subprocess
— Subprocess management¶
Source code: Lib/subprocess.py
The subprocess
module allows you to spawn new processes, connect to their
input/output/error pipes, and obtain their return codes. This module intends to
replace several older modules and functions:
os.system
os.spawn*
Information about how the subprocess
module can be used to replace these
modules and functions can be found in the following sections.
See also
PEP 324 – PEP proposing the subprocess module
Using the subprocess
Module¶
The recommended approach to invoking subprocesses is to use the run()
function for all use cases it can handle. For more advanced use cases, the
underlying Popen
interface can be used directly.
The run()
function was added in Python 3.5; if you need to retain
compatibility with older versions, see the Older high-level API section.
-
subprocess.
run
(args, *, stdin=None, input=None, stdout=None, stderr=None, capture_output=False, shell=False, cwd=None, timeout=None, check=False, encoding=None, errors=None, text=None, env=None, universal_newlines=None, **other_popen_kwargs)¶ Run the command described by args. Wait for command to complete, then return a
CompletedProcess
instance.The arguments shown above are merely the most common ones, described below in Frequently Used Arguments (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the
Popen
constructor - most of the arguments to this function are passed through to that interface. (timeout, input, check, and capture_output are not.)If capture_output is true, stdout and stderr will be captured. When used, the internal
Popen
object is automatically created withstdout=PIPE
andstderr=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, usestdout=PIPE
andstderr=STDOUT
instead of capture_output.The timeout argument is passed to
Popen.communicate()
. If the timeout expires, the child process will be killed and waited for. TheTimeoutExpired
exception will be re-raised after the child process has terminated.The input argument is passed to
Popen.communicate()
and thus to the subprocess’s stdin. If used it must be a byte sequence, or a string if encoding or errors is specified or text is true. When used, the internalPopen
object is automatically created withstdin=PIPE
, and the stdin argument may not be used as well.If check is true, and the process exits with a non-zero exit code, a
CalledProcessError
exception will be raised. Attributes of that exception hold the arguments, the exit code, and stdout and stderr if they were captured.If encoding or errors are specified, or text is true, file objects for stdin, stdout and stderr are opened in text mode using the specified encoding and errors or the
io.TextIOWrapper
default. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.If env is not
None
, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process’ environment. It is passed directly toPopen
.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'')
New in version 3.5.
Changed in version 3.6: Added encoding and errors parameters
Changed in version 3.7: Added the text parameter, as a more understandable alias of universal_newlines. Added the capture_output parameter.
-
class
subprocess.
CompletedProcess
¶ The return value from
run()
, representing a process that has finished.-
args
¶ The arguments used to launch the process. This may be a list or a string.
-
returncode
¶ Exit status of the child process. Typically, an exit status of 0 indicates that it ran successfully.
A negative value
-N
indicates that the child was terminated by signalN
(POSIX only).
-
stdout
¶ Captured stdout from the child process. A bytes sequence, or a string if
run()
was called with an encoding, errors, or text=True.None
if stdout was not captured.If you ran the process with
stderr=subprocess.STDOUT
, stdout and stderr will be combined in this attribute, andstderr
will beNone
.
-
stderr
¶ Captured stderr from the child process. A bytes sequence, or a string if
run()
was called with an encoding, errors, or text=True.None
if stderr was not captured.
-
check_returncode
()¶ If
returncode
is non-zero, raise aCalledProcessError
.
New in version 3.5.
-
-
subprocess.
DEVNULL
¶ Special value that can be used as the stdin, stdout or stderr argument to
Popen
and indicates that the special fileos.devnull
will be used.New in version 3.3.
-
subprocess.
PIPE
¶ Special value that can be used as the stdin, stdout or stderr argument to
Popen
and indicates that a pipe to the standard stream should be opened. Most useful withPopen.communicate()
.
-
subprocess.
STDOUT
¶ Special value that can be used as the stderr argument to
Popen
and indicates that standard error should go into the same handle as standard output.
-
exception
subprocess.
