How Can I Capture Non-Blocking Output from Subprocesses in Python?

Understanding Non-Blocking Output Capture from Subprocesses

In Python, the subprocess module provides a powerful way to interact with system commands. However, when dealing with noisy commands that produce a significant amount of output, it can be challenging to capture this output efficiently and display it line by line.

One common approach is to use the for line in proc.stdout iterator to read the output of the subprocess. However, as the question highlights, this approach can lead to buffering, resulting in delayed display of the output.

Leveraging readline() for Non-Blocking Output

To overcome this buffering issue, the solution lies in utilizing the readline() method of the proc.stdout object. This method allows us to read the output of the subprocess line by line as it becomes available. Here is an updated code snippet that incorporates the readline() approach:

import subprocess

proc = subprocess.Popen(['python', 'fake_utility.py'], stdout=subprocess.PIPE)
while True:
    line = proc.stdout.readline()
    if not line:
        break
    # Perform filtering or other operations on the line as needed
    print("test:", line.rstrip())

In this modified script, we enter a continuous loop that repeatedly reads the stdout of the subprocess using readline(). As long as there is output available, the loop will continue, printing each line as it is received. This ensures that the output is displayed in a non-blocking manner, providing real-time updates to the user.

Handling Subprocess Buffering

It's important to note that the solution still leaves room for potential buffering issues depending on the configuration of the subprocess. For instance, if the subprocess's output is heavily buffered, some delay may still be inevitable. To address this, it may be necessary to adjust the buffering settings of the subprocess or to employ additional strategies, such as flushing the output buffer manually.

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