code: def _validate_excludes(self, excluded_fields, field, value):
          
          if isinstance(excluded_fields, Hashable):
              excluded_fields = [excluded_fields]
  
          # Mark the currently evaluated field as not required for now if it actually is.
          # One of the so marked will be needed to pass when required fields are checked.
          if self.schema[field].get('required', self.require_all):
              self._unrequired_by_excludes.add(field)
  
          for excluded_field in excluded_fields:
              if excluded_field in self.schema and self.schema[field].get(
                  'required', self.require_all
              ):
  
                  self._unrequired_by_excludes.add(excluded_field)
  
          if any(excluded_field in self.document for excluded_field in excluded_fields):
              exclusion_str = ', '.join(
                  "'{0}'".format(field) for field in excluded_fields
              )
              self._error(field, errors.EXCLUDES_FIELD, exclusion_str)
docs: {'type': ('hashable', 'list'),
               'schema': {'type': 'hashable'}}