a multidict type wrapper ...

If you’re here from some search engine, that means you’ve been working with Tornado and WTForms, and you’ve faced the same error -

(Pdb) form = SomeForm(self.request.arguments)
*** TypeError: formdata should be a multidict-type wrapper that supports the 'getlist' method

The thing is, Tornado does provide a dictionary that contains the data like a MultiDict should do for GET/POST/others request arguments -

{'_xsrf': ['b2c0d0bebbf84516a8aa0462bb41ea0f'], 'password': ['password'], 'email': ['mail@gmail.com']}

But it sends it as a dictionary -

(Pdb) type(self.request.arguments)
<type 'dict'>

And Python dictionaries don’t have a getlist() method. And WTForms requires the arguments object to have that method.

Workaround?

class SimpleMultiDict(dict):
    def getlist(self, key):
        return self[key]

    def __repr__(self):
        return type(self).__name__ + '(' + dict.__repr__(self) + ')'

This simply makes a subclass of dict and adds the getlist() method. So, What did we gain?

(Pdb) SimpleMultiDict(self.request.arguments)
SimpleMultiDict({'_xsrf': ['b2c0d0bebbf84516a8aa0462bb41ea0f'], 'password': ['password'], 'email': ['mail@gmail.com']})
(Pdb) arguments = SimpleMultiDict(self.request.arguments)
(Pdb) dir(arguments)
['__class__', (skipping some default methods) , 'clear', 'copy', 'fromkeys', 'get', 'getlist', 'has_key', 'items', 'iteritems', 'iterkeys', 'itervalues', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values', 'viewitems', 'viewkeys', 'viewvalues']

# Notice the getlist() method

(Pdb) arguments.getlist('email')
['mail@gmail.com']
(Pdb) form = LoginForm(arguments)
(Pdb) form.email.data
'mail@gmail.com'

Now there is a point to note here. Tornado sends the arguments data in a simple dictionary that has each of the values inside a list. Here is a pointer if you’re using some framework that sends a simple dictionary, with the arguments not in list, and that also doesn’t have a getlist() method, just change the getlist() method to this -

class SimpleMultiDict(dict):
    def getlist(self, key):
        return self[key] if type(self[key]) == list else [self[key]]

    def __repr__(self):
        return type(self).__name__ + '(' + dict.__repr__(self) + ')'

This just turns the simgle values to a list, just what getlist() is supposed to do.