为什么使用线程的脚本偶尔会打印额外的行？ [英] Why a script that uses threads prints extra lines occasionally?

问题描述

如果 print s 被 print>> sys.stderr，s 替换，则效果消失。

If print s is replaced by print >>sys.stderr, s then the effect vanishes.

import random, sys, time
import threading

lock = threading.Lock()

def echo(s):
    time.sleep(1e-3*random.random()) # instead of threading.Timer()
    with lock:
        print s

for c in 'abc':
    threading.Thread(target=echo, args=(c,)).start()

示例

Example

# Run until empty line is found:
$ while ! python example.py 2>&1|tee out|grep '^$';do echo -n .;done;cat out

输出

Output

....................
b

c
a

输出不应该包含空行，但确实如此。我知道 print 不是线程安全的，但我认为锁应该有用。

The output should not contain empty lines, but it does. I understand that print is not thread-safe, but I would've thought the lock should help.

我的机器：

$ python -mplatform
Linux-2.6.38-11-generic-x86_64-with-Ubuntu-11.04-natty

在py26，py27，pypy上打印额外的行。

Extra lines are printed on py26, py27, pypy.

py24，py25，py31，py32按预期行事（不空线路）。

py24, py25, py31, py32 behave as expected (no empty lines).

• sys.stdout.flush（） print 之后无法解决问题：

• sys.stdout.flush() after the print doesn't solve the problem:

with lock:
    print(s)
    sys.stdout.flush()

更奇怪的是普通的 sys.stdout.write函数（） 不会产生带锁的空行：

even more strange that ordinary sys.stdout.write() doesn't produce empty lines with lock:

with lock:
    sys.stdout.write(s)
    sys.stdout.write('\n') #NOTE: no .flush()

print 功能 按预期工作（无空行）。

print function works as expected (no empty lines).

$ tox

推荐答案

看看这个stackoverflow线程：如何在Python 2.6中获得线程安全打印？。显然，打印到sout不是线程安全的。

Take a look at this stackoverflow thread: How do I get a thread safe print in Python 2.6?. Apparently, printing to sout is not thread-safe.

如果你打开详细线程，你可以更好地看到它：

If you turn on verbose threading, you can see this better:

threading.Thread(target=echo, args=(c,), verbose=True).start()

我输出如下：

MainThread: <Thread(Thread-1, initial)>.start(): starting thread
Thread-1: <Thread(Thread-1, started 6204)>.__bootstrap(): thread started
MainThread: <Thread(Thread-2, initial)>.start(): starting thread
Thread-2: <Thread(Thread-2, started 3752)>.__bootstrap(): thread started
MainThread: <Thread(Thread-3, initial)>.start(): starting thread
Thread-3: <Thread(Thread-3, started 4412)>.__bootstrap(): thread started
MainThread: <Thread(Thread-2, started 3752)>.join(): waiting until thread stops
a
b
Thread-1: <Thread(Thread-1, started 6204)>.__bootstrap(): normal return
Thread-2: <Thread(Thread-2, started 3752)>.__bootstrap(): normal return
MainThread: <Thread(Thread-2, stopped 3752)>.join(): thread stopped
MainThread: <Thread(Thread-3, started 4412)>.join(): waiting until thread stops
Thread-3: <Thread(Thread-3, started 4412)>.__bootstrap(): normal return
MainThread: <Thread(Thread-3, stopped 4412)>.join(): thread stopped
c

你可以看到线程3显示为完成在打印'c'字符之前。这显然不是这种情况，因此这使我假设打印到控制台不是线程安全的。

You can see that thread 3 is shown as finishing before printing the 'c' character. This clearly cannot be the case, so this leads me to assume that printing to the console is not thread-safe.

然而，这并不能解释为什么打印到sys.stderr似乎能正常工作。

This, however, does not explain why printing to sys.stderr appears to work correctly.

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