WTForms create variable number of fields

It was in the docs all along.

def my_view():
    class F(MyBaseForm):
        pass

    F.username = TextField('username')
    for name in iterate_some_model_dynamically():
        setattr(F, name, TextField(name.title()))

    form = F(request.POST, ...)
    # do view stuff

What I didn't realize is that the class attributes must be set before any instantiation occurs. The clarity comes from this bitbucket comment:

This is not a bug, it is by design. There are a lot of problems with adding fields to instantiated forms - For example, data comes in through the Form constructor.

If you reread the thread you link, you'll notice you need to derive the class, add fields to that, and then instantiate the new class. Typically you'll do this inside your view handler.

You're almost there:

CHOICES = [('yes', 'Yes'), ('no', 'No')]

class VariableForm(Form):

    def __new__(cls, questions, **kwargs):
        for index, question in enumerate(questions):
            field_name = "question_{}".format(index)
            field = RadioField(question,
                                  validators=[Required()],
                                  choices=CHOICES)
            setattr(cls, field_name, field)
        return super(VariableForm, cls).__new__(cls, **kwargs)