Patch subprocess.Popen to log args, kwargs, and the cmd string

subprocess_Popen_with_logging.py

from functools import partial

class Popen_with_logging(subprocess.Popen):
    """Patch subprocess.Popen to log args, kwargs, and the cmd string"""
    def __init__(self, *args, cmdprefix='+', **kwargs):
        print = kwargs.get("printfunc",
                           partial(__builtins__.print, file=sys.stderr))
        # print = log.debug ; printfunc=log.debug

        if kwargs.get('shell'):
            if len(args) == 1:
                cmdstr = args[0]
            else:
                cmdstr = " ".join(str(x) for x in args)
        else:  # shell=False
            if len(args) == 1:
                cmdstr = " ".join(str(x) for x in args[0])
            else:
                cmdstr = " ".join(str(x) for x in args)

        event = dict(event='subprocess.Popen', args=args, kwargs=kwargs)
        event['cmdstr'] = cmdstr
        print(event)
        print(f'>>> subprocess.Popen(*{args}, **{kwargs})')
        print(cmdprefix, cmdstr)

        super().__init__(*args, **kwargs)


# TODO: more proper monkeypatch than this:
_subprocess_Popen = subprocess.Popen
subprocess.Popen = Popen_with_logging



def test_subprocess_Popen_with_logging():
    subprocess.Popen(["echo", "test1"])
    subprocess.check_output("echo test1", shell=True)

To log output:

• don't specify stdout or stdin
• specify stdout=sys.stdout and stderr=sys.stderr
• https://stackoverflow.com/questions/72683597/can-i-log-a-subprocess-in-python (python2)
• https://stackoverflow.com/questions/4417546/constantly-print-subprocess-output-while-process-is-running/28319191#28319191
• In Python 3.7+, if you specify bufsize=1 and text=True(instead of the deprecated universal_newlines=True), stdout and stderr will be unbuffered, so that you don't have to explicitly call sys.stdout.flush() to actually see the text on the screen.
  You can explicitly specify unbuffered io for stdout and stderr with the -u option or the PYTHONUNBUFFERED=1 environment variable:

  python3 -u ./script.py
  PYTHONUNBUFFERED=1 python3 ./script.py

  https://docs.python.org/3/using/cmdline.html#cmdoption-u

https://docs.python.org/3/library/subprocess.html#subprocess.Popen

pytest docs > "Accessing captured output from a test function"
https://docs.pytest.org/en/stable/how-to/capture-stdout-stderr.html :

def test_myoutput(capsys):  # or use "capfd" for fd-level
    print("hello")
    sys.stderr.write("world\n")

    captured = capsys.readouterr()
    assert captured.out == "hello\n"
    assert captured.err == "world\n"

    print("next")
    captured = capsys.readouterr()
    assert captured.out == "next\n"

sarge docs > "Why not just use subprocess?"
https://sarge.readthedocs.io/en/latest/overview.html#why-not-just-use-subprocess