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:mod:`!subprocess` --- Subprocess management

Doc/library/subprocess.rst

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:mod:!subprocess --- Subprocess management

.. module:: subprocess :synopsis: Subprocess management.

Source code: :source:Lib/subprocess.py

The :mod:!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 :mod:!subprocess module can be used to replace these modules and functions can be found in the following sections.

.. seealso::

:pep:324 -- PEP proposing the subprocess module

.. include:: ../includes/wasm-mobile-notavail.rst

Using the :mod:!subprocess Module

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

.. function:: 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 :class:CompletedProcess instance.

The arguments shown above are merely the most common ones, described below in :ref: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 :class: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 :class:Popen object is automatically created with stdout and stderr both set to :data:~subprocess.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 :data:~subprocess.PIPE and stderr to :data:~subprocess.STDOUT, instead of using capture_output.

A timeout may be specified in seconds, it is internally passed on to :meth:Popen.communicate. If the timeout expires, the child process will be killed and waited for. The :exc: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 :meth: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 :class:Popen object is automatically created with stdin set to :data:~subprocess.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 :exc: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 :class: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 :class:Popen. This mapping can be str to str on any platform or bytes to bytes on POSIX platforms much like :data:os.environ or :data: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'')

.. versionadded:: 3.5

.. versionchanged:: 3.6

  Added *encoding* and *errors* parameters

.. versionchanged:: 3.7

  Added the *text* parameter, as a more understandable alias of *universal_newlines*.
  Added the *capture_output* parameter.

.. versionchanged:: 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:: CompletedProcess

The return value from :func:run, representing a process that has finished.

.. attribute:: args

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

.. attribute:: 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).

.. attribute:: stdout

  Captured stdout from the child process. A bytes sequence, or a string if
  :func:`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 :attr:`stderr` will be
  ``None``.

.. attribute:: stderr

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

.. method:: check_returncode()

  If :attr:`returncode` is non-zero, raise a :exc:`CalledProcessError`.

.. versionadded:: 3.5

.. data:: DEVNULL

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

.. versionadded:: 3.3

.. data:: PIPE

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

.. data:: STDOUT

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

.. exception:: SubprocessError

Base class for all other exceptions from this module.

.. versionadded:: 3.3

.. exception:: TimeoutExpired

Subclass of :exc:`SubprocessError`, raised when a timeout expires
while waiting for a child process.

.. attribute:: cmd

    Command that was used to spawn the child process.

.. attribute:: timeout

    Timeout in seconds.

.. attribute:: output

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

.. attribute:: stdout

    Alias for output, for symmetry with :attr:`stderr`.

.. attribute:: stderr

    Stderr output of the child process if it was captured by :func:`run`.
    Otherwise, ``None``.  This is always :class:`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.

.. versionadded:: 3.3

.. versionchanged:: 3.5
    *stdout* and *stderr* attributes added

.. exception:: CalledProcessError

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


.. attribute:: returncode

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

.. attribute:: cmd

    Command that was used to spawn the child process.

.. attribute:: output

    Output of the child process if it was captured by :func:`run` or
    :func:`check_output`.  Otherwise, ``None``.

.. attribute:: stdout

    Alias for output, for symmetry with :attr:`stderr`.

.. attribute:: stderr

    Stderr output of the child process if it was captured by :func:`run`.
    Otherwise, ``None``.

.. versionchanged:: 3.5
    *stdout* and *stderr* attributes added

.. _frequently-used-arguments:

Frequently Used Arguments ^^^^^^^^^^^^^^^^^^^^^^^^^

To support a wide variety of use cases, the :class: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 :const: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, :data:PIPE, :data:DEVNULL, an existing file descriptor (a positive integer), and an existing :term:file object with a valid file descriptor. With the default settings of None, no redirection will occur. :data:PIPE indicates that a new pipe to the child should be created. :data:DEVNULL indicates that the special file :data:os.devnull will be used. Additionally, stderr can be :data:STDOUT, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.

.. index:: single: universal newlines; subprocess module

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 :class:io.TextIOWrapper.

For stdin, line ending characters '\n' in the input will be converted to the default line separator :data: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 :class: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.

.. versionchanged:: 3.6 Added the encoding and errors parameters.

.. versionchanged:: 3.7 Added the text parameter as an alias for universal_newlines.

