为什么无缓冲的 stderr 会比有缓冲的 stdout 输出的还晚？

[OhYee]

import sys
sys.stdout.write("stdout1 ")
sys.stderr.write("stderr1 ")
sys.stdout.write("stdout2 ")
sys.stderr.write("stderr2 ")

print()
string = ""
for i in range(10):
    string = string + str(i) + "\n"
print(string)

在使用 python test.py 执行时,应该是有开启缓冲区的,不加上后面的 print 部分,输出顺序是 stderr1 stderr2 stdout1 stdout2

但是为什么加了后面的 print 部分输出变成了

stdout1 stdout2
0
1
2
3
4
5
6
7
8
9

stderr1 stderr2

stderr 不是无缓冲的么,不应该直接就输出出来在最前面么?

[swulling]

@OhYee -u 要看哪个版本，如果是 Python 3 但是是 3.7 以下，那么 -u 也不是 unbuffered，还是 line-buffered
如果是 3.7+，那么
Changed in version 3.7: The text layer of the stdout and stderr streams now is unbuffered.

[OhYee]

@swulling
感谢，试了下 python3.6.7 没有问题，python3.7.1 存在这个问题
看了下两者的区别，应该就是 unbuffered 导致被截断掉了

此贴完结，感谢几位大佬的帮助

————————————

Python3.6

https://docs.python.org/3.6/using/cmdline.html#cmdoption-u
-u
Force the binary layer of the stdout and stderr streams (which is available as their buffer attribute) to be unbuffered. The text I/O layer will still be line-buffered if writing to the console, or block-buffered if redirected to a non-interactive file.

See also PYTHONUNBUFFERED.

https://docs.python.org/3.6/using/cmdline.html#envvar-PYTHONUNBUFFERED
PYTHONUNBUFFERED
If this is set to a non-empty string it is equivalent to specifying the -u option.



Python3.7

https://docs.python.org/3.7/using/cmdline.html#cmdoption-u
-u
Force the stdout and stderr streams to be unbuffered. This option has no effect on the stdin stream.

See also PYTHONUNBUFFERED.

Changed in version 3.7: The text layer of the stdout and stderr streams now is unbuffered.

https://docs.python.org/3.7/using/cmdline.html#envvar-PYTHONUNBUFFERED
PYTHONUNBUFFERED
If this is set to a non-empty string it is equivalent to specifying the -u option.