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subprocess.md

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Run External programs with subprocess

The standard Python library subprocess can be used to run external programs.

subprocess.run()

subprocess.run(
    args, *, stdin=None, input=None, stdout=None, stderr=None,
    capture_output=False, shell=False, cwd=None, timeout=None,
    check=False, encoding=None, errors=None, text=None,
    env=None, universal_newlines=None, **other_popen_kwargs)

NOTE: From Python 3.5 on, subprocess.run() combines all the functionality of older functions subprocess.call(), subprocess.check_call(), subprocess.check_output(). Beneath all the functions, a subprocess.Popen object is used.

The arguments of subprocess.run() is almost the same as the constructor of subprocess.Popen. So for general purpose, subprocess.run() is preferred, and subprocess.Popen is used for finer control.

The first argument args of subprocess.run() is a string or a list of strings with its first element being the command to run.

  • If args is a string and the argument shell=False (by default), args should be the command without any options.
  • If shell=True, storing the command with its options together as a single string into args instead of a list of strings will enable shell's features such as variable expansion on the command.

To capture output of the command, set capture_output=True, thus stdout=subprocess.PIPE and stderr=subprocess.PIPE will be set automatically, and the output will be stored in the returned subprocess.CompletedProcess object (subprocess.CompletedProcess.stdout and subprocess.CompletedProcess.stderr) from subprocess.run(). Otherwise, the output will be printed on the console by default.

To enable exception raising, set check=True, thus a subprocess.CalledProcessError exception holding the exit code (subprocess.CalledProcessError.returncode) and output (subprocess.CalledProcessError.output, subprocess.CalledProcessError.stdout and subprocess.CalledProcessError.stderr) will be raised when the command fails.

import subprocess

try:
    result = subprocess.run('cat xxx', shell=True, check=True, capture_output=True)
except CalledProcessError as e:
    print(e.stderr.decode('utf-8'))
    return

print(result.stdout.decode('utf-8'))

subprocess.Popen

class subprocess.Popen(
    args, bufsize=-1, executable=None,
    stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=True,
    shell=False, cwd=None, env=None, universal_newlines=None,
    startupinfo=None, creationflags=0, restore_signals=True,
    start_new_session=False, pass_fds=(), *,
    encoding=None, errors=None, text=None)
import subprocess

# 
with subprocess.Popen(['ls', '-l'], stdout=subprocess.PIPE, stderr=subprocess.PIPE) as proc:
    print(proc.stdout.read().decode('utf-8'))

# Or
proc = subprocess.Popen(['ls', '-l'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

try:
    out, err = proc.communicate()
except CalledProcessError as e:
    print(e.stderr.decode('utf-8'))
    return

print(out.decode('utf-8'))

Reference