SubprocessError
¶ Base class for all other exceptions from this module.
New in version 3.3.
-
exception
subprocess.
TimeoutExpired
¶ Subclass of
SubprocessError
, raised when a timeout expires while waiting for a child process.-
cmd
¶ Command that was used to spawn the child process.
-
timeout
¶ Timeout in seconds.
-
output
¶ Output of the child process if it was captured by
run()
orcheck_output()
. Otherwise,None
.
New in version 3.3.
Changed in version 3.5: stdout and stderr attributes added
-
-
exception
subprocess.
CalledProcessError
¶ Subclass of
SubprocessError
, raised when a process run bycheck_call()
orcheck_output()
returns a non-zero exit status.-
returncode
¶ Exit status of the child process. If the process exited due to a signal, this will be the negative signal number.
-
cmd
¶ Command that was used to spawn the child process.
-
output
¶ Output of the child process if it was captured by
run()
orcheck_output()
. Otherwise,None
.
Changed in version 3.5: stdout and stderr attributes added
-
Frequently Used Arguments¶
To support a wide variety of use cases, the Popen
constructor (and
the convenience functions) accept a large number of optional arguments. For
most typical use cases, many of these arguments can be safely left at their
default values. The arguments that are most commonly needed are:
args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be
True
(see below) or else the string must simply name the program to be executed without specifying any arguments.stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are
PIPE
,DEVNULL
, an existing file descriptor (a positive integer), an existing file object, andNone
.PIPE
indicates that a new pipe to the child should be created.DEVNULL
indicates that the special fileos.devnull
will be used. With the default settings ofNone
, no redirection will occur; the child’s file handles will be inherited from the parent. Additionally, stderr can beSTDOUT
, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.If encoding or errors are specified, or text (also known as universal_newlines) is true, the file objects stdin, stdout and stderr will be opened in text mode using the encoding and errors specified in the call or the defaults for
io.TextIOWrapper
.For stdin, line ending characters
'\n'
in the input will be converted to the default line separatoros.linesep
. For stdout and stderr, all line endings in the output will be converted to'\n'
. For more information see the documentation of theio.TextIOWrapper
class when the newline argument to its constructor isNone
.If text mode is not used, stdin, stdout and stderr will be opened as binary streams. No encoding or line ending conversion is performed.
New in version 3.6: Added encoding and errors parameters.
New in version 3.7: Added the text parameter as an alias for universal_newlines.
Note
The newlines attribute of the file objects
Popen.stdin
,Popen.stdout
andPopen.stderr
are not updated by thePopen.communicate()
method.If shell is
True
, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of~
to a user’s home directory. However, note that Python itself offers implementations of many shell-like features (in particular,glob
,fnmatch
,os.walk()
,os.path.expandvars()
,os.path.expanduser()
, andshutil
).Changed in version 3.3: When universal_newlines is
True
, the class uses the encodinglocale.getpreferredencoding(False)
instead oflocale.getpreferredencoding()
. See theio.TextIOWrapper
class for more information on this change.Note
Read the Security Considerations section before using
shell=True
.
These options, along with all of the other options, are described in more
detail in the Popen
constructor documentation.
Popen Constructor¶
The underlying process creation and management in this module is handled by
the Popen
class. It offers a lot of flexibility so that developers
are able to handle the less common cases not covered by the convenience
functions.
-
class
subprocess.
Popen
(args, bufsize=-1, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=True, shell=False, cwd=None, env=None, universal_newlines=None, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=(), *, encoding=None, errors=None, text=None)¶ Execute a child program in a new process. On POSIX, the class uses
os.execvp()
-like behavior to execute the child program. On Windows, the class uses the WindowsCreateProcess()
function. The arguments toPopen
are as follows.args should be a sequence of program arguments or else a single string or path-like object. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.
An example of passing some arguments to an external program as a sequence is:
Popen(["/usr/bin/git", "commit", "-m", "Fixes a bug."])
