kbe/src/lib/python/Doc/library/subprocess.rst
subprocess --- Subprocess management.. module:: subprocess :synopsis: Subprocess management.
.. moduleauthor:: Peter Åstrand [email protected] .. sectionauthor:: Peter Åstrand [email protected]
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
subprocess ModuleThe 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.
The :func:run function was added in Python 3.5; if you need to retain
compatibility with older versions, see the :ref:call-function-trio section.
.. 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)
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=PIPE and stderr=PIPE. The stdout and stderr arguments may
not be used as well.
The timeout argument is passed 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 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=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.
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.
.. 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``.
.. 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``.
.. 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` or :func:`check_output` 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 :data:PIPE, :data:DEVNULL, an existing file descriptor (a positive
integer), an existing file object, and None. :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. With the default
settings of None, no redirection will occur; the child's file handles
will be inherited from the parent. 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.
.. versionadded:: 3.6 Added encoding and errors parameters.
.. versionadded:: 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=(), *,
encoding=None, errors=None, text=None)
Execute a child program in a new process. On POSIX, the class uses
:meth:os.execvp-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. 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.
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::
:meth:`shlex.split` can be useful when determining the correct
tokenization for *args*, especially in complex cases::
>>> 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.
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 universal_newlines=True i.e., in a text mode).. 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 :const: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.
stdin, stdout and stderr specify the executed program's standard input,
standard output and standard error file handles, respectively. Valid values
are :data:PIPE, :data:DEVNULL, an existing file descriptor (a positive
integer), an existing :term:file object, and None. :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. With the
default settings of None, no redirection will occur; the child's file
handles will be inherited from the parent. 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.
If you must use it, keep it trivial! Minimize the number of libraries
you call into.
.. 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* parameter can take the place of a previously
common use of *preexec_fn* to call os.setsid() in the child.
If close_fds is true, all file descriptors except :const:0, :const:1 and
:const: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)
.. versionadded:: 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 :class:str and
:term:path-like <path-like object> object. In particular, 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.
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. (POSIX only)
.. versionchanged:: 3.2 start_new_session was added.
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.
.. 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 :envvar:`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.
creationflags, if given, 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`
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())
.. 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.
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.
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:call 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.
Unlike some other popen functions, this implementation will never
implicitly 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.
When using shell=True, the :func:shlex.quote function can be
used to properly escape whitespace and shell metacharacters in strings
that are going to be used to construct shell commands.
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::
The function is implemented using a busy loop (non-blocking call and
short sleeps). Use the :mod:`asyncio` module for an asynchronous wait:
see :class:`asyncio.create_subprocess_exec`.
.. versionchanged:: 3.3 timeout was added.
.. 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. 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.
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.
.. versionchanged:: 3.3 timeout was added.
.. method:: Popen.send_signal(signal)
Sends the signal signal to the child.
.. 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 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 available:
.. 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 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 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 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, set by :meth:poll and :meth:wait (and indirectly
by :meth:communicate). A None value indicates that the process
hasn't terminated yet.
A negative value -N indicates that the child was terminated by signal
N (POSIX only).
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:: 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 an 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:
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)
Run the command described by args. Wait for command to complete, then
return the :attr:~Popen.returncode attribute.
This is equivalent to::
run(...).returncode
(except that the input and check parameters are not supported)
The arguments shown above are merely the most
common ones. The full function signature is largely 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.
.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)
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.
This is equivalent to::
run(..., check=True)
(except that the input parameter is not supported)
The arguments shown above are merely the most
common ones. The full function signature is largely 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.
.. function:: check_output(args, *, stdin=None, stderr=None, shell=False,
cwd=None, encoding=None, errors=None,
universal_newlines=None, timeout=None, text=None)
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 the most common ones.
The full function signature is largely the same as that of :func:run -
most arguments are passed directly through to that interface.
However, explicitly passing input=None to inherit the parent's
standard input file handle is not supported.
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 universal_newlines to
True as described above in :ref:frequently-used-arguments.
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.
.. _subprocess-replacements:
subprocess ModuleIn 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 /bin/sh shell backquote ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. 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")
sts = call("mycmd" + " myarg", shell=True)
Notes:
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, :func:os.popen2, :func: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 :mod: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)
:class:popen2.Popen3 and :class:popen2.Popen4 basically work as
:class:subprocess.Popen, except that:
:class: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 :class:Popen to guarantee this behavior on
all platforms or past Python versions.
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)
Return (exitcode, output) of executing cmd in a shell.
Execute the string cmd in a shell with :meth:Popen.check_output and
return a 2-tuple (exitcode, output). The locale encoding is used;
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:: POSIX & 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`.
.. function:: getoutput(cmd)
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:: POSIX & Windows.
.. versionchanged:: 3.3.4 Windows support added
.. _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):
Arguments are delimited by white space, which is either a space or a tab.
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.
A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.
Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
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.