Python – subprocess.Popen error

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
# -> 

Your observation suggests that myapp is terminating without reading (all of the) input. Not knowing anything about myapp, that's hard to confirm, but consider for example

$ echo 'hello world' | tr 'l' 'L'
heLLo worLd

now...:

>>> cmd = ['/usr/bin/tr']
>>> p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, stdin=subprocess.PIPE)
>>> out,err = p.communicate('hello world')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/subprocess.py", line 668, in communicate
    return self._communicate(input)
  File "/usr/lib/python2.5/subprocess.py", line 1218, in _communicate
    bytes_written = self._write_no_intr(self.stdin.fileno(), buffer(input, input_offset, 512))
  File "/usr/lib/python2.5/subprocess.py", line 997, in _write_no_intr
    return os.write(fd, s)
OSError: [Errno 32] Broken pipe

because...:

>>> p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
>>> /usr/bin/tr: missing operand
Try `/usr/bin/tr --help' for more information.

and if we fix the bug:

>>> cmd = ['/usr/bin/tr', 'l', 'L']
>>> p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
>>> out,err = p.communicate('hello world')>>> print out
heLLo worLd
>>> print err
None

...it fixes everything. What happens if you omit the stderr redirection -- do you perchance see any error messages from myapp...?