我编写了一个文件的子类，它a）提供了方便地锁定它的方法（使用fcntl，所以它只支持unix，但这对我atm来说还可以）和b）在读取或写入时断言文件已适当锁定。
现在我不是这些东西的专家（我刚刚读过一篇关于它的论文 [de]）并且希望得到一些反馈：它是否安全，是否存在竞争条件，是否还有其他可以做得更好的事情......在这里是代码：
from fcntl import flock, LOCK_EX, LOCK_SH, LOCK_UN, LOCK_NB
class LockedFile(file):
"""
A wrapper around `file` providing locking. Requires a shared lock to read
and a exclusive lock to write.
Main differences:
* Additional methods: lock_ex, lock_sh, unlock
* Refuse to read when not locked, refuse to write when not locked
exclusivly.
* mode cannot be `w` since then the file would be truncated before
it could be locked.
You have to lock the file yourself, it won't be done for you implicitly.
Only you know what lock you need.
Example usage::
def get_config():
f = LockedFile(CONFIG_FILENAME, 'r')
f.lock_sh()
config = parse_ini(f.read())
f.close()
def set_config(key, value):
f = LockedFile(CONFIG_FILENAME, 'r+')
f.lock_ex()
config = parse_ini(f.read())
config[key] = value
f.truncate()
f.write(make_ini(config))
f.close()
"""
def __init__(self, name, mode='r', *args, **kwargs):
if 'w' in mode:
raise ValueError('Cannot open file in `w` mode')
super(LockedFile, self).__init__(name, mode, *args, **kwargs)
self.locked = None
def lock_sh(self, **kwargs):
"""
Acquire a shared lock on the file. If the file is already locked
exclusively, do nothing.
:returns: Lock status from before the call (one of 'sh', 'ex', None).
:param nonblocking: Don't wait for the lock to be available.
"""
if self.locked == 'ex':
return # would implicitly remove the exclusive lock
return self._lock(LOCK_SH, **kwargs)
def lock_ex(self, **kwargs):
"""
Acquire an exclusive lock on the file.
:returns: Lock status from before the call (one of 'sh', 'ex', None).
:param nonblocking: Don't wait for the lock to be available.
"""
return self._lock(LOCK_EX, **kwargs)
def unlock(self):
"""
Release all locks on the file.
Flushes if there was an exclusive lock.
:returns: Lock status from before the call (one of 'sh', 'ex', None).
"""
if self.locked == 'ex':
self.flush()
return self._lock(LOCK_UN)
def _lock(self, mode, nonblocking=False):
flock(self, mode | bool(nonblocking) * LOCK_NB)
before = self.locked
self.locked = {LOCK_SH: 'sh', LOCK_EX: 'ex', LOCK_UN: None}[mode]
return before
def _assert_read_lock(self):
assert self.locked, "File is not locked"
def _assert_write_lock(self):
assert self.locked == 'ex', "File is not locked exclusively"
def read(self, *args):
self._assert_read_lock()
return super(LockedFile, self).read(*args)
def readline(self, *args):
self._assert_read_lock()
return super(LockedFile, self).readline(*args)
def readlines(self, *args):
self._assert_read_lock()
return super(LockedFile, self).readlines(*args)
def xreadlines(self, *args):
self._assert_read_lock()
return super(LockedFile, self).xreadlines(*args)
def __iter__(self):
self._assert_read_lock()
return super(LockedFile, self).__iter__()
def next(self):
self._assert_read_lock()
return super(LockedFile, self).next()
def write(self, *args):
self._assert_write_lock()
return super(LockedFile, self).write(*args)
def writelines(self, *args):
self._assert_write_lock()
return super(LockedFile, self).writelines(*args)
def flush(self):
self._assert_write_lock()
return super(LockedFile, self).flush()
def truncate(self, *args):
self._assert_write_lock()
return super(LockedFile, self).truncate(*args)
def close(self):
self.unlock()
return super(LockedFile, self).close()
（文档字符串中的示例也是我当前的用例）
感谢您阅读到这里，甚至可能回答:)