一个lock file的python实现

如果多个进程，或者多个独立程序要写同一个文件，那么就存在大家同时写文件的可能，这就不妙了，数据可能会出问题。最近在网上找到一个开源的python实现，有效简洁，列出来分析下代码看看：
文件名：lockfile.py，内容如下，有部分注释加了中文，添加了一些注释。

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:  
                #独占式打开文件  
                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. 
        """  
        #关闭文件，删除文件  
        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.") 
"""  

用法比较有意思，使用with关键字。对with关键字来说，FileLock类先执行enter函数，然后，执行with块里的那些代码，执行完了之后，再执行exit函数，等价于相当于如下形式：

try:
    执行 __enter__的内容
    执行 with_block.
finally:
    执行 __exit__内容

FileLock在enter函数独占式创建或打开一个文件，这个文件不会被其他程序或者进程再次创建或者打开，由此形成lock，执行完代码，在exit里，关闭并删除文件