lock fileのpython実装

複数のプロセス、または複数の独立したプログラムが同じファイルを書く場合は、みんなが同時にファイルを書く可能性があります.これはまずいです.データに問題が発生する可能性があります.最近ネット上で1つのオープンソースのpython実装を見つけて、有効で簡潔で、列を作って分析して下のコードを見てみます:ファイル名:lockfile.py、内容は以下の通りで、一部の注釈に中国語を加え、いくつかの注釈を追加しました.

import os  
import time  
import errno  

class FileLockException(Exception):  
    pass  

class FileLock(object):  
    """ A file locking mechanism that has context-manager support so  
        you can use it in a with statement. This should be relatively cross 
        compatible as it doesn't rely on msvcrt or fcntl for the locking. 
    """  

    def __init__(self, file_name, timeout=10, delay=.05):  
        """ Prepare the file locker. Specify the file to lock and optionally 
            the maximum timeout and the delay between each attempt to lock. 
        """  
        self.is_locked = False  
        self.lockfile = os.path.join(os.getcwd(), "%s.lock" % file_name)  
        self.file_name = file_name  
        self.timeout = timeout  
        self.delay = delay  


    def acquire(self):  
        """ Acquire the lock, if possible. If the lock is in use, it check again 
            every `wait` seconds. It does this until it either gets the lock or 
            exceeds `timeout` number of seconds, in which case it throws  
            an exception. 
        """  
        start_time = time.time()  
        while True:  
            try:  
                #         
                self.fd = os.open(self.lockfile, os.O_CREAT|os.O_EXCL|os.O_RDWR)  
                break;  
            except OSError as e:  
                if e.errno != errno.EEXIST:  
                    raise   
                if (time.time() - start_time) >= self.timeout:  
                    raise FileLockException("Timeout occured.")  
                time.sleep(self.delay)  
        self.is_locked = True  


    def release(self):  
        """ Get rid of the lock by deleting the lockfile.  
            When working in a `with` statement, this gets automatically  
            called at the end. 
        """  
        #    ，      
        if self.is_locked:  
            os.close(self.fd)  
            os.unlink(self.lockfile)  
            self.is_locked = False  


    def __enter__(self):  
        """ Activated when used in the with statement.  
            Should automatically acquire a lock to be used in the with block. 
        """  
        if not self.is_locked:  
            self.acquire()  
        return self  


    def __exit__(self, type, value, traceback):  
        """ Activated at the end of the with statement. 
            It automatically releases the lock if it isn't locked. 
        """  
        if self.is_locked:  
            self.release()  


    def __del__(self):  
        """ Make sure that the FileLock instance doesn't leave a lockfile 
            lying around. 
        """  
        self.release()  

""" 
#use as: 
from filelock import FileLock 
with FileLock("myfile.txt"): 
    # work with the file as it is now locked 
    print("Lock acquired.") 
"""  

使い方が面白いのでwithキーワードを使います.withキーワードの場合、FileLockクラスはまずenter関数を実行し、その後、withブロックのコードを実行し、実行が完了した後、exit関数を実行します.以下の形式に相当します.

try:
       __enter__   
       with_block.
finally:
       __exit__  

FileLockはenter関数でファイルを独占的に作成または開く.このファイルは他のプログラムやプロセスによって再び作成または開くことはなく、lockを形成し、コードを実行し、exitでファイルを閉じて削除する.