How to Display Real-time Program Output with Python's `subprocess`?

Accessing Real-Time Program Output with Subprocess

Question:

How can we obtain real-time program output using subprocess in Python? Specifically, how can we display the progress of a command-line program without buffering the output?

Answer:

To get real-time output from a program executed with subprocess, we can use the following technique:

import subprocess

p = subprocess.Popen('svnadmin verify /var/svn/repos/config',
                     stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True,
                     bufsize=0)  # Set bufsize to 0 for no buffering

while True:
    line = p.stdout.readline()
    if not line:
        break
    print(line.replace('\n', ''))

Explanation:

• We set bufsize to 0 in Popen to disable output buffering.
• In the while loop, we continuously read lines from p.stdout using readline().
• If no line is available, the loop breaks and the program exits.
• We can process or display each line as they are received, providing real-time output.

Note:

• This method may not work on all systems or with all programs.
• For Python 3.8 and above, consider using asyncio instead of raw file descriptors for handling subprocess output.

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