On POSIX, if args is a string, the string is interpreted as the name or path of the program to execute. However, this can only be done if not passing arguments to the program.
Note
It may not be obvious how to break a shell command into a sequence of arguments, especially in complex cases.
shlex.split()
can illustrate how to determine the correct tokenization for args:>>> import shlex, subprocess >>> command_line = input() /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'" >>> args = shlex.split(command_line) >>> print(args) ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"] >>> p = subprocess.Popen(args) # Success!
Note in particular that options (such as -input) and arguments (such as eggs.txt) that are separated by whitespace in the shell go in separate list elements, while arguments that need quoting or backslash escaping when used in the shell (such as filenames containing spaces or the echo command shown above) are single list elements.
On Windows, if args is a sequence, it will be converted to a string in a manner described in Converting an argument sequence to a string on Windows. This is because the underlying
CreateProcess()
operates on strings.Changed in version 3.6: args parameter accepts a path-like object if shell is
False
and a sequence containing path-like objects on POSIX.Changed in version 3.8: args parameter accepts a path-like object if shell is
False
and a sequence containing bytes and path-like objects on Windows.The shell argument (which defaults to
False
) specifies whether to use the shell as the program to execute. If shell isTrue
, it is recommended to pass args as a string rather than as a sequence.On POSIX with
shell=True
, the shell defaults to/bin/sh
. If args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say,Popen
does the equivalent of:Popen(['/bin/sh', '-c', args[0], args[1], ...])
On Windows with
shell=True
, theCOMSPEC
environment variable specifies the default shell. The only time you need to specifyshell=True
on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy). You do not needshell=True
to run a batch file or console-based executable.Note
Read the Security Considerations section before using
shell=True
.bufsize will be supplied as the corresponding argument to the
open()
function when creating the stdin/stdout/stderr pipe file objects:0
means unbuffered (read and write are one system call and can return short)1
means line buffered (only usable ifuniversal_newlines=True
i.e., in a text mode)any other positive value means use a buffer of approximately that size
negative bufsize (the default) means the system default of io.DEFAULT_BUFFER_SIZE will be used.
Changed in version 3.3.1: bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In versions prior to Python 3.2.4 and 3.3.1 it incorrectly defaulted to
0
which was unbuffered and allowed short reads. This was unintentional and did not match the behavior of Python 2 as most code expected.The executable argument specifies a replacement program to execute. It is very seldom needed. When
shell=False
, executable replaces the program to execute specified by args. However, the original args is still passed to the program. Most programs treat the program specified by args as the command name, which can then be different from the program actually executed. On POSIX, the args name becomes the display name for the executable in utilities such as ps. Ifshell=True
, on POSIX the executable argument specifies a replacement shell for the default/bin/sh
.Changed in version 3.6: executable parameter accepts a path-like object on POSIX.
Changed in version 3.8: executable parameter accepts a bytes and path-like object on Windows.
stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are
PIPE
,DEVNULL
, an existing file descriptor (a positive integer), an existing file object, andNone
.PIPE
indicates that a new pipe to the child should be created.DEVNULL
indicates that the special fileos.devnull
will be used. With the default settings ofNone
, no redirection will occur; the child’s file handles will be inherited from the parent. Additionally, stderr can beSTDOUT
, 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.
Changed in version 3.8: The preexec_fn parameter is no longer supported in subinterpreters. The use of the parameter in a subinterpreter raises
RuntimeError
. The new restriction may affect applications that are deployed in mod_wsgi, uWSGI, and other embedded environments.If close_fds is true, all file descriptors except
0
,1
and2
will be closed before the child process is executed. Otherwise when close_fds is false, file descriptors obey their inheritable flag as described in Inheritance of File Descriptors.On Windows, if close_fds is true then no handles will be inherited by the child process unless explicitly passed in the
handle_list
element ofSTARTUPINFO.lpAttributeList
, or by standard handle redirection.Changed in version 3.2: The default for close_fds was changed from
False
to what is described above.Changed in version 3.7: On Windows the default for close_fds was changed from
False
toTrue
when redirecting the standard handles. It’s now possible to set close_fds toTrue
when redirecting the standard handles.pass_fds is an optional sequence of file descriptors to keep open between the parent and child. Providing any pass_fds forces close_fds to be
True
. (POSIX only)Changed in version 3.2: The pass_fds parameter was added.
