def clean_username(self):
        """
        Validate that the username is alphanumeric and is not already in use.
        Also validates that the username is not listed in
        ``USERENA_FORBIDDEN_USERNAMES`` list.

        """
        try:
            user = get_user_model().objects.get(username__iexact=self.cleaned_data['username'])
        except get_user_model().DoesNotExist:
            pass
        else:
            if UserenaSignup.objects.filter(user__username__iexact=self.cleaned_data['username']).exclude(activation_key=userena_settings.USERENA_ACTIVATED):
                raise forms.ValidationError(_('This username is already taken but not confirmed. Please check you email for verification steps.'))
            raise forms.ValidationError(_('This username is already taken.'))
        if self.cleaned_data['username'].lower() in userena_settings.USERENA_FORBIDDEN_USERNAMES:
            raise forms.ValidationError(_('This username is not allowed.'))
        return self.cleaned_data['username']
dataset/ETHPy150Open clione/django-kanban/src/core/userena/forms.py/SignupForm.clean_username/original