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

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?

There's a beautiful solution if you're using Python 3.4 or better. Use the input argument instead of the stdin argument, which accepts a bytes argument:

output = subprocess.check_output(
    ["sed", "s/foo/bar/"],
    input=b"foo",
)

This works for check_output and run, but not call or check_call for some reason.

On Python 3.7+ do this:

my_data = "whatever you want\nshould match this f"
subprocess.run(["grep", "f"], text=True, input=my_data)

and you'll probably want to add capture_output=True to get the output of running the command as a string.

On older versions of Python, replace text=True with universal_newlines=True:

subprocess.run(["grep", "f"], universal_newlines=True, input=my_data)

from subprocess import Popen, PIPE
from tempfile import SpooledTemporaryFile as tempfile
f = tempfile()
f.write('one\ntwo\nthree\nfour\nfive\nsix\n')
f.seek(0)
print Popen(['/bin/grep','f'],stdout=PIPE,stdin=f).stdout.read()
f.close()

"""
Ex: Dialog (2-way) with a Popen()
"""

p = subprocess.Popen('Your Command Here',
                 stdout=subprocess.PIPE,
                 stderr=subprocess.STDOUT,
                 stdin=PIPE,
                 shell=True,
                 bufsize=0)
p.stdin.write('START\n')
out = p.stdout.readline()
while out:
  line = out
  line = line.rstrip("\n")

  if "WHATEVER1" in line:
      pr = 1
      p.stdin.write('DO 1\n')
      out = p.stdout.readline()
      continue

  if "WHATEVER2" in line:
      pr = 2
      p.stdin.write('DO 2\n')
      out = p.stdout.readline()
      continue
"""
..........
"""

out = p.stdout.readline()

p.wait()

Apparently a cStringIO.StringIO object doesn't quack close enough to a file duck to suit subprocess.Popen

I'm afraid not. The pipe is a low-level OS concept, so it absolutely requires a file object that is represented by an OS-level file descriptor. Your workaround is the right one.

I figured out this workaround:

>>> p = subprocess.Popen(['grep','f'],stdout=subprocess.PIPE,stdin=subprocess.PIPE)
>>> p.stdin.write(b'one\ntwo\nthree\nfour\nfive\nsix\n') #expects a bytes type object
>>> p.communicate()[0]
'four\nfive\n'
>>> p.stdin.close()

Is there a better one?

Beware that Popen.communicate(input=s)may give you trouble ifsis too big, because apparently the parent process will buffer it before forking the child subprocess, meaning it needs "twice as much" used memory at that point (at least according to the "under the hood" explanation and linked documentation found here). In my particular case,swas a generator that was first fully expanded and only then written tostdin so the parent process was huge right before the child was spawned, and no memory was left to fork it:

File "/opt/local/stow/python-2.7.2/lib/python2.7/subprocess.py", line 1130, in _execute_child self.pid = os.fork() OSError: [Errno 12] Cannot allocate memory

I'm a bit surprised nobody suggested creating a pipe, which is in my opinion the far simplest way to pass a string to stdin of a subprocess:

read, write = os.pipe()
os.write(write, "stdin input here")
os.close(write)

subprocess.check_call(['your-command'], stdin=read)

p = Popen(['grep', 'f'], stdout=PIPE, stdin=PIPE, stderr=STDOUT)    
p.stdin.write('one\n')
time.sleep(0.5)
p.stdin.write('two\n')
time.sleep(0.5)
p.stdin.write('three\n')
time.sleep(0.5)
testresult = p.communicate()[0]
time.sleep(0.5)
print(testresult)

I am using python3 and found out that you need to encode your string before you can pass it into stdin:

p = Popen(['grep', 'f'], stdout=PIPE, stdin=PIPE, stderr=PIPE)
out, err = p.communicate(input='one\ntwo\nthree\nfour\nfive\nsix\n'.encode())
print(out)