嵌套Python上下文管理器

Question

在this question中，我定義了一個包含上下文管理器的上下文管理器。什麼是最簡單的方法來完成這種嵌套？我最終打電話self.temporary_file.__enter__()在self.__enter__()。但是，在self.__exit__中，我很確定我必須在finally塊中調用self.temporary_file.__exit__(type_, value, traceback)以防發生異常。如果self.__exit__中出現問題，我應該設置type_，value和traceback參數嗎？我檢查了contextlib，但找不到任何工具來幫助解決這個問題。從問題

原始代碼：

import itertools as it 
import tempfile 

class WriteOnChangeFile: 
    def __init__(self, filename): 
     self.filename = filename 

    def __enter__(self): 
     self.temporary_file = tempfile.TemporaryFile('r+') 
     self.f = self.temporary_file.__enter__() 
     return self.f 

    def __exit__(self, type_, value, traceback): 
     try: 
      try: 
       with open(self.filename, 'r') as real_f: 
        self.f.seek(0) 
        overwrite = any(
         l != real_l 
         for l, real_l in it.zip_longest(self.f, real_f)) 
      except IOError: 
       overwrite = True 
      if overwrite: 
       with open(self.filename, 'w') as real_f: 
        self.f.seek(0) 
        for l in self.f: 
         real_f.write(l) 
     finally: 
      self.temporary_file.__exit__(type_, value, traceback) 

Answer 1

最簡單的方法來創建上下文管理器是contextlib.contextmanager。事情是這樣的：

@contextlib.contextmanager 
def write_on_change_file(filename): 
    with tempfile.TemporaryFile('r+') as temporary_file: 
     yield temporary_file 
     try: 
      ... some saving logic that you had in __exit__ ... 

然後使用with write_on_change_file(...) as f:。
with聲明的正文將被執行「而不是」yield。如果您想要捕捉身體中發生的任何異常，請將yield本身包裝在try塊中。

臨時文件將始終正確關閉（當其with塊結束時）。