How do I pass a string into subprocess.Popen (using the stdin argument)?

Question

If I do the following:

import subprocess from cStringIO import StringIO subprocess.Popen(['grep','f'],stdout=subprocess.PIPE,stdin=StringIO('one\ntwo\nthree\nfour\nfive\nsix\n')).communicate()[0] 

I get:

Traceback (most recent call last):   File "<stdin>", line 1, in ?   File "/build/toolchain/mac32/python-2.4.3/lib/python2.4/subprocess.py", line 533, in __init__     (p2cread, p2cwrite,   File "/build/toolchain/mac32/python-2.4.3/lib/python2.4/subprocess.py", line 830, in _get_handles     p2cread = stdin.fileno() AttributeError: 'cStringIO.StringI' object has no attribute 'fileno' 

Apparently a cStringIO.StringIO object doesn't quack close enough to a file duck to suit subprocess.Popen. How do I work around this?

Answer 1

Popen.communicate() documentation:

Note that if you want to send data to the process’s stdin, you need to create the Popen object with stdin=PIPE. Similarly, to get anything other than None in the result tuple, you need to give stdout=PIPE and/or stderr=PIPE too.

Replacing os.popen*

    pipe = os.popen(cmd, 'w', bufsize)     # ==>     pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin 

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.

So your example could be written as follows:

from subprocess import Popen, PIPE, STDOUT  p = Popen(['grep', 'f'], stdout=PIPE, stdin=PIPE, stderr=STDOUT)     grep_stdout = p.communicate(input=b'one\ntwo\nthree\nfour\nfive\nsix\n')[0] print(grep_stdout.decode()) # -> four # -> five # -> 

---

On Python 3.5+ (3.6+ for encoding), you could use subprocess.run, to pass input as a string to an external command and get its exit status, and its output as a string back in one call:

#!/usr/bin/env python3 from subprocess import run, PIPE  p = run(['grep', 'f'], stdout=PIPE,         input='one\ntwo\nthree\nfour\nfive\nsix\n', encoding='ascii') print(p.returncode) # -> 0 print(p.stdout) # -> four # -> five # ->