Python Learning subprocess Module

Subprocess. Popen

This module mainly provides a class Popen:

            Executable=none,      stdin=none,       creationflags=0)  

This pile of stuff is so crazy:

1 row buffer Other positive buffer size negative value with default system buffering (generally full buffering)
generally not, args string or list the first item represents the program name
stdin stdout stderr	none no redirection, inherit parent process Pipe creation pipeline File Object File descriptor (integer) StdErr can also be set to STDOUT
hook function, executed between fork and exec. (Unix)
unix whether to close files other than 0/1/2 before executing the new process Windows does not inherit or inherit the file descriptor of the parent process
Unix is equivalent to args before adding   "/bin/sh"   "-C" window, equivalent to adding "cmd.exe /c"
Cwd	Set up working directory
Env	Setting environment variables
window passed to CreateProcess structure
Creationflags	Under Windows, pass Create_new_console Create your own console window

• The most disturbing thing was the args parameter. Can be a string, which can be a list.

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

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

• But the following can work

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

This is because it is equivalent to

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

is a parameter 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 a list need to be merged into a string before being passed to the API function.

• Like 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 use)

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

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

• Check_call () calls the preceding call and throws an exception if the return value is nonzero

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 Object

This object provides a number of method functions available. And it's been used before. Wait ()/poll ()/communicate ()

Poll ()	Check if end, set return value
Wait ()	Wait for end, set return value
communicate ()	parameter is standard input, return standard output and standard error
send_signal ()	send signal (mostly useful under Unix)
terminate ()	terminate process, UNIX corresponding sigterm signal, Windows downgrade API function TerminateProcess ()
kill ()	kill process (Unix corresponds to Sigkill signal), Windows below
Stdin StdOut StdErr	parameter, it is useful to specify pipe in the
Pid	Process ID
ReturnCode	Process return value

Reference

Python Learning subprocess Module