python, subprocess: reading output from subprocess

Question

I have following script:

#!/usr/bin/python

while True:
    x = raw_input()
    print x[::-1]

I am calling it from ipython:

In [5]: p = Popen('./script.py', stdin=PIPE)

In [6]: p.stdin.write('abc\n')
cba

and it works fine.

However, when I do this:

In [7]: p = Popen('./script.py', stdin=PIPE, stdout=PIPE)

In [8]: p.stdin.write('abc\n')

In [9]: p.stdout.read()

the interpreter hangs. What am I doing wrong? I would like to be able to both write and read from another process multiple times, to pass some tasks to this process. What do I need to do differently?

EDIT 1

If I use communicate, I get this:

In [7]: p = Popen('./script.py', stdin=PIPE, stdout=PIPE)

In [8]: p.communicate('abc\n')
Traceback (most recent call last):
  File "./script.py", line 4, in <module>
    x = raw_input()
EOFError: EOF when reading a line
Out[8]: ('cba\n', None)

EDIT 2

I tried flushing:

#!/usr/bin/python

import sys

while True:
        x = raw_input()
        print x[::-1]
        sys.stdout.flush()

and here:

In [5]: from subprocess import PIPE, Popen

In [6]: p = Popen('./script.py', stdin=PIPE, stdout=PIPE)

In [7]: p.stdin.write('abc')

In [8]: p.stdin.flush()

In [9]: p.stdout.read()

but it hangs again.

Answer 1

When you are through writing to p.stdin, close it: p.stdin.close()

Answer 2

Use communicate() instead of .stdout.read().

Example:

from subprocess import Popen, PIPE
p = Popen('./script.py', stdin=PIPE, stdout=PIPE, stderr=PIPE)
input = 'abc\n'
stdout, stderr = p.communicate(input)

This recommendation comes from the Popen objects section in the subprocess documentation:

Warning: Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Answer 3

I believe there are two problems at work here:

1) Your parent script calls p.stdout.read(), which will read all data until end-of-file. However, your child script runs in an infinite loop so end-of-file will never happen. Probably you want p.stdout.readline()?

2) In interactive mode, most programs do buffer only one line at a time. When run from another program, they buffer much more. The buffering improves efficiency in many cases, but causes problems when two programs need to communicate interactively.

After p.stdin.write('abc\n') add:

p.stdin.flush()

In your subprocess script, after print x[::-1] add the following within the loop:

sys.stdout.flush()

(and import sys at the top)

Answer 4

The subprocess method check_output can be useful for this:

output = subprocess.check_output('./script.py')

And output will be the stdout from the process. If you need stderr, too:

output = subprocess.check_output('./script.py', stderr=subprocess.STDOUT)

Because you avoid managing pipes directly, it may circumvent your issue.

Answer 5

If you'd like to pass several lines to script.py then you need to read/write simultaneously:

#!/usr/bin/env python
import sys
from subprocess import PIPE, Popen
from threading  import Thread

def print_output(out, ntrim=80):
    for line in out:
        print len(line)
        if len(line) > ntrim: # truncate long output
            line = line[:ntrim-2]+'..'
        print line.rstrip() 


if __name__=="__main__":
    p = Popen(['python', 'script.py'], stdin=PIPE, stdout=PIPE)
    Thread(target=print_output, args=(p.stdout,)).start()
    for s in ['abc', 'def', 'ab'*10**7, 'ghi']:
        print >>p.stdin, s
    p.stdin.close()
    sys.exit(p.wait()) #NOTE: read http://docs.python.org/library/subprocess.html#subprocess.Popen.wait

Output:

4
cba
4
fed
20000001
bababababababababababababababababababababababababababababababababababababababa..
4
ihg

Where script.py:

#!/usr/bin/env python
"""Print reverse lines."""
while True:
    try: x = raw_input()
    except EOFError:
        break # no more input
    else:
        print x[::-1]

Or

#!/usr/bin/env python
"""Print reverse lines."""
import sys

for line in sys.stdin:
    print line.rstrip()[::-1]

Or

#!/usr/bin/env python
"""Print reverse lines."""
import fileinput

for line in fileinput.input(): # accept files specified as command line arguments
    print line.rstrip()[::-1]

Answer 6

You're probably tripping over Python's output buffering. Here's what python --help has to say about it.

-u     : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x
         see man page for details on internal buffering relating to '-u'