Usage of stdout.close() in python's subprocess module when piping

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In the python subprocess module, if we wanted to run the shell command

foo | grep bar

from within python, we might use

p1 = Popen(["foo"], stdout = PIPE)
p2 = Popen(["grep", "bar"], stdin = p1.stdout, stdout = PIPE)
p1.stdout.close()
output = p2.communicate()[0]

I'm confused about the line p1.stdout.close(). If you'll forgive me, I'll trace through how I think the program works, and the error will hopefully reveal itself.

It seems to me that when the line output = p2.communicate()[0] is enacted by python, python tries to call p2, it recognizes that it needs output from p1. So it calls p1, which executes foo and throws the output on the stack so that p2 can finish executing. And then p2 finishes.

But nowhere in this trace does p1.stdout.close() actually happen. So what is actually happening? It seems to me that this ordering of lines might matter too, so that the following wouldn't work:

p1 = Popen(["foo"], stdout = PIPE)
p1.stdout.close()
p2 = Popen(["grep", "bar"], stdin = p1.stdout, stdout = PIPE)
output = p2.communicate()[0]

And that's the status of my understanding.

asked 25 secs ago

mixedmath

118