"subprocess" — Subprocess management
************************************

**Source code:** Lib/subprocess.py

======================================================================

The "subprocess" module allows you to spawn new processes, connect to
their input/output/error pipes, and obtain their return codes.  This
module intends to replace several older modules and functions:

   os.system
   os.spawn*

Information about how the "subprocess" module can be used to replace
these modules and functions can be found in the following sections.

See also: **PEP 324** – PEP proposing the subprocess module

Availability: not Android, not iOS, not WASI.

This module is not supported on mobile platforms or WebAssembly
platforms.


Using the "subprocess" Module
=============================

The recommended approach to invoking subprocesses is to use the
"run()" function for all use cases it can handle. For more advanced
use cases, the underlying "Popen" interface can be used directly.

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)

   Run the command described by *args*.  Wait for command to complete,
   then return a "CompletedProcess" instance.

   The arguments shown above are merely the most common ones,
   described below in Frequently Used Arguments (hence the use of
   keyword-only notation in the abbreviated signature). The full
   function signature is largely the same as that of the "Popen"
   constructor - most of the arguments to this function are passed
   through to that interface. (*timeout*,  *input*, *check*, and
   *capture_output* are not.)

   If *capture_output* is true, stdout and stderr will be captured.
   When used, the internal "Popen" object is automatically created
   with *stdout* and *stderr* both set to "PIPE". The *stdout* and
   *stderr* arguments may not be supplied at the same time as
   *capture_output*. If you wish to capture and combine both streams
   into one, set *stdout* to "PIPE" and *stderr* to "STDOUT", instead
   of using *capture_output*.

   A *timeout* may be specified in seconds, it is internally passed on
   to "Popen.communicate()". If the timeout expires, the child process
   will be killed and waited for. The "TimeoutExpired" exception will
   be re-raised after the child process has terminated. The initial
   process creation itself cannot be interrupted on many platform APIs
   so you are not guaranteed to see a timeout exception until at least
   after however long process creation takes.

   The *input* argument is passed to "Popen.communicate()" and thus to
   the subprocess’s stdin.  If used it must be a byte sequence, or a
   string if *encoding* or *errors* is specified or *text* is true.
   When used, the internal "Popen" object is automatically created
   with *stdin* set to "PIPE", and the *stdin* argument may not be
   used as well.

   If *check* is true, and the process exits with a non-zero exit
   code, a "CalledProcessError" exception will be raised. Attributes
   of that exception hold the arguments, the exit code, and stdout and
   stderr if they were captured.

   If *encoding* or *errors* are specified, or *text* is true, file
   objects for stdin, stdout and stderr are opened in text mode using
   the specified *encoding* and *errors* or the "io.TextIOWrapper"
   default. The *universal_newlines* argument is equivalent  to *text*
   and is provided for backwards compatibility. By default, file
   objects are opened in binary mode.

   If *env* is not "None", it must be a mapping that defines the
   environment variables for the new process; these are used instead
   of the default behavior of inheriting the current process’
   environment. It is passed directly to "Popen". This mapping can be
   str to str on any platform or bytes to bytes on POSIX platforms
   much like "os.environ" or "os.environb".

   Examples:

      >>> subprocess.run(["ls", "-l"])  # doesn't capture output
      CompletedProcess(args=['ls', '-l'], returncode=0)

      >>> subprocess.run("exit 1", shell=True, check=True)
      Traceback (most recent call last):
        ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

      >>> subprocess.run(["ls", "-l", "/dev/null"], capture_output=True)
      CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
      stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n', stderr=b'')

   Added in version 3.5.

   Changed in version 3.6: Added *encoding* and *errors* parameters

   Changed in version 3.7: Added the *text* parameter, as a more
   understandable alias of *universal_newlines*. Added the
   *capture_output* parameter.

   Changed in version 3.12: Changed Windows shell search order for
   "shell=True". The current directory and "%PATH%" are replaced with
   "%COMSPEC%" and "%SystemRoot%\System32\cmd.exe". As a result,
   dropping a malicious program named "cmd.exe" into a current
   directory no longer works.

class subprocess.CompletedProcess

   The return value from "run()", representing a process that has
   finished.

   args

      The arguments used to launch the process. This may be a list or
      a string.

   returncode

      Exit status of the child process. Typically, an exit status of 0
      indicates that it ran successfully.

      A negative value "-N" indicates that the child was terminated by
      signal "N" (POSIX only).

   stdout

      Captured stdout from the child process. A bytes sequence, or a
      string if "run()" was called with an encoding, errors, or
      text=True. "None" if stdout was not captured.

      If you ran the process with "stderr=subprocess.STDOUT", stdout
      and stderr will be combined in this attribute, and "stderr" will
      be "None".

   stderr

      Captured stderr from the child process. A bytes sequence, or a
      string if "run()" was called with an encoding, errors, or
      text=True. "None" if stderr was not captured.

   check_returncode()

      If "returncode" is non-zero, raise a "CalledProcessError".

   Added in version 3.5.

subprocess.DEVNULL

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to "Popen" and indicates that the special file
   "os.devnull" will be used.

   Added in version 3.3.

subprocess.PIPE

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to "Popen" and indicates that a pipe to the standard
   stream should be opened.  Most useful with "Popen.communicate()".

subprocess.STDOUT

   Special value that can be used as the *stderr* argument to "Popen"
   and indicates that standard error should go into the same handle as
   standard output.

exception subprocess.SubprocessError

   Base class for all other exceptions from this module.

   Added in version 3.3.

exception subprocess.TimeoutExpired

   Subclass of "SubprocessError", raised when a timeout expires while
   waiting for a child process.

   cmd

      Command that was used to spawn the child process.

   timeout

      Timeout in seconds.

   output

      Output of the child process if it was captured by "run()" or
      "check_output()".  Otherwise, "None".  This is always "bytes"
      when any output was captured regardless of the "text=True"
      setting.  It may remain "None" instead of "b''" when no output
      was observed.

   stdout

      Alias for output, for symmetry with "stderr".

   stderr

      Stderr output of the child process if it was captured by
      "run()". Otherwise, "None".  This is always "bytes" when stderr
      output was captured regardless of the "text=True" setting.  It
      may remain "None" instead of "b''" when no stderr output was
      observed.

   Added in version 3.3.

