一個lock file的python實現

如果多個進程，或者多個獨立程序要寫同一個文件，那麼就存在大家同時寫文件的可能，這就不妙了，數據可能會出問題。最近在網上找到一個開源的python實現，有效簡潔，列出來分析下代碼看看：
文件名：lockfile.py，內容如下，有部分註釋加了中文，添加了一些註釋。

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:  
                #獨佔式打開文件  
                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. 
        """  
        #關閉文件，刪除文件  
        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.") 
"""  

用法比較有意思，使用with關鍵字。對with關鍵字來說，FileLock類先執行enter函數，然後，執行with塊裏的那些代碼，執行完了之後，再執行exit函數，等價於相當於如下形式：

try:
    執行 __enter__的內容
    執行 with_block.
finally:
    執行 __exit__內容

FileLock在enter函數獨佔式創建或打開一個文件，這個文件不會被其他程序或者進程再次創建或者打開，由此形成lock，執行完代碼，在exit裏，關閉並刪除文件