.. note::

  The newlines attribute of the file objects :attr:`Popen.stdin`,
  :attr:`Popen.stdout` and :attr:`Popen.stderr` are not updated by
  the :meth:`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, :mod:glob, :mod:fnmatch, :func:os.walk, :func:os.path.expandvars, :func:os.path.expanduser, and :mod:shutil).

.. versionchanged:: 3.3 When universal_newlines is True, the class uses the encoding :func:locale.getpreferredencoding(False) <locale.getpreferredencoding> instead of locale.getpreferredencoding(). See the :class: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 :class:Popen constructor documentation.

Popen Constructor ^^^^^^^^^^^^^^^^^

The underlying process creation and management in this module is handled by the :class: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:: 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 :meth:os.execvpe-like behavior to execute the child program. On Windows, the class uses the Windows CreateProcess() function. The arguments to :class:Popen are as follows.

args should be a sequence of program arguments or else a single string or :term: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 :envvar:`PATH`, use
  :meth:`shutil.which`. On all platforms, passing :data:`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 :meth:`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. :meth:`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 :ref:converting-argument-sequence. This is because the underlying CreateProcess() operates on strings.

.. versionchanged:: 3.6 args parameter accepts a :term:path-like object if shell is False and a sequence containing path-like objects on POSIX.

.. versionchanged:: 3.8 args parameter accepts a :term: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 :file:/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, :class:Popen does the equivalent of::

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

On Windows with shell=True, the :envvar: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. :command:dir or :command: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 :func: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.

.. versionchanged:: 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 :program:ps. If shell=True, on POSIX the executable argument specifies a replacement shell for the default :file:/bin/sh.

.. versionchanged:: 3.6 executable parameter accepts a :term:path-like object on POSIX.

.. versionchanged:: 3.8 executable parameter accepts a bytes and :term:path-like object on Windows.

.. versionchanged:: 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, :data:PIPE, :data:DEVNULL, an existing file descriptor (a positive integer), and an existing :term:file object with a valid file descriptor. With the default settings of None, no redirection will occur. :data:PIPE indicates that a new pipe to the child should be created. :data:DEVNULL indicates that the special file :data:os.devnull will be used. Additionally, stderr can be :data: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 :func:`os.setsid` or :func:`os.setpgid` in the child.

.. versionchanged:: 3.8

  The *preexec_fn* parameter is no longer supported in subinterpreters.
  The use of the parameter in a subinterpreter raises
  :exc:`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 :ref:fd_inheritance.

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 :attr:STARTUPINFO.lpAttributeList, or by standard handle redirection.

.. versionchanged:: 3.2 The default for close_fds was changed from :const:False to what is described above.

.. versionchanged:: 3.7 On Windows the default for close_fds was changed from :const:False to :const:True when redirecting the standard handles. It's now possible to set close_fds to :const: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 :const:True. (POSIX only)

.. versionchanged:: 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 :term:path-like <path-like object> 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.

.. versionchanged:: 3.6 cwd parameter accepts a :term:path-like object on POSIX.

.. versionchanged:: 3.7 cwd parameter accepts a :term:path-like object on Windows.

.. versionchanged:: 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)

.. versionchanged:: 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 .. versionchanged:: 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 .. versionchanged:: 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 :func: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 .. versionadded:: 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 :func:grp.getgrnam and the values in gr_gid will be used. Integer values will be passed verbatim. (POSIX only)

.. availability:: POSIX .. versionadded:: 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 :func:pwd.getpwnam and the value in pw_uid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)

.. note::

  Specifying *user* will not drop existing supplementary group memberships!
  The caller must also pass ``extra_groups=()`` to reduce the group membership
  of the child process for security purposes.

.. availability:: POSIX .. versionadded:: 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 .. versionadded:: 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 :data:os.environ or :data: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%``.

.. _side-by-side assembly: https://en.wikipedia.org/wiki/Side-by-Side_Assembly

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 :ref: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.

.. versionadded:: 3.6 encoding and errors were added.

.. versionadded:: 3.7 text was added as a more readable alias for universal_newlines.

