"for line in file" is *still* broken in Python 2.7 on pipes

[AndyLutomirski]

BPO	15532
Nosy	@ned-deily

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2012-08-02.05:04:28.299>
created_at = <Date 2012-08-01.21:56:37.731>
labels = ['type-bug', 'expert-IO']
title = '"for line in file" is *still* broken in Python 2.7 on pipes'
updated_at = <Date 2012-08-02.05:04:28.278>
user = 'https://bugs.python.org/AndyLutomirski'

bugs.python.org fields:

activity = <Date 2012-08-02.05:04:28.278>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2012-08-02.05:04:28.299>
closer = 'ned.deily'
components = ['IO']
creation = <Date 2012-08-01.21:56:37.731>
creator = 'Andy.Lutomirski'
dependencies = []
files = []
hgrepos = []
issue_num = 15532
keywords = []
message_count = 2.0
messages = ['167170', '167189']
nosy_count = 2.0
nosy_names = ['ned.deily', 'Andy.Lutomirski']
pr_nums = []
priority = 'normal'
resolution = 'works for me'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue15532'
versions = ['Python 2.7']

[AndyLutomirski]

This program:

import subprocess, sys
p = subprocess.Popen(['bash', '-c', 'while true; do echo x; sleep 1; done'], bufsize=0, stdout=subprocess.PIPE)

for line in p.stdout:
    sys.stdout.buffer.write(line)
    sys.stdout.flush()

sits around and does nothing on Python 2.7.3. It works (i.e. prints 'x' once per second) on Python 3.

This was http://bugs.python.org/issue3907 and is supposedly fixed, but it's not.

[ned-deily]

Notice in the reply to bpo-3907, "with 2.6, you have to use io.open() explicitly". This is still true in Python 2.7, i.e. the new 3.x-compatible io library is not used by default in Python 2. If you want to use it with subprocess.Popen, one way is to supply a pipe with an io wrapper, perhaps something like:

import io, os
i, o = os.pipe()
piperead = io.open(i,'rb',buffering=0)
p = subprocess.Popen(..., stdout=o)

See Chapter 15 of the Python 2.7 Standard Library doc for more information on the io module: http://docs.python.org/library/io.html