If cwd is not
None
, the function changes the working directory to cwd before executing the child. cwd can be a string, bytes or path-like object. 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.Changed in version 3.6: cwd parameter accepts a path-like object on POSIX.
Changed in version 3.7: cwd parameter accepts a path-like object on Windows.
Changed in version 3.8: cwd parameter accepts a bytes object on Windows.
If restore_signals is true (the default) all signals that Python has set to SIG_IGN are restored to SIG_DFL in the child process before the exec. Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. (POSIX only)
Changed in version 3.2: restore_signals was added.
If start_new_session is true the setsid() system call will be made in the child process prior to the execution of the subprocess. (POSIX only)
Changed in version 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
SystemRoot
.If encoding or errors are specified, or text is true, the file objects stdin, stdout and stderr are opened in text mode with the specified encoding and errors, as described above in Frequently Used Arguments. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.
New in version 3.6: encoding and errors were added.
New in version 3.7: text was added as a more readable alias for universal_newlines.
If given, startupinfo will be a
STARTUPINFO
object, which is passed to the underlyingCreateProcess
function. creationflags, if given, can be one or more of the following flags:Popen objects are supported as context managers via the
with
statement: on exit, standard file descriptors are closed, and the process is waited for.with Popen(["ifconfig"], stdout=PIPE) as proc: log.write(proc.stdout.read())
Popen and the other functions in this module that use it raise an auditing event
subprocess.Popen
with argumentsexecutable
,args
,cwd
, andenv
. The value forargs
may be a single string or a list of strings, depending on platform.Changed in version 3.2: Added context manager support.
Changed in version 3.6: Popen destructor now emits a
ResourceWarning
warning if the child process is still running.Changed in version 3.8: Popen can use
os.posix_spawn()
in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, Popen constructor usingos.posix_spawn()
no longer raise an exception on errors like missing program, but the child process fails with a non-zeroreturncode
.
Exceptions¶
Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent.
The most common exception raised is OSError
. This occurs, for example,
when trying to execute a non-existent file. Applications should prepare for
OSError
exceptions.
A ValueError
will be raised if Popen
is called with invalid
arguments.
check_call()
and check_output()
will raise
CalledProcessError
if the called process returns a non-zero return
code.
All of the functions and methods that accept a timeout parameter, such as
call()
and Popen.communicate()
will raise TimeoutExpired
if
the timeout expires before the process exits.
Exceptions defined in this module all inherit from SubprocessError
.
New in version 3.3: The
SubprocessError
base class was added.
Security Considerations¶
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
vulnerabilities.
When using shell=True
, the 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.
Popen Objects¶
Instances of the Popen
class have the following methods:
-
Popen.
poll
()¶ Check if child process has terminated. Set and return
returncode
attribute. Otherwise, returnsNone
.
-
Popen.
wait
(timeout=None)¶ Wait for child process to terminate. Set and return
returncode
attribute.If the process does not terminate after timeout seconds, raise a
TimeoutExpired
exception. It is safe to catch this exception and retry the wait.Note
This will deadlock when using
stdout=PIPE
orstderr=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. UsePopen.communicate()
when using pipes to avoid that.Note
The function is implemented using a busy loop (non-blocking call and short sleeps). Use the
asyncio
module for an asynchronous wait: seeasyncio.create_subprocess_exec
.Changed in version 3.3: timeout was added.
