The introductory Guide to the subprocess module under Python _python

After being familiar with the qprocess of Qt, looking back at the Python subprocess finally did not feel as scary as before.

Like Qprocess, Subprocess's goal is to start a new process and communicate with it.
subprocess. Popen

This module mainly provides a class Popen:

Class subprocess. Popen (args, 
   bufsize=0, 
   executable=none,
   stdin=none,
   stdout=none, 
   stderr=none, 
   preexec_ Fn=none, 
   Close_fds=false, 
   shell=false, 
   cwd=none, 
   env=none, 
   Universal_newlines=false, 
   Startupinfo=none, 
   creationflags=0)

This pile of stuff really freaks me out:

Subprocess. Popen (["Gedit", "Abc.txt"])
subprocess. Popen ("Gedit abc.txt")

Of these two, the latter will not work. Because if it is a string, it must be the path to the program. (Consider the UNIX API function exec, accept a list of strings)

But the following can work

Subprocess. Popen ("Gedit abc.txt", shell=true)

This is because it is equivalent

Subprocess. Popen (["/bin/sh", "-C", "Gedit Abc.txt"])

It all became the parameters of sh, it doesn't matter.

Under Windows, the following can work

Subprocess. Popen (["Notepad.exe", "Abc.txt"])
subprocess. Popen ("notepad.exe abc.txt")

This is because the API function under Windows CreateProcess accepts a string. Even parameters in the form of lists need to be merged into strings before being passed to API functions.

Similar to the above

Subprocess. Popen ("notepad.exe abc.txt" shell=true)

Equivalent to

Subprocess. Popen ("cmd.exe/c" + "notepad.exe abc.txt" shell=true)

subprocess.call*

The module also provides several convenience functions (which in itself is a good example of popen usage)

Call () executes the program and waits for it to complete

def call (*popenargs, **kwargs): Return
  Popen (*popenargs, **kwargs). Wait ()

Check_call () invokes the preceding call and throws an exception if the return value is Non-zero

def check_call (*popenargs, **kwargs):
  retcode = Call (*popenargs, **kwargs)
  if retcode:
    cmd = Kwargs.get (" Args ")
    raise Calledprocesserror (Retcode, cmd) return
  0

Check_output () executes the program and returns its standard output

def check_output (*popenargs, **kwargs):
  process = Popen (*popenargs, Stdout=pipe, **kwargs)
  output, Unused_ Err = Process.communicate ()
  retcode = Process.poll ()
  if retcode:
    cmd = Kwargs.get ("args")
    raise Calledprocesserror (Retcode, cmd, output=output) return
  output

Popen objects

This object provides a number of method functions available. And we've already used the Wait ()/poll ()/communicate ()