关闭stdin之前的Python子进程Popen readline

Question

import subprocess as sp

p = sp.Popen("/bin/cat", stdin=sp.PIPE, stdout=sp.PIPE)

p.stdin.write(b"foobaz!\n")
p.stdin.close()
print(p.stdout.readline())

This code works as expected (cat echoes the stdin, and we can read the stdout)

If we comment out the p.stdin.close() , the .readline() blocks indefinitely. This behaves the same calling .read(1) instead. Is there a way to readline using the subprocess module without sending EOF to stdin?

I am aware there are other libraries that might solve my problem, and am looking into them as well. For this question, I am only curious if subprocess has this capability, and how to achieve it through subprocess .

Answer 1

The problem is with the buffering. You can use bufsize=1 if you also use universal_newlines=True (though you would then write strings instead of bytes):

p = sp.Popen("/bin/cat", stdin=sp.PIPE, stdout=sp.PIPE, 
             bufsize=1, universal_newlines=True)
p.stdin.write("foobaz!\n")
print(p.stdout.readline())

The above is tested in Python 3.5.2. Note - I have separately just read an answer to unrelated question that suggests that in more recent Python versions text=True is equivalent of universal_newlines=True , but don't have a more recent installation to try it with at time of writing.