-
Popen.
communicate
(input=None, timeout=None)¶ Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate and set the
returncode
attribute. The optional input argument should be data to be sent to the child process, orNone
, if no data should be sent to the child. If streams were opened in text mode, input must be a string. Otherwise, it must be bytes.communicate()
returns a tuple(stdout_data, stderr_data)
. The data will be strings if streams were opened in text mode; otherwise, bytes.Note that if you want to send data to the process’s stdin, you need to create the Popen object with
stdin=PIPE
. Similarly, to get anything other thanNone
in the result tuple, you need to givestdout=PIPE
and/orstderr=PIPE
too.If the process does not terminate after timeout seconds, a
TimeoutExpired
exception will be raised. Catching this exception and retrying communication will not lose any output.The child process is not killed if the timeout expires, so in order to cleanup properly a well-behaved application should kill the child process and finish communication:
proc = subprocess.Popen(...) try: outs, errs = proc.communicate(timeout=15) except TimeoutExpired: proc.kill() outs, errs = proc.communicate()
Note
The data read is buffered in memory, so do not use this method if the data size is large or unlimited.
Changed in version 3.3: timeout was added.
-
Popen.
send_signal
(signal)¶ Sends the signal signal to the child.
Note
On Windows, SIGTERM is an alias for
terminate()
. CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP.
-
Popen.
terminate
()¶ Stop the child. On POSIX OSs the method sends SIGTERM to the child. On Windows the Win32 API function
TerminateProcess()
is called to stop the child.
-
Popen.
kill
()¶ Kills the child. On POSIX OSs the function sends SIGKILL to the child. On Windows
kill()
is an alias forterminate()
.
The following attributes are also available:
-
Popen.
args
¶ The args argument as it was passed to
Popen
– a sequence of program arguments or else a single string.New in version 3.3.
-
Popen.
stdin
¶ If the stdin argument was
PIPE
, this attribute is a writeable stream object as returned byopen()
. If the encoding or errors arguments were specified or the universal_newlines argument wasTrue
, the stream is a text stream, otherwise it is a byte stream. If the stdin argument was notPIPE
, this attribute isNone
.
-
Popen.
stdout
¶ If the stdout argument was
PIPE
, this attribute is a readable stream object as returned byopen()
. Reading from the stream provides output from the child process. If the encoding or errors arguments were specified or the universal_newlines argument wasTrue
, the stream is a text stream, otherwise it is a byte stream. If the stdout argument was notPIPE
, this attribute isNone
.
-
Popen.
stderr
¶ If the stderr argument was
PIPE
, this attribute is a readable stream object as returned byopen()
. Reading from the stream provides error output from the child process. If the encoding or errors arguments were specified or the universal_newlines argument wasTrue
, the stream is a text stream, otherwise it is a byte stream. If the stderr argument was notPIPE
, this attribute isNone
.
Warning
Use communicate()
rather than .stdin.write
,
.stdout.read
or .stderr.read
to avoid
deadlocks due to any of the other OS pipe buffers filling up and blocking the
child process.
-
Popen.
pid
¶ The process ID of the child process.
Note that if you set the shell argument to
True
, this is the process ID of the spawned shell.
-
Popen.
returncode
¶ The child return code, set by
poll()
andwait()
(and indirectly bycommunicate()
). ANone
value indicates that the process hasn’t terminated yet.A negative value
-N
indicates that the child was terminated by signalN
(POSIX only).
Windows Popen Helpers¶
The STARTUPINFO
class and following constants are only available
on Windows.
-
class
subprocess.
STARTUPINFO
(*, dwFlags=0, hStdInput=None, hStdOutput=None, hStdError=None, wShowWindow=0, lpAttributeList=None)¶ Partial support of the Windows STARTUPINFO structure is used for
Popen
creation. The following attributes can be set by passing them as keyword-only arguments.Changed in version 3.7: Keyword-only argument support was added.
-
dwFlags
¶ A bit field that determines whether certain
STARTUPINFO
attributes are used when the process creates a window.si = subprocess.STARTUPINFO() si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW
-
hStdInput
¶ If
dwFlags
specifiesSTARTF_USESTDHANDLES
, this attribute is the standard input handle for the process. IfSTARTF_USESTDHANDLES
is not specified, the default for standard input is the keyboard buffer.