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

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

  • :data:CREATE_NEW_CONSOLE
  • :data:CREATE_NEW_PROCESS_GROUP
  • :data:ABOVE_NORMAL_PRIORITY_CLASS
  • :data:BELOW_NORMAL_PRIORITY_CLASS
  • :data:HIGH_PRIORITY_CLASS
  • :data:IDLE_PRIORITY_CLASS
  • :data:NORMAL_PRIORITY_CLASS
  • :data:REALTIME_PRIORITY_CLASS
  • :data:CREATE_NO_WINDOW
  • :data:DETACHED_PROCESS
  • :data:CREATE_DEFAULT_ERROR_MODE
  • :data:CREATE_BREAKAWAY_FROM_JOB

pipesize can be used to change the size of the pipe when :data: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.

.. versionchanged:: 3.10 Added the pipesize parameter.

Popen objects are supported as context managers via the :keyword: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())

.. audit-event:: subprocess.Popen executable,args,cwd,env subprocess.Popen

  Popen and the other functions in this module that use it raise an
  :ref:`auditing event <auditing>` ``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.

.. versionchanged:: 3.2 Added context manager support.

.. versionchanged:: 3.6 Popen destructor now emits a :exc:ResourceWarning warning if the child process is still running.

.. versionchanged:: 3.8 Popen can use :func:os.posix_spawn in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, Popen constructor using :func:os.posix_spawn no longer raise an exception on errors like missing program, but the child process fails with a non-zero :attr:~Popen.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 :exc:OSError. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for :exc:OSError exceptions. Note that, when shell=True, :exc: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 :exc:ValueError will be raised if :class:Popen is called with invalid arguments.

:func:check_call and :func:check_output will raise :exc:CalledProcessError if the called process returns a non-zero return code.

All of the functions and methods that accept a timeout parameter, such as :func:run and :meth:Popen.communicate will raise :exc:TimeoutExpired if the timeout expires before the process exits.

Exceptions defined in this module all inherit from :exc:SubprocessError.

.. versionadded:: 3.3 The :exc:SubprocessError base class was added.

.. _subprocess-security:

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 <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>_ vulnerabilities. On :ref:some platforms <shlex-quote-warning>, it is possible to use :func:shlex.quote for this escaping.

On Windows, batch files (:file:*.bat or :file:*.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 :class:Popen class have the following methods:

.. method:: Popen.poll()

Check if child process has terminated. Set and return :attr:~Popen.returncode attribute. Otherwise, returns None.

.. method:: Popen.wait(timeout=None)

Wait for child process to terminate. Set and return :attr:~Popen.returncode attribute.

If the process does not terminate after timeout seconds, raise a :exc: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 :meth:`Popen.communicate` when using pipes to avoid that.

.. note::

  When ``timeout`` is not ``None`` and the platform supports it, an
  efficient event-driven mechanism is used to wait for process termination:

  - Linux >= 5.3 uses :func:`os.pidfd_open` + :func:`select.poll`
  - macOS and other BSD variants use :func:`select.kqueue` +
    ``KQ_FILTER_PROC`` + ``KQ_NOTE_EXIT``
  - Windows uses ``WaitForSingleObject``

  If none of these mechanisms are available, the function falls back to a
  busy loop (non-blocking call and short sleeps).

.. note::

  Use the :mod:`asyncio` module for an asynchronous wait: see
  :class:`asyncio.create_subprocess_exec`.

.. versionchanged:: 3.3 timeout was added.

.. versionchanged:: 3.15 if timeout is not None, use efficient event-driven implementation on Linux >= 5.3 and macOS / BSD.

.. method:: 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 :attr:~Popen.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.

:meth: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 :exc:TimeoutExpired exception will be raised. Catching this exception and retrying communication will not lose any output. Supplying input to a subsequent post-timeout :meth:communicate call is in undefined behavior and may become an error in the future.

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()

After a call to :meth:~Popen.communicate raises :exc:TimeoutExpired, do not call :meth:~Popen.wait. Use an additional :meth:~Popen.communicate call to finish handling pipes and populate the :attr:~Popen.returncode attribute.

.. note::

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

.. versionchanged:: 3.3 timeout was added.

.. method:: 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 :meth:`terminate`. CTRL_C_EVENT and
  CTRL_BREAK_EVENT can be sent to processes started with a *creationflags*
  parameter which includes ``CREATE_NEW_PROCESS_GROUP``.

.. method:: Popen.terminate()

Stop the child. On POSIX OSs the method sends :py:const:~signal.SIGTERM to the child. On Windows the Win32 API function :c:func:!TerminateProcess is called to stop the child.

