code: def _base_signup_form_class():
      
      if not app_settings.SIGNUP_FORM_CLASS:
          return _DummyCustomSignupForm
      try:
          fc_module, fc_classname = app_settings.SIGNUP_FORM_CLASS.rsplit('.', 1)
      except ValueError:
          raise exceptions.ImproperlyConfigured('%s does not point to a form'
                                                ' class'
                                                % app_settings.SIGNUP_FORM_CLASS)
      try:
          mod = import_module(fc_module)
      except ImportError as e:
          raise exceptions.ImproperlyConfigured('Error importing form class %s:'
                                                ' "%s"' % (fc_module, e))
      try:
          fc_class = getattr(mod, fc_classname)
      except AttributeError:
          raise exceptions.ImproperlyConfigured('Module "%s" does not define a'
                                                ' "%s" class' % (fc_module,
                                                                 fc_classname))
      if not hasattr(fc_class, 'signup'):
          if hasattr(fc_class, 'save'):
              warnings.warn("The custom signup form must offer"
                            " a `def signup(self, request, user)` method",
                            DeprecationWarning)
          else:
              raise exceptions.ImproperlyConfigured(
                  'The custom signup form must implement a "signup" method')
      return fc_class
docs: Currently, we inherit from the custom form, if any. This is all
      not very elegant, though it serves a purpose:
  
      - There are two signup forms: one for local accounts, and one for
        social accounts
      - Both share a common base (BaseSignupForm)
  
      - Given the above, how to put in a custom signup form? Which form
        would your custom form derive from, the local or the social one?