   Changed in version 3.5: *stdout* and *stderr* attributes added

exception subprocess.CalledProcessError

   Subclass of "SubprocessError", raised when a process run by
   "check_call()", "check_output()", or "run()" (with "check=True")
   returns a non-zero exit status.

   returncode

      Exit status of the child process.  If the process exited due to
      a signal, this will be the negative signal number.

   cmd

      Command that was used to spawn the child process.

   output

      Output of the child process if it was captured by "run()" or
      "check_output()".  Otherwise, "None".

   stdout

      Alias for output, for symmetry with "stderr".

   stderr

      Stderr output of the child process if it was captured by
      "run()". Otherwise, "None".

   Changed in version 3.5: *stdout* and *stderr* attributes added


Frequently Used Arguments
-------------------------

To support a wide variety of use cases, the "Popen" constructor (and
the convenience functions) accept a large number of optional
arguments. For most typical use cases, many of these arguments can be
safely left at their default values. The arguments that are most
commonly needed are:

   *args* is required for all calls and should be a string, or a
   sequence of program arguments. Providing a sequence of arguments is
   generally preferred, as it allows the module to take care of any
   required escaping and quoting of arguments (e.g. to permit spaces
   in file names). If passing a single string, either *shell* must be
   "True" (see below) or else the string must simply name the program
   to be executed without specifying any arguments.

   *stdin*, *stdout* and *stderr* specify the executed program’s
   standard input, standard output and standard error file handles,
   respectively.  Valid values are "None", "PIPE", "DEVNULL", an
   existing file descriptor (a positive integer), and an existing
   *file object* with a valid file descriptor.  With the default
   settings of "None", no redirection will occur.  "PIPE" indicates
   that a new pipe to the child should be created.  "DEVNULL"
   indicates that the special file "os.devnull" will be used.
   Additionally, *stderr* can be "STDOUT", which indicates that the
   stderr data from the child process should be captured into the same
   file handle as for *stdout*.

   If *encoding* or *errors* are specified, or *text* (also known as
   *universal_newlines*) is true, the file objects *stdin*, *stdout*
   and *stderr* will be opened in text mode using the *encoding* and
   *errors* specified in the call or the defaults for
   "io.TextIOWrapper".

   For *stdin*, line ending characters "'\n'" in the input will be
   converted to the default line separator "os.linesep". For *stdout*
   and *stderr*, all line endings in the output will be converted to
   "'\n'".  For more information see the documentation of the
   "io.TextIOWrapper" class when the *newline* argument to its
   constructor is "None".

   If text mode is not used, *stdin*, *stdout* and *stderr* will be
   opened as binary streams. No encoding or line ending conversion is
   performed.

   Changed in version 3.6: Added the *encoding* and *errors*
   parameters.

   Changed in version 3.7: Added the *text* parameter as an alias for
   *universal_newlines*.

   Note:

     The newlines attribute of the file objects "Popen.stdin",
     "Popen.stdout" and "Popen.stderr" are not updated by the
     "Popen.communicate()" method.

   If *shell* is "True", the specified command will be executed
   through the shell.  This can be useful if you are using Python
   primarily for the enhanced control flow it offers over most system
   shells and still want convenient access to other shell features
   such as shell pipes, filename wildcards, environment variable
   expansion, and expansion of "~" to a user’s home directory.
   However, note that Python itself offers implementations of many
   shell-like features (in particular, "glob", "fnmatch", "os.walk()",
   "os.path.expandvars()", "os.path.expanduser()", and "shutil").

   Changed in version 3.3: When *universal_newlines* is "True", the
   class uses the encoding "locale.getpreferredencoding(False)"
   instead of "locale.getpreferredencoding()".  See the
   "io.TextIOWrapper" class for more information on this change.

   Note:

     Read the Security Considerations section before using
     "shell=True".

These options, along with all of the other options, are described in
more detail in the "Popen" constructor documentation.


Popen Constructor
-----------------

The underlying process creation and management in this module is
handled by the "Popen" class. It offers a lot of flexibility so that
developers are able to handle the less common cases not covered by the
convenience functions.

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=(), *, group=None, extra_groups=None, user=None, umask=-1, encoding=None, errors=None, text=None, pipesize=-1, process_group=None)

   Execute a child program in a new process.  On POSIX, the class uses
   "os.execvpe()"-like behavior to execute the child program.  On
   Windows, the class uses the Windows "CreateProcess()" function.
   The arguments to "Popen" are as follows.

   *args* should be a sequence of program arguments or else a single
   string or *path-like object*. By default, the program to execute is
   the first item in *args* if *args* is a sequence.  If *args* is a
   string, the interpretation is platform-dependent and described
   below.  See the *shell* and *executable* arguments for additional
   differences from the default behavior.  Unless otherwise stated, it
   is recommended to pass *args* as a sequence.

   Warning:

     For maximum reliability, use a fully qualified path for the
     executable. To search for an unqualified name on "PATH", use
     "shutil.which()". On all platforms, passing "sys.executable" is
     the recommended way to launch the current Python interpreter
     again, and use the "-m" command-line format to launch an
     installed module.Resolving the path of *executable* (or the first
     item of *args*) is platform dependent. For POSIX, see
     "os.execvpe()", and note that when resolving or searching for the
     executable path, *cwd* overrides the current working directory
     and *env* can override the "PATH" environment variable. For
     Windows, see the documentation of the "lpApplicationName" and
     "lpCommandLine" parameters of WinAPI "CreateProcess", and note
     that when resolving or searching for the executable path with
     "shell=False", *cwd* does not override the current working
     directory and *env* cannot override the "PATH" environment
     variable. Using a full path avoids all of these variations.

   An example of passing some arguments to an external program as a
   sequence is:

      Popen(["/usr/bin/git", "commit", "-m", "Fixes a bug."])

   On POSIX, if *args* is a string, the string is interpreted as the
   name or path of the program to execute.  However, this can only be
   done if not passing arguments to the program.

   Note:

     It may not be obvious how to break a shell command into a
     sequence of arguments, especially in complex cases.
     "shlex.split()" can illustrate how to determine the correct
     tokenization for *args*:

        >>> import shlex, subprocess
        >>> command_line = input()
        /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
        >>> args = shlex.split(command_line)
        >>> print(args)
        ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
        >>> p = subprocess.Popen(args) # Success!