.. method:: Popen.kill()

Kills the child. On POSIX OSs the function sends SIGKILL to the child. On Windows :meth:kill is an alias for :meth:terminate.

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

.. attribute:: Popen.args

The args argument as it was passed to :class:Popen -- a sequence of program arguments or else a single string.

.. versionadded:: 3.3

.. attribute:: Popen.stdin

If the stdin argument was :data:PIPE, this attribute is a writeable stream object as returned by :func: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 :data:PIPE, this attribute is None.

.. attribute:: Popen.stdout

If the stdout argument was :data:PIPE, this attribute is a readable stream object as returned by :func: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 :data:PIPE, this attribute is None.

.. attribute:: Popen.stderr

If the stderr argument was :data:PIPE, this attribute is a readable stream object as returned by :func: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 :data:PIPE, this attribute is None.

.. warning::

Use :meth:~Popen.communicate rather than :attr:.stdin.write <Popen.stdin>, :attr:.stdout.read <Popen.stdout> or :attr:.stderr.read <Popen.stderr> to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

.. attribute:: 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.

.. attribute:: Popen.returncode

The child return code. Initially None, :attr:returncode is set by a call to the :meth:poll, :meth:wait, or :meth: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).

When shell=True, the return code reflects the exit status of the shell itself (e.g. /bin/sh), which may map signals to codes such as 128+N. See the documentation of the shell (for example, the Bash manual's Exit Status) for details.

Windows Popen Helpers

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

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

Partial support of the Windows STARTUPINFO <https://msdn.microsoft.com/en-us/library/ms686331(v=vs.85).aspx>__ structure is used for :class:Popen creation. The following attributes can be set by passing them as keyword-only arguments.

.. versionchanged:: 3.7 Keyword-only argument support was added.

.. attribute:: dwFlags

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

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

.. attribute:: hStdInput

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

.. attribute:: hStdOutput

  If :attr:`dwFlags` specifies :data:`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.

.. attribute:: hStdError

  If :attr:`dwFlags` specifies :data:`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.

.. attribute:: wShowWindow

  If :attr:`dwFlags` specifies :data:`STARTF_USESHOWWINDOW`, this attribute
  can be any of the values that can be specified in the ``nCmdShow``
  parameter for the
  `ShowWindow <https://msdn.microsoft.com/en-us/library/ms633548(v=vs.85).aspx>`__
  function, except for ``SW_SHOWDEFAULT``. Otherwise, this attribute is
  ignored.

  :data:`SW_HIDE` is provided for this attribute. It is used when
  :class:`Popen` is called with ``shell=True``.

.. attribute:: lpAttributeList

  A dictionary of additional attributes for process creation as given in
  ``STARTUPINFOEX``, see
  `UpdateProcThreadAttribute <https://msdn.microsoft.com/en-us/library/windows/desktop/ms686880(v=vs.85).aspx>`__.

  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
     :func:`os.set_handle_inheritable` when passed to the :class:`Popen`
     constructor, else :class:`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 :func:`os.system`.  This also applies to
        standard handle redirection, which temporarily creates inheritable
        handles.

  .. versionadded:: 3.7

Windows Constants ^^^^^^^^^^^^^^^^^

The :mod:!subprocess module exposes the following constants.

.. data:: STD_INPUT_HANDLE

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

.. data:: STD_OUTPUT_HANDLE

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

.. data:: STD_ERROR_HANDLE

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

.. data:: SW_HIDE

Hides the window. Another window will be activated.

.. data:: STARTF_USESTDHANDLES

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

.. data:: STARTF_USESHOWWINDOW

Specifies that the :attr:STARTUPINFO.wShowWindow attribute contains additional information.

.. data:: STARTF_FORCEONFEEDBACK

A :attr: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.

.. versionadded:: 3.13

.. data:: STARTF_FORCEOFFFEEDBACK

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

.. versionadded:: 3.13

.. data:: CREATE_NEW_CONSOLE

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

.. data:: CREATE_NEW_PROCESS_GROUP

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

This flag is ignored if :data:CREATE_NEW_CONSOLE is specified.

