code: def _validate_mandatory_keys(mandatory, validated, data, to_validate):
      
      errors = []
      for key, sub_schema in mandatory.items():
          if key not in data:
              errors.append('missing key: %r' % (key,))
              continue
          try:
              validated[key] = sub_schema(data[key])
          except NotValid as ex:
              errors.extend(['%r: %s' % (key, arg) for arg in ex.args])
          to_validate.remove(key)
      return errors
docs: Validate the manditory keys.