     Note in particular that options (such as *-input*) and arguments
     (such as *eggs.txt*) that are separated by whitespace in the
     shell go in separate list elements, while arguments that need
     quoting or backslash escaping when used in the shell (such as
     filenames containing spaces or the *echo* command shown above)
     are single list elements.

   On Windows, if *args* is a sequence, it will be converted to a
   string in a manner described in Converting an argument sequence to
   a string on Windows.  This is because the underlying
   "CreateProcess()" operates on strings.

   Changed in version 3.6: *args* parameter accepts a *path-like
   object* if *shell* is "False" and a sequence containing path-like
   objects on POSIX.

   Changed in version 3.8: *args* parameter accepts a *path-like
   object* if *shell* is "False" and a sequence containing bytes and
   path-like objects on Windows.

   The *shell* argument (which defaults to "False") specifies whether
   to use the shell as the program to execute.  If *shell* is "True",
   it is recommended to pass *args* as a string rather than as a
   sequence.

   On POSIX with "shell=True", the shell defaults to "/bin/sh".  If
   *args* is a string, the string specifies the command to execute
   through the shell.  This means that the string must be formatted
   exactly as it would be when typed at the shell prompt.  This
   includes, for example, quoting or backslash escaping filenames with
   spaces in them.  If *args* is a sequence, the first item specifies
   the command string, and any additional items will be treated as
   additional arguments to the shell itself.  That is to say, "Popen"
   does the equivalent of:

      Popen(['/bin/sh', '-c', args[0], args[1], ...])

   On Windows with "shell=True", the "COMSPEC" environment variable
   specifies the default shell.  The only time you need to specify
   "shell=True" on Windows is when the command you wish to execute is
   built into the shell (e.g. **dir** or **copy**).  You do not need
   "shell=True" to run a batch file or console-based executable.

   Note:

     Read the Security Considerations section before using
     "shell=True".

   *bufsize* will be supplied as the corresponding argument to the
   "open()" function when creating the stdin/stdout/stderr pipe file
   objects:

   * "0" means unbuffered (read and write are one system call and can
     return short)

   * "1" means line buffered (only usable if "text=True" or
     "universal_newlines=True")

   * any other positive value means use a buffer of approximately that
     size

   * negative bufsize (the default) means the system default of
     io.DEFAULT_BUFFER_SIZE will be used.

   Changed in version 3.3.1: *bufsize* now defaults to -1 to enable
   buffering by default to match the behavior that most code expects.
   In versions prior to Python 3.2.4 and 3.3.1 it incorrectly
   defaulted to "0" which was unbuffered and allowed short reads.
   This was unintentional and did not match the behavior of Python 2
   as most code expected.

   The *executable* argument specifies a replacement program to
   execute.   It is very seldom needed.  When "shell=False",
   *executable* replaces the program to execute specified by *args*.
   However, the original *args* is still passed to the program.  Most
   programs treat the program specified by *args* as the command name,
   which can then be different from the program actually executed.  On
   POSIX, the *args* name becomes the display name for the executable
   in utilities such as **ps**.  If "shell=True", on POSIX the
   *executable* argument specifies a replacement shell for the default
   "/bin/sh".

   Changed in version 3.6: *executable* parameter accepts a *path-like
   object* on POSIX.

   Changed in version 3.8: *executable* parameter accepts a bytes and
   *path-like object* on Windows.

   Changed in version 3.12: Changed Windows shell search order for
   "shell=True". The current directory and "%PATH%" are replaced with
   "%COMSPEC%" and "%SystemRoot%\System32\cmd.exe". As a result,
   dropping a malicious program named "cmd.exe" into a current
   directory no longer works.

   *stdin*, *stdout* and *stderr* specify the executed program’s
   standard input, standard output and standard error file handles,
   respectively.  Valid values are "None", "PIPE", "DEVNULL", an
   existing file descriptor (a positive integer), and an existing
   *file object* with a valid file descriptor.  With the default
   settings of "None", no redirection will occur.  "PIPE" indicates
   that a new pipe to the child should be created.  "DEVNULL"
   indicates that the special file "os.devnull" will be used.
   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.
   (POSIX only)

   Warning:

     The *preexec_fn* parameter is NOT SAFE to use in the presence of
     threads in your application.  The child process could deadlock
     before exec is called.

   Note:

     If you need to modify the environment for the child use the *env*
     parameter rather than doing it in a *preexec_fn*. The
     *start_new_session* and *process_group* parameters should take
     the place of code using *preexec_fn* to call "os.setsid()" or
     "os.setpgid()" in the child.

   Changed in version 3.8: The *preexec_fn* parameter is no longer
   supported in subinterpreters. The use of the parameter in a
   subinterpreter raises "RuntimeError". The new restriction may
   affect applications that are deployed in mod_wsgi, uWSGI, and other
   embedded environments.

   If *close_fds* is true, all file descriptors except "0", "1" and
   "2" will be closed before the child process is executed.  Otherwise
   when *close_fds* is false, file descriptors obey their inheritable
   flag as described in Inheritance of File Descriptors.

   On Windows, if *close_fds* is true then no handles will be
   inherited by the child process unless explicitly passed in the
   "handle_list" element of "STARTUPINFO.lpAttributeList", or by
   standard handle redirection.

   Changed in version 3.2: The default for *close_fds* was changed
   from "False" to what is described above.

   Changed in version 3.7: On Windows the default for *close_fds* was
   changed from "False" to "True" when redirecting the standard
   handles. It’s now possible to set *close_fds* to "True" when
   redirecting the standard handles.

   *pass_fds* is an optional sequence of file descriptors to keep open
   between the parent and child.  Providing any *pass_fds* forces
   *close_fds* to be "True".  (POSIX only)

   Changed in version 3.2: The *pass_fds* parameter was added.

   If *cwd* is not "None", the function changes the working directory
   to *cwd* before executing the child.  *cwd* can be a string, bytes
   or *path-like* object.  On POSIX, the function looks for
   *executable* (or for the first item in *args*) relative to *cwd* if
   the executable path is a relative path.

   Changed in version 3.6: *cwd* parameter accepts a *path-like
   object* on POSIX.

   Changed in version 3.7: *cwd* parameter accepts a *path-like
   object* on Windows.

