我编写了一个文件的子类，a）提供了方便地锁定它的方法（使用fcntl，所以它只支持unix，但对我来说是好的）和b）当读取或写入断言文件是适当锁定。
现在我不是这方面的专家（我刚刚阅读了one paper [de]），并希望得到一些反馈：它是否安全，是否存在竞争条件，还有其他可以做的事情吗？更好......这是代码：
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()
（docstring中的示例也是我目前的用例）
感谢您一直读到这里，甚至可能回答：）
答案 0 :(得分：2)
我也不是专家，但有一件事你应该改变，还有其他人要考虑：
首先，使用assert这种方式是一个坏主意：如果使用-O或-OO运行python，则关闭断言，并且您的两个assert_*_lock()方法始终返回True。
第二 - 你需要一些测试。 :)我冒昧地添加了一个自定义错误类并编写了几个测试。前四次通过，最后一次失败;这提出了一个问题，如果正常打开文件（如某些其他非LockedFile对象）并将数据写入其中会发生什么？
哦，最后 - LockableFile这个名字对我来说更有意义，因为该文件可以处于解锁状态。
以下是我所做的更改：
class LockedFileError(OSError): # might want IOError instead
pass
if __name__ == '__main__':
import unittest
import tempfile
import shutil
import os
class TestLockedFile(unittest.TestCase):
def setUp(self):
self.dir = tempfile.mkdtemp()
self.testfile = testfile = os.path.join(self.dir, 'opened.txt')
temp = open(testfile, 'w')
temp.write('[global]\nsetting1=99\nsetting2=42\n')
temp.close()
def tearDown(self):
shutil.rmtree(self.dir, ignore_errors=True)
def test_01(self):
"writes fail if not locked exclusively"
testfile = self.testfile
temp = LockedFile(testfile, 'r+')
self.assertRaises(LockedFileError, temp.write, 'arbitrary data')
temp.lock_sh()
self.assertRaises(LockedFileError, temp.write, 'arbitrary data')
def test_02(self):
"reads fail if not locked"
testfile = self.testfile
temp = LockedFile(testfile, 'r')
self.assertRaises(LockedFileError, temp.read)
def test_03(self):
"writes succeed if locked exclusively"
testfile = self.testfile
temp = LockedFile(testfile, 'r+')
temp.lock_ex()
temp.write('arbitrary data\n')
def test_04(self):
"reads succeed if locked"
testfile = self.testfile
temp = LockedFile(testfile, 'r')
temp.lock_sh()
temp.readline()
temp.lock_ex()
temp.readline()
def test_05(self):
"other writes fail if locked exclusively"
testfile = self.testfile
temp = LockedFile(testfile, 'r')
temp.lock_ex()
testing = open(testfile, 'r+')
# not sure if this should be OSError, IOError, or something else...
self.assertRaises(OSError, testing.write, 'this should fail\n')
unittest.main()
应该编写更多的测试来覆盖LockedFile与读取，写入和其他尝试读取/写入同一实际文件的非LockedFile文件对象的各种组合。