does order matter for p.stdout.read() and p.wait()?
Questions about Python's subprocess.Popen() objects
(Please assume a scenario where the number of bytes being generated for stdout/stderr does not fill up the OS pipe buffers and create a deadlock waiting for the OS pipe buffers to accept more data)
1) Does it matter what order p.stdout.read() and p.wait() are in?
2) Does read() on a stdout/stderr subprocess.PIPE block until the process has terminated?
3) Are the stdout/stderr subprocess.PIPE file objects and data available even after the process has terminated?
import subprocess
process = subprocess.Popen(args="ls", stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
stdout = process.stdout.read()
# Does the above read() block until the process has terminated?
stderr = process.stderr.read()
return_code = process.wait()
process = subprocess.Popen(args="ls", stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
return_code = process.wait()
# Are stdout and stderr pipes available now, even after the process terminated?
stdout = process.stdout.read()
stderr = process.stderr.read()
Q: Does it matter what order p.stdout.read() and p.wait() are in?
A: No.
Q: Does read() on a stdout/stderr subprocess.PIPE block until the process has terminated?
A: If no limit on the number of bytes to read is specified, then it will block until the stream is closed (which is likely when the process terminates).
Q: Are the stdout/stderr subprocess.PIPE file objects and data available even after the process has terminated?
A: Yes.
You may want to pay special attention to this warning from the subprocess documentation:
Warning: This will deadlock when using stdout=PIPE and/or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use communicate() to avoid that.
You might want to consider using a library like sarge which allows flexibility when doing I/O with subprocesses. (Disclosure: I'm the maintainer. I wrote it partly in response to the kind of difficulties you're having.)
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