相較於return後再次呼叫該函式都是新的開始，yield類似一個紀錄點，local state會被保存

• local variables
• the instruction pointer
• the internal evaluation stack
• the state of any exception handling

保存後暫停，等到下一次執行時恢復上一次所保存的狀態
在iter_content定義了一個generator function，關於yield的使用分成兩個部分

1. 判斷self.raw是否含有stream屬性後，使用存取該屬性為chunk並回傳
2. 定義一個無窮迴圈，使用self.raw.read存取並回傳

等到所有內容被消化完成並且沒有發生錯誤，設定self._content_consumed為True

        def generate():
            # Special case for urllib3.
            if hasattr(self.raw, 'stream'):
                try:
                    for chunk in self.raw.stream(chunk_size, decode_content=True):
                        yield chunk

                except ProtocolError as e:
                    self._error = ChunkedEncodingError(e)

                except DecodeError as e:
                    self._error = ContentDecodingError(e)

                except ReadTimeoutError as e:
                    self._error = ConnectionError(e)

                finally:
                    # if we had an error - throw the saved error
                    if self._error:
                        raise self._error

            else:
                # Standard file-like object.
                while True:
                    chunk = self.raw.read(chunk_size)
                    if not chunk:
                        break
                    yield chunk

            self._content_consumed = True

參考

• python - Yield expressions