   Changed in version 3.8: *cwd* parameter accepts a bytes object on
   Windows.

   If *restore_signals* is true (the default) all signals that Python
   has set to SIG_IGN are restored to SIG_DFL in the child process
   before the exec. Currently this includes the SIGPIPE, SIGXFZ and
   SIGXFSZ signals. (POSIX only)

   Changed in version 3.2: *restore_signals* was added.

   If *start_new_session* is true the "setsid()" system call will be
   made in the child process prior to the execution of the subprocess.

   Availability: POSIX

   Changed in version 3.2: *start_new_session* was added.

   If *process_group* is a non-negative integer, the "setpgid(0,
   value)" system call will be made in the child process prior to the
   execution of the subprocess.

   Availability: POSIX

   Changed in version 3.11: *process_group* was added.

   If *group* is not "None", the setregid() system call will be made
   in the child process prior to the execution of the subprocess. If
   the provided value is a string, it will be looked up via
   "grp.getgrnam()" and the value in "gr_gid" will be used. If the
   value is an integer, it will be passed verbatim. (POSIX only)

   Availability: POSIX

   Added in version 3.9.

   If *extra_groups* is not "None", the setgroups() system call will
   be made in the child process prior to the execution of the
   subprocess. Strings provided in *extra_groups* will be looked up
   via "grp.getgrnam()" and the values in "gr_gid" will be used.
   Integer values will be passed verbatim. (POSIX only)

   Availability: POSIX

   Added in version 3.9.

   If *user* is not "None", the setreuid() system call will be made in
   the child process prior to the execution of the subprocess. If the
   provided value is a string, it will be looked up via
   "pwd.getpwnam()" and the value in "pw_uid" will be used. If the
   value is an integer, it will be passed verbatim. (POSIX only)

   Availability: POSIX

   Added in version 3.9.

   If *umask* is not negative, the umask() system call will be made in
   the child process prior to the execution of the subprocess.

   Availability: POSIX

   Added in version 3.9.

   If *env* is not "None", it must be a mapping that defines the
   environment variables for the new process; these are used instead
   of the default behavior of inheriting the current process’
   environment. This mapping can be str to str on any platform or
   bytes to bytes on POSIX platforms much like "os.environ" or
   "os.environb".

   Note:

     If specified, *env* must provide any variables required for the
     program to execute.  On Windows, in order to run a side-by-side
     assembly the specified *env* **must** include a valid
     "SystemRoot".

   If *encoding* or *errors* are specified, or *text* is true, the
   file objects *stdin*, *stdout* and *stderr* are opened in text mode
   with the specified *encoding* and *errors*, as described above in
   Frequently Used Arguments. The *universal_newlines* argument is
   equivalent  to *text* and is provided for backwards compatibility.
   By default, file objects are opened in binary mode.

   Added in version 3.6: *encoding* and *errors* were added.

   Added in version 3.7: *text* was added as a more readable alias for
   *universal_newlines*.

   If given, *startupinfo* will be a "STARTUPINFO" object, which is
   passed to the underlying "CreateProcess" function.

   If given, *creationflags*, can be one or more of the following
   flags:

   * "CREATE_NEW_CONSOLE"

   * "CREATE_NEW_PROCESS_GROUP"

   * "ABOVE_NORMAL_PRIORITY_CLASS"

   * "BELOW_NORMAL_PRIORITY_CLASS"

   * "HIGH_PRIORITY_CLASS"

   * "IDLE_PRIORITY_CLASS"

   * "NORMAL_PRIORITY_CLASS"

   * "REALTIME_PRIORITY_CLASS"

   * "CREATE_NO_WINDOW"

   * "DETACHED_PROCESS"

   * "CREATE_DEFAULT_ERROR_MODE"

   * "CREATE_BREAKAWAY_FROM_JOB"

   *pipesize* can be used to change the size of the pipe when "PIPE"
   is used for *stdin*, *stdout* or *stderr*. The size of the pipe is
   only changed on platforms that support this (only Linux at this
   time of writing). Other platforms will ignore this parameter.

   Changed in version 3.10: Added the *pipesize* parameter.

   Popen objects are supported as context managers via the "with"
   statement: on exit, standard file descriptors are closed, and the
   process is waited for.

      with Popen(["ifconfig"], stdout=PIPE) as proc:
          log.write(proc.stdout.read())

   Popen and the other functions in this module that use it raise an
   auditing event "subprocess.Popen" with arguments "executable",
   "args", "cwd", and "env". The value for "args" may be a single
   string or a list of strings, depending on platform.

   Changed in version 3.2: Added context manager support.

   Changed in version 3.6: Popen destructor now emits a
   "ResourceWarning" warning if the child process is still running.

   Changed in version 3.8: Popen can use "os.posix_spawn()" in some
   cases for better performance. On Windows Subsystem for Linux and
   QEMU User Emulation, Popen constructor using "os.posix_spawn()" no
   longer raise an exception on errors like missing program, but the
   child process fails with a non-zero "returncode".


Exceptions
----------

Exceptions raised in the child process, before the new program has
started to execute, will be re-raised in the parent.

The most common exception raised is "OSError".  This occurs, for
example, when trying to execute a non-existent file.  Applications
should prepare for "OSError" exceptions. Note that, when "shell=True",
"OSError" will be raised by the child only if the selected shell
itself was not found. To determine if the shell failed to find the
requested application, it is necessary to check the return code or
output from the subprocess.

A "ValueError" will be raised if "Popen" is called with invalid
arguments.

"check_call()" and "check_output()" will raise "CalledProcessError" if
the called process returns a non-zero return code.

All of the functions and methods that accept a *timeout* parameter,
such as "run()" and "Popen.communicate()" will raise "TimeoutExpired"
if the timeout expires before the process exits.

Exceptions defined in this module all inherit from "SubprocessError".

Added in version 3.3: The "SubprocessError" base class was added.


Security Considerations
=======================

Unlike some other popen functions, this library will not implicitly
choose to call a system shell.  This means that all characters,
including shell metacharacters, can safely be passed to child
processes. If the shell is invoked explicitly, via "shell=True", it is
the application’s responsibility to ensure that all whitespace and
metacharacters are quoted appropriately to avoid shell injection
vulnerabilities. On some platforms, it is possible to use
"shlex.quote()" for this escaping.

