Using a combination of FieldList and FormField results in exception

Question

I'm using a combination of jinja2 and wtforms for my project, where I'm required to use FormField in a FieldList. The following code does not work but throws exception.

class FormTranslations(object):
    def gettext(self, string):
        return gettext(string)
    def ngettext(self, singular, plural, n):
        return ngettext(singular, plural, n)

class BaseForm(Form):
    def __init__(self, request_handler):
        super(BaseForm, self).__init__(request_handler.request.POST)
    def _get_translations(self):
        return FormTranslations()

class SubForm(BaseForm):
    name = fields.StringField()
    qty = fields.IntegerField()

class MainForm(BaseForm):
    value = fields.IntegerField()
    items = fields.FieldList(fields.FormField(SubForm), min_entries=2)


#Instantiate and initialize the MainForm:
f = MainForm(self)

Exception:
…
…
…

File "/src/external/wtforms/form.py", line 178, in __call__
return type.__call__(cls, *args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'formdata'

Sometime it is formdata. Some other times, it is obj or prefix that seems to be the unexpected keyword.

What is wrong with my code?

Answer 1

The issue is the constructor for your subform (via BaseForm) accepts different parameters to that of the built-in wtforms "Form" constructor.

The wtforms builtin form init has this signature: def __init__(self, formdata=None, obj=None, prefix='', **kwargs):

The FormField object constructs the encapsulated form with the following logic: if isinstance(data, dict): self.form = self.form_class(formdata=formdata, prefix=prefix, **data) else: self.form = self.form_class(formdata=formdata, obj=data, prefix=prefix) Thus the BaseForm Constructor needs to accept the appropriate parameters to be encapsulated within the FormField object.

The solution appears to be either to inherit "Form" in your SubForm, or add the required support to BaseForm.

I'm currently working through what appears to be the same issue in a webapp2 application and I'm testing having the subform inherit from Form instead of BaseForm with some success.