How Can I Achieve Non-Blocking Reads from Subprocess Pipes in Python?

Non-Blocking Reads on Subprocess Pipes in Python

Subprocess pipes provide a way to communicate with external programs from within Python. However, reading from standard output by default blocks the process until data is available. For applications requiring non-blocking reads, there are several approaches to consider.

Avoid fcntl, select, and asyncproc

While commonly suggested, fcntl, select, and asyncproc may not be suitable for this scenario. fcntl and select require platform-specific code, while asyncproc relies on multiprocessing, which can introduce additional overhead and interact poorly with pipes.

Queue-Based Solution

A reliable and portable solution is to use a Queue.get_nowait() call. Here's how it works:

from queue import Queue, Empty
from subprocess import PIPE, Popen
from threading import Thread

# Initialize a subprocess and a queue for output
p = Popen(['myprogram.exe'], stdout=PIPE, bufsize=1)
q = Queue()

# Create a thread to enqueue output from the subprocess
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True
t.start()

# Read the queue in a non-blocking manner
try:
    line = q.get_nowait()
except Empty:
    print('No output yet')
else:
    # ... process the output line

In this approach, a separate thread is used to enqueue output from the subprocess into the queue. The main process can then attempt to get data from the queue without blocking. If the queue is empty, it returns an Empty exception.

This solution is both portable and efficient, and it allows for non-blocking reads on subprocess pipes without requiring platform-specific code or additional dependencies.

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