On Windows, batch files ("*.bat" or "*.cmd") may be launched by the
operating system in a system shell regardless of the arguments passed
to this library. This could result in arguments being parsed according
to shell rules, but without any escaping added by Python. If you are
intentionally launching a batch file with arguments from untrusted
sources, consider passing "shell=True" to allow Python to escape
special characters. See gh-114539 for additional discussion.


Popen Objects
=============

Instances of the "Popen" class have the following methods:

Popen.poll()

   Check if child process has terminated.  Set and return "returncode"
   attribute. Otherwise, returns "None".

Popen.wait(timeout=None)

   Wait for child process to terminate.  Set and return "returncode"
   attribute.

   If the process does not terminate after *timeout* seconds, raise a
   "TimeoutExpired" exception.  It is safe to catch this exception and
   retry the wait.

   Note:

     This will deadlock when using "stdout=PIPE" or "stderr=PIPE" and
     the child process generates enough output to a pipe such that it
     blocks waiting for the OS pipe buffer to accept more data. Use
     "Popen.communicate()" when using pipes to avoid that.

   Note:

     When the "timeout" parameter is not "None", then (on POSIX) the
     function is implemented using a busy loop (non-blocking call and
     short sleeps). Use the "asyncio" module for an asynchronous wait:
     see "asyncio.create_subprocess_exec".

   Changed in version 3.3: *timeout* was added.

Popen.communicate(input=None, timeout=None)

   Interact with process: Send data to stdin.  Read data from stdout
   and stderr, until end-of-file is reached.  Wait for process to
   terminate and set the "returncode" attribute.  The optional *input*
   argument should be data to be sent to the child process, or "None",
   if no data should be sent to the child.  If streams were opened in
   text mode, *input* must be a string. Otherwise, it must be bytes.

   "communicate()" returns a tuple "(stdout_data, stderr_data)". The
   data will be strings if streams were opened in text mode;
   otherwise, bytes.

   Note that if you want to send data to the process’s stdin, you need
   to create the Popen object with "stdin=PIPE".  Similarly, to get
   anything other than "None" in the result tuple, you need to give
   "stdout=PIPE" and/or "stderr=PIPE" too.

   If the process does not terminate after *timeout* seconds, a
   "TimeoutExpired" exception will be raised.  Catching this exception
   and retrying communication will not lose any output.

   The child process is not killed if the timeout expires, so in order
   to cleanup properly a well-behaved application should kill the
   child process and finish communication:

      proc = subprocess.Popen(...)
      try:
          outs, errs = proc.communicate(timeout=15)
      except TimeoutExpired:
          proc.kill()
          outs, errs = proc.communicate()

   Note:

     The data read is buffered in memory, so do not use this method if
     the data size is large or unlimited.

   Changed in version 3.3: *timeout* was added.

Popen.send_signal(signal)

   Sends the signal *signal* to the child.

   Do nothing if the process completed.

   Note:

     On Windows, SIGTERM is an alias for "terminate()". CTRL_C_EVENT
     and CTRL_BREAK_EVENT can be sent to processes started with a
     *creationflags* parameter which includes
     "CREATE_NEW_PROCESS_GROUP".

Popen.terminate()

   Stop the child. On POSIX OSs the method sends "SIGTERM" to the
   child. On Windows the Win32 API function "TerminateProcess()" is
   called to stop the child.

Popen.kill()

   Kills the child. On POSIX OSs the function sends SIGKILL to the
   child. On Windows "kill()" is an alias for "terminate()".

The following attributes are also set by the class for you to access.
Reassigning them to new values is unsupported:

Popen.args

   The *args* argument as it was passed to "Popen" – a sequence of
   program arguments or else a single string.

   Added in version 3.3.

Popen.stdin

   If the *stdin* argument was "PIPE", this attribute is a writeable
   stream object as returned by "open()". If the *encoding* or
   *errors* arguments were specified or the *text* or
   *universal_newlines* argument was "True", the stream is a text
   stream, otherwise it is a byte stream. If the *stdin* argument was
   not "PIPE", this attribute is "None".

Popen.stdout

   If the *stdout* argument was "PIPE", this attribute is a readable
   stream object as returned by "open()". Reading from the stream
   provides output from the child process. If the *encoding* or
   *errors* arguments were specified or the *text* or
   *universal_newlines* argument was "True", the stream is a text
   stream, otherwise it is a byte stream. If the *stdout* argument was
   not "PIPE", this attribute is "None".

Popen.stderr

   If the *stderr* argument was "PIPE", this attribute is a readable
   stream object as returned by "open()". Reading from the stream
   provides error output from the child process. If the *encoding* or
   *errors* arguments were specified or the *text* or
   *universal_newlines* argument was "True", the stream is a text
   stream, otherwise it is a byte stream. If the *stderr* argument was
   not "PIPE", this attribute is "None".

Warning:

  Use "communicate()" rather than ".stdin.write", ".stdout.read" or
  ".stderr.read" to avoid deadlocks due to any of the other OS pipe
  buffers filling up and blocking the child process.

Popen.pid

   The process ID of the child process.

   Note that if you set the *shell* argument to "True", this is the
   process ID of the spawned shell.

Popen.returncode

   The child return code. Initially "None", "returncode" is set by a
   call to the "poll()", "wait()", or "communicate()" methods if they
   detect that the process has terminated.

   A "None" value indicates that the process hadn’t yet terminated at
   the time of the last method call.

   A negative value "-N" indicates that the child was terminated by
   signal "N" (POSIX only).


Windows Popen Helpers
=====================

The "STARTUPINFO" class and following constants are only available on
Windows.

class subprocess.STARTUPINFO(*, dwFlags=0, hStdInput=None, hStdOutput=None, hStdError=None, wShowWindow=0, lpAttributeList=None)

   Partial support of the Windows STARTUPINFO structure is used for
   "Popen" creation.  The following attributes can be set by passing
   them as keyword-only arguments.

