code: def _check_boolean(parameter_name, value, parameter_config):
    
    if parameter_config.get('type') != 'boolean':
      return
  
    if value.lower() not in ('1', 'true', '0', 'false'):
      raise errors.BasicTypeParameterError(parameter_name, value, 'boolean')
docs: Checks if a boolean value is valid.
  
    This is called by the transform_parameter_value function and shouldn't be
    called directly.
  
    This checks that the string value passed in can be converted to a valid
    boolean value.
  
    Args:
      parameter_name: A string containing the name of the parameter, which is
        either just a variable name or the name with the index appended. For
        example 'var' or 'var[2]'.
      value: A string containing the value passed in for the parameter.
      parameter_config: The dictionary containing information specific to the
        parameter in question. This is retrieved from request.parameters in
        the method config.
  
    Raises:
      BasicTypeParameterError: If the given value is not a valid boolean
        value.