How Can I Perform Non-Blocking Reads from a Subprocess's Output Stream in Python?

Non-Blocking Reads on Subprocess Output Streams in Python

In Python, the subprocess module provides a convenient way to launch and interact with external processes. However, it's crucial to handle I/O interactions between the parent and child processes efficiently.

Consider the following scenario: you wish to perform non-blocking reads on the standard output stream of a subprocess. By default, p.stdout.readline() blocks until data becomes available in the buffer. To address this, we can employ a technique that involves a separate thread and a queue.

First, create a queue to store output lines:

from Queue import Queue

Next, start a thread that reads lines from the subprocess stdout and enqueues them:

def enqueue_output(out, queue):
    for line in iter(out.readline, b''):
        queue.put(line)
    out.close()

Initialize the subprocess and start the thread:

p = Popen(['myprogram.exe'], stdout=PIPE)
q = Queue()
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True
t.start()

To read a line non-blocking, use the following:

try:
    line = q.get_nowait()  # or q.get(timeout=.1)
except Empty:
    print('no output yet')
else:  # got line
    # do something with the line

This method is cross-platform compatible and ensures non-blocking reads, allowing you to efficiently process data from the subprocess.

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