   Changed in version 3.7: Keyword-only argument support was added.

   dwFlags

      A bit field that determines whether certain "STARTUPINFO"
      attributes are used when the process creates a window.

         si = subprocess.STARTUPINFO()
         si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW

   hStdInput

      If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
      the standard input handle for the process. If
      "STARTF_USESTDHANDLES" is not specified, the default for
      standard input is the keyboard buffer.

   hStdOutput

      If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
      the standard output handle for the process. Otherwise, this
      attribute is ignored and the default for standard output is the
      console window’s buffer.

   hStdError

      If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
      the standard error handle for the process. Otherwise, this
      attribute is ignored and the default for standard error is the
      console window’s buffer.

   wShowWindow

      If "dwFlags" specifies "STARTF_USESHOWWINDOW", this attribute
      can be any of the values that can be specified in the "nCmdShow"
      parameter for the ShowWindow function, except for
      "SW_SHOWDEFAULT". Otherwise, this attribute is ignored.

      "SW_HIDE" is provided for this attribute. It is used when
      "Popen" is called with "shell=True".

   lpAttributeList

      A dictionary of additional attributes for process creation as
      given in "STARTUPINFOEX", see UpdateProcThreadAttribute.

      Supported attributes:

      **handle_list**
         Sequence of handles that will be inherited. *close_fds* must
         be true if non-empty.

         The handles must be temporarily made inheritable by
         "os.set_handle_inheritable()" when passed to the "Popen"
         constructor, else "OSError" will be raised with Windows error
         "ERROR_INVALID_PARAMETER" (87).

         Warning:

           In a multithreaded process, use caution to avoid leaking
           handles that are marked inheritable when combining this
           feature with concurrent calls to other process creation
           functions that inherit all handles such as "os.system()".
           This also applies to standard handle redirection, which
           temporarily creates inheritable handles.

      Added in version 3.7.


Windows Constants
-----------------

The "subprocess" module exposes the following constants.

subprocess.STD_INPUT_HANDLE

   The standard input device. Initially, this is the console input
   buffer, "CONIN$".

subprocess.STD_OUTPUT_HANDLE

   The standard output device. Initially, this is the active console
   screen buffer, "CONOUT$".

subprocess.STD_ERROR_HANDLE

   The standard error device. Initially, this is the active console
   screen buffer, "CONOUT$".

subprocess.SW_HIDE

   Hides the window. Another window will be activated.

subprocess.STARTF_USESTDHANDLES

   Specifies that the "STARTUPINFO.hStdInput",
   "STARTUPINFO.hStdOutput", and "STARTUPINFO.hStdError" attributes
   contain additional information.

subprocess.STARTF_USESHOWWINDOW

   Specifies that the "STARTUPINFO.wShowWindow" attribute contains
   additional information.

subprocess.STARTF_FORCEONFEEDBACK

   A "STARTUPINFO.dwFlags" parameter to specify that the *Working in
   Background* mouse cursor will be displayed while a process is
   launching. This is the default behavior for GUI processes.

   Added in version 3.13.

subprocess.STARTF_FORCEOFFFEEDBACK

   A "STARTUPINFO.dwFlags" parameter to specify that the mouse cursor
   will not be changed when launching a process.

   Added in version 3.13.

subprocess.CREATE_NEW_CONSOLE

   The new process has a new console, instead of inheriting its
   parent’s console (the default).

subprocess.CREATE_NEW_PROCESS_GROUP

   A "Popen" "creationflags" parameter to specify that a new process
   group will be created. This flag is necessary for using "os.kill()"
   on the subprocess.

   This flag is ignored if "CREATE_NEW_CONSOLE" is specified.

subprocess.ABOVE_NORMAL_PRIORITY_CLASS

   A "Popen" "creationflags" parameter to specify that a new process
   will have an above average priority.

   Added in version 3.7.

subprocess.BELOW_NORMAL_PRIORITY_CLASS

   A "Popen" "creationflags" parameter to specify that a new process
   will have a below average priority.

   Added in version 3.7.

subprocess.HIGH_PRIORITY_CLASS

   A "Popen" "creationflags" parameter to specify that a new process
   will have a high priority.

   Added in version 3.7.

subprocess.IDLE_PRIORITY_CLASS

   A "Popen" "creationflags" parameter to specify that a new process
   will have an idle (lowest) priority.

   Added in version 3.7.

subprocess.NORMAL_PRIORITY_CLASS

   A "Popen" "creationflags" parameter to specify that a new process
   will have a normal priority. (default)

   Added in version 3.7.

subprocess.REALTIME_PRIORITY_CLASS

   A "Popen" "creationflags" parameter to specify that a new process
   will have realtime priority. You should almost never use
   REALTIME_PRIORITY_CLASS, because this interrupts system threads
   that manage mouse input, keyboard input, and background disk
   flushing. This class can be appropriate for applications that
   “talk” directly to hardware or that perform brief tasks that should
   have limited interruptions.

   Added in version 3.7.

subprocess.CREATE_NO_WINDOW

   A "Popen" "creationflags" parameter to specify that a new process
   will not create a window.

   Added in version 3.7.

subprocess.DETACHED_PROCESS

   A "Popen" "creationflags" parameter to specify that a new process
   will not inherit its parent’s console. This value cannot be used
   with CREATE_NEW_CONSOLE.

   Added in version 3.7.

subprocess.CREATE_DEFAULT_ERROR_MODE

   A "Popen" "creationflags" parameter to specify that a new process
   does not inherit the error mode of the calling process. Instead,
   the new process gets the default error mode. This feature is
   particularly useful for multithreaded shell applications that run
   with hard errors disabled.

   Added in version 3.7.

subprocess.CREATE_BREAKAWAY_FROM_JOB

   A "Popen" "creationflags" parameter to specify that a new process
   is not associated with the job.

   Added in version 3.7.


Older high-level API
====================

Prior to Python 3.5, these three functions comprised the high level
API to subprocess. You can now use "run()" in many cases, but lots of
existing code calls these functions.

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, **other_popen_kwargs)

   Run the command described by *args*.  Wait for command to complete,
   then return the "returncode" attribute.

   Code needing to capture stdout or stderr should use "run()"
   instead:

      run(...).returncode

   To suppress stdout or stderr, supply a value of "DEVNULL".

   The arguments shown above are merely some common ones. The full
   function signature is the same as that of the "Popen" constructor -
   this function passes all supplied arguments other than *timeout*
   directly through to that interface.

   Note:

     Do not use "stdout=PIPE" or "stderr=PIPE" with this function.
     The child process will block if it generates enough output to a
     pipe to fill up the OS pipe buffer as the pipes are not being
     read from.