-
hStdOutput
¶ If
dwFlags
specifiesSTARTF_USESTDHANDLES
, this attribute is the standard output handle for the process. Otherwise, this attribute is ignored and the default for standard output is the console window’s buffer.
-
hStdError
¶ If
dwFlags
specifiesSTARTF_USESTDHANDLES
, this attribute is the standard error handle for the process. Otherwise, this attribute is ignored and the default for standard error is the console window’s buffer.
-
wShowWindow
¶ If
dwFlags
specifiesSTARTF_USESHOWWINDOW
, this attribute can be any of the values that can be specified in thenCmdShow
parameter for the ShowWindow function, except forSW_SHOWDEFAULT
. Otherwise, this attribute is ignored.SW_HIDE
is provided for this attribute. It is used whenPopen
is called withshell=True
.
-
lpAttributeList
¶ A dictionary of additional attributes for process creation as given in
STARTUPINFOEX
, see UpdateProcThreadAttribute.Supported attributes:
- handle_list
Sequence of handles that will be inherited. close_fds must be true if non-empty.
The handles must be temporarily made inheritable by
os.set_handle_inheritable()
when passed to thePopen
constructor, elseOSError
will be raised with Windows errorERROR_INVALID_PARAMETER
(87).Warning
In a multithreaded process, use caution to avoid leaking handles that are marked inheritable when combining this feature with concurrent calls to other process creation functions that inherit all handles such as
os.system()
. This also applies to standard handle redirection, which temporarily creates inheritable handles.
New in version 3.7.
-
Windows Constants¶
The subprocess
module exposes the following constants.
-
subprocess.
STD_INPUT_HANDLE
¶ The standard input device. Initially, this is the console input buffer,
CONIN$
.
-
subprocess.
STD_OUTPUT_HANDLE
¶ The standard output device. Initially, this is the active console screen buffer,
CONOUT$
.
-
subprocess.
STD_ERROR_HANDLE
¶ The standard error device. Initially, this is the active console screen buffer,
CONOUT$
.
-
subprocess.
SW_HIDE
¶ Hides the window. Another window will be activated.
-
subprocess.
STARTF_USESTDHANDLES
¶ Specifies that the
STARTUPINFO.hStdInput
,STARTUPINFO.hStdOutput
, andSTARTUPINFO.hStdError
attributes contain additional information.
-
subprocess.
STARTF_USESHOWWINDOW
¶ Specifies that the
STARTUPINFO.wShowWindow
attribute contains additional information.
-
subprocess.
CREATE_NEW_CONSOLE
¶ The new process has a new console, instead of inheriting its parent’s console (the default).
-
subprocess.
CREATE_NEW_PROCESS_GROUP
¶ A
Popen
creationflags
parameter to specify that a new process group will be created. This flag is necessary for usingos.kill()
on the subprocess.This flag is ignored if
CREATE_NEW_CONSOLE
is specified.
-
subprocess.
ABOVE_NORMAL_PRIORITY_CLASS
¶ A
Popen
creationflags
parameter to specify that a new process will have an above average priority.New in version 3.7.
-
subprocess.
BELOW_NORMAL_PRIORITY_CLASS
¶ A
Popen
creationflags
parameter to specify that a new process will have a below average priority.New in version 3.7.
-
subprocess.
HIGH_PRIORITY_CLASS
¶ A
Popen
creationflags
parameter to specify that a new process will have a high priority.New in version 3.7.
-
subprocess.
IDLE_PRIORITY_CLASS
¶ A
Popen
creationflags
parameter to specify that a new process will have an idle (lowest) priority.New in version 3.7.
-
subprocess.
NORMAL_PRIORITY_CLASS
¶ A
Popen
creationflags
parameter to specify that a new process will have an normal priority. (default)New in version 3.7.
-
subprocess.