.. data:: ABOVE_NORMAL_PRIORITY_CLASS

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

.. versionadded:: 3.7

.. data:: BELOW_NORMAL_PRIORITY_CLASS

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

.. versionadded:: 3.7

.. data:: HIGH_PRIORITY_CLASS

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

.. versionadded:: 3.7

.. data:: IDLE_PRIORITY_CLASS

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

.. versionadded:: 3.7

.. data:: NORMAL_PRIORITY_CLASS

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

.. versionadded:: 3.7

.. data:: REALTIME_PRIORITY_CLASS

A :class: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.

.. versionadded:: 3.7

.. data:: CREATE_NO_WINDOW

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

.. versionadded:: 3.7

.. data:: DETACHED_PROCESS

A :class: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.

.. versionadded:: 3.7

.. data:: CREATE_DEFAULT_ERROR_MODE

A :class: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.

.. versionadded:: 3.7

.. data:: CREATE_BREAKAWAY_FROM_JOB

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

.. versionadded:: 3.7

.. _call-function-trio:

Older high-level API

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

.. function:: 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 :attr:~Popen.returncode attribute.

Code needing to capture stdout or stderr should use :func:run instead::

   run(...).returncode

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

The arguments shown above are merely some common ones. The full function signature is the same as that of the :class: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.

.. versionchanged:: 3.3 timeout was added.

.. versionchanged:: 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.

.. function:: 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 :exc:CalledProcessError. The :exc:CalledProcessError object will have the return code in the :attr:~CalledProcessError.returncode attribute. If :func: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 :func:run instead::

   run(..., check=True)

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

The arguments shown above are merely some common ones. The full function signature is the same as that of the :class: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.

.. versionchanged:: 3.3 timeout was added.

.. versionchanged:: 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.

.. function:: 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 :exc:CalledProcessError. The :exc:CalledProcessError object will have the return code in the :attr:~CalledProcessError.returncode attribute and any output in the :attr:~CalledProcessError.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 :func:run - most arguments are passed directly through to that interface. One API deviation from :func: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 :ref:frequently-used-arguments and :func: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'

.. versionadded:: 3.1

.. versionchanged:: 3.3 timeout was added.

.. versionchanged:: 3.4 Support for the input keyword argument was added.

.. versionchanged:: 3.6 encoding and errors were added. See :func:run for details.

.. versionadded:: 3.7 text was added as a more readable alias for universal_newlines.

.. versionchanged:: 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-replacements:

Replacing Older Functions with the :mod:!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 :exc:OSError instead.

In addition, the replacements using :func:check_output will fail with a :exc:CalledProcessError if the requested operation produces a non-zero return code. The output is still available as the :attr:~CalledProcessError.output attribute of the raised exception.

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

Replacing :program:/bin/sh shell command substitution ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: bash

output=$(mycmd myarg)

becomes::

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

Replacing shell pipeline ^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: bash

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:

.. code-block:: bash

output=$(dmesg | grep hda)

becomes::

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

Replacing :func: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 :func:call return value is encoded differently to that of :func:os.system.

  • The :func:os.system function ignores SIGINT and SIGQUIT signals while the command is running, but the caller must do this separately when using the :mod:!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 :func:os.spawn <os.spawnl> 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 :func:os.popen ^^^^^^^^^^^^^^^^^^^^^^^^^^

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")

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.

.. function:: getstatusoutput(cmd, *, encoding=None, errors=None)

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

Execute the string cmd in a shell with :func:check_output and return a 2-tuple (exitcode, output). encoding and errors are used to decode output; see the notes on :ref: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.

.. versionchanged:: 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
  :attr:`~Popen.returncode`.

.. versionchanged:: 3.11 Added the encoding and errors parameters.

.. function:: getoutput(cmd, *, encoding=None, errors=None)

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

Like :func: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.

.. versionchanged:: 3.3.4 Windows support added

.. versionchanged:: 3.11 Added the encoding and errors parameters.

Notes

.. _subprocess-timeout-behavior:

Timeout Behavior ^^^^^^^^^^^^^^^^

When using the timeout parameter in functions like :func:run, :meth:Popen.wait, or :meth: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 :exc:TimeoutExpired exceptions because process creation and system scheduling inherently require time.

.. _converting-argument-sequence:

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.

.. seealso::

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

.. _disable_posix_spawn:

Disable use of posix_spawn() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

::

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

It is safe to set this to false on any Python version. It will have no effect on older or newer versions where unsupported. Do not assume the attribute is available to read. Despite the name, a true value does not indicate 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.

.. versionadded:: 3.8 _USE_POSIX_SPAWN