   Changed in version 3.3: *timeout* was added.

   Changed in version 3.12: Changed Windows shell search order for
   "shell=True". The current directory and "%PATH%" are replaced with
   "%COMSPEC%" and "%SystemRoot%\System32\cmd.exe". As a result,
   dropping a malicious program named "cmd.exe" into a current
   directory no longer works.

subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, **other_popen_kwargs)

   Run command with arguments.  Wait for command to complete. If the
   return code was zero then return, otherwise raise
   "CalledProcessError". The "CalledProcessError" object will have the
   return code in the "returncode" attribute. If "check_call()" was
   unable to start the process it will propagate the exception that
   was raised.

   Code needing to capture stdout or stderr should use "run()"
   instead:

      run(..., check=True)

   To suppress stdout or stderr, supply a value of "DEVNULL".

   The arguments shown above are merely some common ones. The full
   function signature is the same as that of the "Popen" constructor -
   this function passes all supplied arguments other than *timeout*
   directly through to that interface.

   Note:

     Do not use "stdout=PIPE" or "stderr=PIPE" with this function.
     The child process will block if it generates enough output to a
     pipe to fill up the OS pipe buffer as the pipes are not being
     read from.

   Changed in version 3.3: *timeout* was added.

   Changed in version 3.12: Changed Windows shell search order for
   "shell=True". The current directory and "%PATH%" are replaced with
   "%COMSPEC%" and "%SystemRoot%\System32\cmd.exe". As a result,
   dropping a malicious program named "cmd.exe" into a current
   directory no longer works.

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, cwd=None, encoding=None, errors=None, universal_newlines=None, timeout=None, text=None, **other_popen_kwargs)

   Run command with arguments and return its output.

   If the return code was non-zero it raises a "CalledProcessError".
   The "CalledProcessError" object will have the return code in the
   "returncode" attribute and any output in the "output" attribute.

   This is equivalent to:

      run(..., check=True, stdout=PIPE).stdout

   The arguments shown above are merely some common ones. The full
   function signature is largely the same as that of "run()" - most
   arguments are passed directly through to that interface. One API
   deviation from "run()" behavior exists: passing "input=None" will
   behave the same as "input=b''" (or "input=''", depending on other
   arguments) rather than using the parent’s standard input file
   handle.

   By default, this function will return the data as encoded bytes.
   The actual encoding of the output data may depend on the command
   being invoked, so the decoding to text will often need to be
   handled at the application level.

   This behaviour may be overridden by setting *text*, *encoding*,
   *errors*, or *universal_newlines* to "True" as described in
   Frequently Used Arguments and "run()".

   To also capture standard error in the result, use
   "stderr=subprocess.STDOUT":

      >>> subprocess.check_output(
      ...     "ls non_existent_file; exit 0",
      ...     stderr=subprocess.STDOUT,
      ...     shell=True)
      'ls: non_existent_file: No such file or directory\n'

   Added in version 3.1.

   Changed in version 3.3: *timeout* was added.

   Changed in version 3.4: Support for the *input* keyword argument
   was added.

   Changed in version 3.6: *encoding* and *errors* were added.  See
   "run()" for details.

   Added in version 3.7: *text* was added as a more readable alias for
   *universal_newlines*.

   Changed in version 3.12: Changed Windows shell search order for
   "shell=True". The current directory and "%PATH%" are replaced with
   "%COMSPEC%" and "%SystemRoot%\System32\cmd.exe". As a result,
   dropping a malicious program named "cmd.exe" into a current
   directory no longer works.


Replacing Older Functions with the "subprocess" Module
======================================================

In this section, “a becomes b” means that b can be used as a
replacement for a.

Note:

  All “a” functions in this section fail (more or less) silently if
  the executed program cannot be found; the “b” replacements raise
  "OSError" instead.In addition, the replacements using
  "check_output()" will fail with a "CalledProcessError" if the
  requested operation produces a non-zero return code. The output is
  still available as the "output" attribute of the raised exception.

In the following examples, we assume that the relevant functions have
already been imported from the "subprocess" module.


Replacing **/bin/sh** shell command substitution
------------------------------------------------

   output=$(mycmd myarg)

becomes:

   output = check_output(["mycmd", "myarg"])


Replacing shell pipeline
------------------------

   output=$(dmesg | grep hda)

becomes:

   p1 = Popen(["dmesg"], stdout=PIPE)
   p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
   p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
   output = p2.communicate()[0]

The "p1.stdout.close()" call after starting the p2 is important in
order for p1 to receive a SIGPIPE if p2 exits before p1.

Alternatively, for trusted input, the shell’s own pipeline support may
still be used directly:

   output=$(dmesg | grep hda)

becomes:

   output = check_output("dmesg | grep hda", shell=True)


Replacing "os.system()"
-----------------------

   sts = os.system("mycmd" + " myarg")
   # becomes
   retcode = call("mycmd" + " myarg", shell=True)

Notes:

* Calling the program through the shell is usually not required.

* The "call()" return value is encoded differently to that of
  "os.system()".

* The "os.system()" function ignores SIGINT and SIGQUIT signals while
  the command is running, but the caller must do this separately when
  using the "subprocess" module.

A more realistic example would look like this:

   try:
       retcode = call("mycmd" + " myarg", shell=True)
       if retcode < 0:
           print("Child was terminated by signal", -retcode, file=sys.stderr)
       else:
           print("Child returned", retcode, file=sys.stderr)
   except OSError as e:
       print("Execution failed:", e, file=sys.stderr)


Replacing the "os.spawn" family
-------------------------------

P_NOWAIT example:

   pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
   ==>
   pid = Popen(["/bin/mycmd", "myarg"]).pid

P_WAIT example:

   retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
   ==>
   retcode = call(["/bin/mycmd", "myarg"])

Vector example:

   os.spawnvp(os.P_NOWAIT, path, args)
   ==>
   Popen([path] + args[1:])

Environment example:

   os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
   ==>
   Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})


Replacing "os.popen()", "os.popen2()", "os.popen3()"
----------------------------------------------------

   (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
   ==>
   p = Popen(cmd, shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, close_fds=True)
   (child_stdin, child_stdout) = (p.stdin, p.stdout)

   (child_stdin,
    child_stdout,
    child_stderr) = os.popen3(cmd, mode, bufsize)
   ==>
   p = Popen(cmd, shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
   (child_stdin,
    child_stdout,
    child_stderr) = (p.stdin, p.stdout, p.stderr)

   (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
   ==>
   p = Popen(cmd, shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
   (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)

Return code handling translates as follows:

   pipe = os.popen(cmd, 'w')
   ...
   rc = pipe.close()
   if rc is not None and rc >> 8:
       print("There were some errors")
   ==>
   process = Popen(cmd, stdin=PIPE)
   ...
   process.stdin.close()
   if process.wait() != 0:
       print("There were some errors")


Replacing functions from the "popen2" module
--------------------------------------------

Note:

  If the cmd argument to popen2 functions is a string, the command is
  executed through /bin/sh.  If it is a list, the command is directly
  executed.