REALTIME_PRIORITY_CLASS
¶ A
Popen
creationflags
parameter to specify that a new process will have realtime priority. You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts system threads that manage mouse input, keyboard input, and background disk flushing. This class can be appropriate for applications that “talk” directly to hardware or that perform brief tasks that should have limited interruptions.New in version 3.7.
-
subprocess.
CREATE_NO_WINDOW
¶ A
Popen
creationflags
parameter to specify that a new process will not create a window.New in version 3.7.
-
subprocess.
DETACHED_PROCESS
¶ A
Popen
creationflags
parameter to specify that a new process will not inherit its parent’s console. This value cannot be used with CREATE_NEW_CONSOLE.New in version 3.7.
-
subprocess.
CREATE_DEFAULT_ERROR_MODE
¶ A
Popen
creationflags
parameter to specify that a new process does not inherit the error mode of the calling process. Instead, the new process gets the default error mode. This feature is particularly useful for multithreaded shell applications that run with hard errors disabled.New in version 3.7.
Older high-level API¶
Prior to Python 3.5, these three functions comprised the high level API to
subprocess. You can now use run()
in many cases, but lots of existing code
calls these functions.
-
subprocess.
call
(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, **other_popen_kwargs)¶ Run the command described by args. Wait for command to complete, then return the
returncode
attribute.Code needing to capture stdout or stderr should use
run()
instead:run(...).returncode
To suppress stdout or stderr, supply a value of
DEVNULL
.The arguments shown above are merely some common ones. The full function signature is the same as that of the
Popen
constructor - this function passes all supplied arguments other than timeout directly through to that interface.Note
Do not use
stdout=PIPE
orstderr=PIPE
with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.Changed in version 3.3: timeout was added.
-
subprocess.
check_call
(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, **other_popen_kwargs)¶ Run command with arguments. Wait for command to complete. If the return code was zero then return, otherwise raise
CalledProcessError
. TheCalledProcessError
object will have the return code in thereturncode
attribute.Code needing to capture stdout or stderr should use
run()
instead:run(..., check=True)
To suppress stdout or stderr, supply a value of
DEVNULL
.The arguments shown above are merely some common ones. The full function signature is the same as that of the
Popen
constructor - this function passes all supplied arguments other than timeout directly through to that interface.Note
Do not use
stdout=PIPE
orstderr=PIPE
with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.Changed in version 3.3: timeout was added.
-
subprocess.
check_output
(args, *, stdin=None, stderr=None, shell=False, cwd=None, encoding=None, errors=None, universal_newlines=None, timeout=None, text=None, **other_popen_kwargs)¶ Run command with arguments and return its output.
If the return code was non-zero it raises a
CalledProcessError
. TheCalledProcessError
object will have the return code in thereturncode
attribute and any output in theoutput
attribute.This is equivalent to:
run(..., check=True, stdout=PIPE).stdout
The arguments shown above are merely some common ones. The full function signature is largely the same as that of
run()
- most arguments are passed directly through to that interface. One API deviation fromrun()
behavior exists: passinginput=None
will behave the same asinput=b''
(orinput=''
, depending on other arguments) rather than using the parent’s standard input file handle.By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level.
This behaviour may be overridden by setting text, encoding, errors, or universal_newlines to
True
as described in Frequently Used Arguments andrun()
.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'
New in version 3.1.
Changed in version 3.3: timeout was added.
Changed in version 3.4: Support for the input keyword argument was added.
Changed in version 3.6: encoding and errors were added. See
run()
for details.New in version 3.7: text was added as a more readable alias for universal_newlines.
Replacing Older Functions with the subprocess
Module¶
In this section, “a becomes b” means that b can be used as a replacement for a.
Note
All “a” functions in this section fail (more or less) silently if the
executed program cannot be found; the “b” replacements raise OSError
instead.
In addition, the replacements using check_output()
will fail with a
CalledProcessError
if the requested operation produces a non-zero
return code. The output is still available as the
output
attribute of the raised exception.
In the following examples, we assume that the relevant functions have already
been imported from the subprocess
module.
