code: def validate_field(self, value):
          '''
          Method that must validate the value
          It must return None if the value is valid and a error msg otherelse.
          Ex: If expected input must be int, validate should a return a msg like
          "The filed must be a integer value"
          '''
          if self.choices:
              value = self.normalize_field(value)
              if value in self.choices:
                  return None
              return _('Must be one of: %(choices)s') % {'choices': '; '.join(self.choices)}
          if self.default is not None:
              if value is None or value == '':
                  value = self.default
          if self.required and (value is None or value == ''):
              return _('Required field'f validate_field(self, value):
          '''
          Method that must validate the value
          It must return None if the value is valid and a error msg otherelse.
          Ex: If expected input must be int, validate should a return a msg like
          "The filed must be a integer value"
          '''
          if self.choices:
              value = self.normalize_field(value)
              if value in self.choices:
                  return None
              return _('Must be one of: %(choices)s') % {'choices': '; '.join(self.choices)}
          if self.default is not None:
              if value is None or value == '':
                  value = self.default
          if self.required and (value is None or value == ''):
              return _('Required field')
docs: Method that must validate the value
          It must return None if the value is valid and a error msg otherelse.
          Ex: If expected input must be int, validate should a return a msg like
          "The filed must be a integer value"