   (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
   ==>
   p = Popen("somestring", shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, close_fds=True)
   (child_stdout, child_stdin) = (p.stdout, p.stdin)

   (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
   ==>
   p = Popen(["mycmd", "myarg"], bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, close_fds=True)
   (child_stdout, child_stdin) = (p.stdout, p.stdin)

"popen2.Popen3" and "popen2.Popen4" basically work as
"subprocess.Popen", except that:

* "Popen" raises an exception if the execution fails.

* The *capturestderr* argument is replaced with the *stderr* argument.

* "stdin=PIPE" and "stdout=PIPE" must be specified.

* popen2 closes all file descriptors by default, but you have to
  specify "close_fds=True" with "Popen" to guarantee this behavior on
  all platforms or past Python versions.


Legacy Shell Invocation Functions
=================================

This module also provides the following legacy functions from the 2.x
"commands" module. These operations implicitly invoke the system shell
and none of the guarantees described above regarding security and
exception handling consistency are valid for these functions.

subprocess.getstatusoutput(cmd, *, encoding=None, errors=None)

   Return "(exitcode, output)" of executing *cmd* in a shell.

   Execute the string *cmd* in a shell with "Popen.check_output()" and
   return a 2-tuple "(exitcode, output)". *encoding* and *errors* are
   used to decode output; see the notes on Frequently Used Arguments
   for more details.

   A trailing newline is stripped from the output. The exit code for
   the command can be interpreted as the return code of subprocess.
   Example:

      >>> subprocess.getstatusoutput('ls /bin/ls')
      (0, '/bin/ls')
      >>> subprocess.getstatusoutput('cat /bin/junk')
      (1, 'cat: /bin/junk: No such file or directory')
      >>> subprocess.getstatusoutput('/bin/junk')
      (127, 'sh: /bin/junk: not found')
      >>> subprocess.getstatusoutput('/bin/kill $$')
      (-15, '')

   Availability: Unix, Windows.

   Changed in version 3.3.4: Windows support was added.The function
   now returns (exitcode, output) instead of (status, output) as it
   did in Python 3.3.3 and earlier.  exitcode has the same value as
   "returncode".

   Changed in version 3.11: Added the *encoding* and *errors*
   parameters.

subprocess.getoutput(cmd, *, encoding=None, errors=None)

   Return output (stdout and stderr) of executing *cmd* in a shell.

   Like "getstatusoutput()", except the exit code is ignored and the
   return value is a string containing the command’s output.  Example:

      >>> subprocess.getoutput('ls /bin/ls')
      '/bin/ls'

   Availability: Unix, Windows.

   Changed in version 3.3.4: Windows support added

   Changed in version 3.11: Added the *encoding* and *errors*
   parameters.


Notes
=====


Timeout Behavior
----------------

When using the "timeout" parameter in functions like "run()",
"Popen.wait()", or "Popen.communicate()", users should be aware of the
following behaviors:

1. **Process Creation Delay**: The initial process creation itself
   cannot be interrupted on many platform APIs. This means that even
   when specifying a timeout, you are not guaranteed to see a timeout
   exception until at least after however long process creation takes.

2. **Extremely Small Timeout Values**: Setting very small timeout
   values (such as a few milliseconds) may result in almost immediate
   "TimeoutExpired" exceptions because process creation and system
   scheduling inherently require time.


Converting an argument sequence to a string on Windows
------------------------------------------------------

On Windows, an *args* sequence is converted to a string that can be
parsed using the following rules (which correspond to the rules used
by the MS C runtime):

1. Arguments are delimited by white space, which is either a space or
   a tab.

2. A string surrounded by double quotation marks is interpreted as a
   single argument, regardless of white space contained within.  A
   quoted string can be embedded in an argument.

3. A double quotation mark preceded by a backslash is interpreted as a
   literal double quotation mark.

4. Backslashes are interpreted literally, unless they immediately
   precede a double quotation mark.

5. If backslashes immediately precede a double quotation mark, every
   pair of backslashes is interpreted as a literal backslash.  If the
   number of backslashes is odd, the last backslash escapes the next
   double quotation mark as described in rule 3.

See also:

  "shlex"
     Module which provides function to parse and escape command lines.


Disabling use of "vfork()" or "posix_spawn()"
---------------------------------------------

On Linux, "subprocess" defaults to using the "vfork()" system call
internally when it is safe to do so rather than "fork()". This greatly
improves performance.

If you ever encounter a presumed highly unusual situation where you
need to prevent "vfork()" from being used by Python, you can set the
"subprocess._USE_VFORK" attribute to a false value.

   subprocess._USE_VFORK = False  # See CPython issue gh-NNNNNN.

Setting this has no impact on use of "posix_spawn()" which could use
"vfork()" internally within its libc implementation.  There is a
similar "subprocess._USE_POSIX_SPAWN" attribute if you need to prevent
use of that.

   subprocess._USE_POSIX_SPAWN = False  # See CPython issue gh-NNNNNN.

It is safe to set these to false on any Python version. They will have
no effect on older versions when unsupported. Do not assume the
attributes are available to read. Despite their names, a true value
does not indicate that the corresponding function will be used, only
that it may be.

Please file issues any time you have to use these private knobs with a
way to reproduce the issue you were seeing. Link to that issue from a
comment in your code.

Added in version 3.8: "_USE_POSIX_SPAWN"

Added in version 3.11: "_USE_VFORK"