Replacing /bin/sh shell command substitution¶
output=$(mycmd myarg)
becomes:
output = check_output(["mycmd", "myarg"])
Replacing shell pipeline¶
output=$(dmesg | grep hda)
becomes:
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]
The p1.stdout.close()
call after starting the p2 is important in order for
p1 to receive a SIGPIPE if p2 exits before p1.
Alternatively, for trusted input, the shell’s own pipeline support may still be used directly:
output=$(dmesg | grep hda)
becomes:
output=check_output("dmesg | grep hda", shell=True)
Replacing os.system()
¶
sts = os.system("mycmd" + " myarg")
# becomes
sts = call("mycmd" + " myarg", shell=True)
Notes:
Calling the program through the shell is usually not required.
A more realistic example would look like this:
try:
retcode = call("mycmd" + " myarg", shell=True)
if retcode < 0:
print("Child was terminated by signal", -retcode, file=sys.stderr)
else:
print("Child returned", retcode, file=sys.stderr)
except OSError as e:
print("Execution failed:", e, file=sys.stderr)
Replacing the os.spawn
family¶
P_NOWAIT example:
pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid
P_WAIT example:
retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])
Vector example:
os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])
Environment example:
os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
Replacing os.popen()
, os.popen2()
, os.popen3()
¶
(child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdin, child_stdout) = (p.stdin, p.stdout)
(child_stdin,
child_stdout,
child_stderr) = os.popen3(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
(child_stdin,
child_stdout,
child_stderr) = (p.stdin, p.stdout, p.stderr)
(child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
Return code handling translates as follows:
pipe = os.popen(cmd, 'w')
...
rc = pipe.close()
if rc is not None and rc >> 8:
print("There were some errors")
==>
process = Popen(cmd, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
print("There were some errors")
Replacing functions from the popen2
module¶
Note
If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed.
(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen("somestring", shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)
(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)
popen2.Popen3
and popen2.Popen4
basically work as
subprocess.Popen
, except that:
Popen
raises an exception if the execution fails.The capturestderr argument is replaced with the stderr argument.
stdin=PIPE
andstdout=PIPE
must be specified.popen2 closes all file descriptors by default, but you have to specify
close_fds=True
withPopen
to guarantee this behavior on all platforms or past Python versions.
Legacy Shell Invocation Functions¶
This module also provides the following legacy functions from the 2.x
commands
module. These operations implicitly invoke the system shell and
none of the guarantees described above regarding security and exception
handling consistency are valid for these functions.
-
subprocess.
getstatusoutput
(cmd)¶ Return
(exitcode, output)
of executing cmd in a shell.Execute the string cmd in a shell with
Popen.check_output()
and return a 2-tuple(exitcode, output)
. The locale encoding is used; see the notes on Frequently Used Arguments for more details.A trailing newline is stripped from the output. The exit code for the command can be interpreted as the return code of subprocess. Example:
>>> subprocess.getstatusoutput('ls /bin/ls') (0, '/bin/ls') >>> subprocess.getstatusoutput('cat /bin/junk') (1, 'cat: /bin/junk: No such file or directory') >>> subprocess.getstatusoutput('/bin/junk') (127, 'sh: /bin/junk: not found') >>> subprocess.getstatusoutput('/bin/kill $$') (-15, '')
Availability: POSIX & Windows.
Changed in version 3.3.4: Windows support was added.
The function now returns (exitcode, output) instead of (status, output) as it did in Python 3.3.3 and earlier. exitcode has the same value as
returncode
.
-
subprocess.
getoutput
(cmd)¶ Return output (stdout and stderr) of executing cmd in a shell.
Like
getstatusoutput()
, except the exit code is ignored and the return value is a string containing the command’s output. Example:>>> subprocess.getoutput('ls /bin/ls') '/bin/ls'
Availability: POSIX & Windows.
Changed in version 3.3.4: Windows support added
Notes¶
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.
See also
shlex
Module which provides function to parse and escape command lines.