def get(self, function_id, key):
    """Gets stored value given a function id and key.

    Args:
      function_id: identifier for the preprocessing function used.
      key: identifier for the variable stored.
    Returns:
      value: the corresponding value, expected to be a tensor or
             nested structure of tensors.
    Raises:
      ValueError: if function_id is not one of the 23 valid function ids.
    """
    if function_id not in self._VALID_FNS:
      raise ValueError('Function id not recognized: %s.' % str(function_id))
    return self._history[function_id].get(key)
Gets stored value given a function id and key.

    Args:
      function_id: identifier for the preprocessing function used.
      key: identifier for the variable stored.
    Returns:
      value: the corresponding value, expected to be a tensor or
             nested structure of tensors.
    Raises:
      ValueError: if function_id is not one of the 23 valid function ids.
    
inference_api/src/main/object_detection/core/preprocessor_cache.py