I'd like to read the first byte of a subprocess' stdout to know that it has started running. After that I'd like to discard all further output, so that I don't have to worry about the buffer.
What is the best way to do this?
Clarification: I'd like the subprocess to continue running alongside my program, I don't want to wait for it to terminate or anything like that. Ideally there would be some simple way to do this, without resorting to threading, forking or multiprocessing.
If I ignore the output stream, or .close() it, it causes errors if it is sent more data than it can fit in its buffer.
If you're using Python 3.3+, you can use the DEVNULL special value for stdout and stderr to discard subprocess output.
from subprocess import Popen, DEVNULL
process = Popen(["mycmd", "myarg"], stdout=DEVNULL, stderr=DEVNULL)
Or if you're using Python 2.4+, you can simulate this with:
import os
from subprocess import Popen
DEVNULL = open(os.devnull, 'wb')
process = Popen(["mycmd", "myarg"], stdout=DEVNULL, stderr=DEVNULL)
However this doesn't give you the opportunity to read the first byte of stdout.
This seems to work, but it doesn't feel idiomatic.
#!/usr/bin/env python3.1
import threading
import subprocess
def discard_stream_while_running(stream, process):
while process.poll() is None:
stream.read(1024)
def discard_subprocess_pipes(process, out=True, err=True, in_=True):
if out and process.stdout is not None and not process.stdout.closed:
t = threading.Thread(target=discard_stream_while_running, args=(process.stdout, process))
t.start()
if err and process.stderr is not None and not process.stderr.closed:
u = threading.Thread(target=discard_stream_while_running, args=(process.stderr, process))
u.start()
if in_ and process.stdin is not None and not process.stdin.closed:
process.stdin.close()
Example/test usage
if __name__ == "__main__":
import tempfile
import textwrap
import time
with tempfile.NamedTemporaryFile("w+t", prefix="example-", suffix=".py") as f:
f.write(textwrap.dedent("""
import sys
import time
sys.stderr.write("{} byte(s) read through stdin.\\n"
.format(len(sys.stdin.read())))
# Push a couple of MB/s to stdout, messages to stderr.
while True:
sys.stdout.write("Hello Parent\\n" * 1000000)
sys.stderr.write("Subprocess Writing Data\\n")
time.sleep(0.5)
"""))
f.flush()
p = subprocess.Popen(["python3.1", f.name],
stdout=subprocess.PIPE,
stdin=subprocess.PIPE)
p.stdin.write("Hello Child\n".encode())
discard_subprocess_pipes(p) # <-- Here
for s in range(16, 0, -1):
print("Main Process Running For", s, "More Seconds")
time.sleep(1)
