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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def clean_username(self):
 """ Validate that the username is unique and not listed in ``defaults.ACCOUNTS_FORBIDDEN_USERNAMES`` list. """

        try: 
            user = get_user_model().objects.get(username=self.cleaned_data["username"])
        except get_user_model().DoesNotExist: 
            pass
        else: 
            raise forms.ValidationError(
                            self.error_messages['duplicate_username'])

        if self.cleaned_data['username'].lower() \
            in defaults.ACCOUNTS_FORBIDDEN_USERNAMES:
            raise forms.ValidationError(_(u'This username is not allowed.'))
        return self.cleaned_data['username']