CustomFormatter python3 - Return string to format with replacement fields not used in string

CustomFormatter python3 - Return string to format with replacement fields not used in string.txt

import string
class PartialFormatter(string.Formatter):
    
    def get_field(self, field_name, args, kwargs):
        try:
            val=super().get_field(field_name, args, kwargs)
        except (KeyError, AttributeError):
            val = '{{{nome_campo}}}'.format(nome_campo=field_name),field_name
        return val 
        
    def get_value(self, key, args, kwargs):
        valor=super().get_value(key, args, kwargs)
        if not valor:
            valor = '{{{chave}}}'.format(chave=key)
        return valor