## 17.1.1 Using the subprocess Module

This module defines one class called `Popen`:

class `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)

  Arguments are:

  args should be a string, or a sequence of program arguments. The program to execute is normally the first item in the args sequence or string, but can be explicitly set by using the executable argument.

  On Unix, with shell=False (default): In this case, the Popen class uses `os.execvp()` to execute the child program. args should normally be a sequence. A string will be treated as a sequence with the string as the only item (the program to execute).

  On Unix, with shell=True: If args is a string, it specifies the command string to execute through the shell. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional shell arguments.

  On Windows: the `Popen` class uses CreateProcess() to execute the child program, which operates on strings. If args is a sequence, it will be converted to a string using the `list2cmdline` method. Please note that not all MS Windows applications interpret the command line the same way: `list2cmdline` is designed for applications using the same rules as the MS C runtime.

  bufsize, if given, has the same meaning as the corresponding argument to the built-in open() function: `0` means unbuffered, `1` means line buffered, any other positive value means use a buffer of (approximately) that size. A negative bufsize means to use the system default, which usually means fully buffered. The default value for bufsize is `0` (unbuffered).

  The executable argument specifies the program to execute. It is very seldom needed: Usually, the program to execute is defined by the args argument. If `shell=True`, the executable argument specifies which shell to use. On Unix, the default shell is /bin/sh. On Windows, the default shell is specified by the COMSPEC environment variable.

  stdin, stdout and stderr specify the executed programs' standard input, standard output and standard error file handles, respectively. Valid values are `PIPE`, an existing file descriptor (a positive integer), an existing file object, and `None`. `PIPE` indicates that a new pipe to the child should be created. With `None`, no redirection will occur; the child's file handles will be inherited from the parent. Additionally, stderr can be `STDOUT`, which indicates that the stderr data from the applications should be captured into the same file handle as for stdout.

  If preexec_fn is set to a callable object, this object will be called in the child process just before the child is executed. (Unix only)

  If close_fds is true, all file descriptors except `0`, `1` and `2` will be closed before the child process is executed. (Unix only)

  If shell is `True`, the specified command will be executed through the shell.

  If cwd is not `None`, the child's current directory will be changed to cwd before it is executed. Note that this directory is not considered when searching the executable, so you can't specify the program's path relative to cwd.

  If env is not `None`, it defines the environment variables for the new process.

  If universal_newlines is `True`, the file objects stdout and stderr are opened as text files, but lines may be terminated by any of `'\n'`, the Unix end-of-line convention, `'\r'`, the Macintosh convention or `'\r\n'`, the Windows convention. All of these external representations are seen as `'\n'` by the Python program. Note: This feature is only available if Python is built with universal newline support (the default). Also, the newlines attribute of the file objects `stdout`, `stdin` and `stderr` are not updated by the communicate() method.

  The startupinfo and creationflags, if given, will be passed to the underlying CreateProcess() function. They can specify things such as appearance of the main window and priority for the new process. (Windows only)