如果创建，Python 子进程 stderr/stdout 字段为 None

Question

当我用一个文件流作为stderr(或stdout)创建一个python子进程时，对应的字段是None:

s = subprocess.Popen(['ls','qwer'],
                     stdout=subprocess.PIPE,
                     stdin=subprocess.PIPE,
                     stderr=open("zzzzz",'wb'))
s.stderr is None
==> True

(我希望 s.stderr 是文件流)。

• 这是记录在案的/故意的吗？
• 理由是什么？

Answer 1

是的，这是故意的。当您向 Popen constructor 提供 stdout/stdin/stderr kwargs 时，您要告诉它在子进程中将这些管道指向何处:

stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively.

但是 Popen 对象上的 stdout/stdin/stderr 属性 are documented as only having a value when you use PIPE :

Popen.stdin

If the stdin argument was PIPE, this attribute is a file object that provides input to the child process. Otherwise, it is None.

Popen.stdout

If the stdout argument was PIPE, this attribute is a file object that provides output from the child process. Otherwise, it is None.

Popen.stderr

If the stderr argument was PIPE, this attribute is a file object that provides error output from the child process. Otherwise, it is None.

至于当你传递一个不是PIPE 的句柄时，为什么Popen.std* 是None，我不确定。也许是因为它被认为是多余的，因为您必须首先将一个打开的句柄传递给构造函数？