Skip to content

C
cpython
Commit ca170d50 authored by Martin Panter's avatar Martin Panter
Browse files
Options

Issue #26240: Clean up the subprocess module doc string 

Patch by Tim Mitchell.
parent 29a505eb
Hide whitespace changes
Inline Side-by-side
Showing with 80 additions and 348 deletions
Lib/subprocess.py
View file @ ca170d50
......@@ -7,352 +7,38 @@
# Licensed to PSF under a Contributor Agreement.
# See http://www.python.org/2.4/license for licensing details.
r"""subprocess - Subprocesses with accessible I/O streams
r"""Subprocesses with accessible I/O streams
This module allows you to spawn 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 below.
Using the subprocess module
===========================
This module defines one class called Popen:
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=False,
startupinfo=None, creationflags=0,
restore_signals=True, start_new_session=False, pass_fds=()):
Arguments are:
args should be a string, or a sequence of program arguments. The
program to execute is normally the first item in the args sequence or
string, but can be explicitly set by using the executable argument.
On POSIX, with shell=False (default): In this case, the Popen class
uses os.execvp() to execute the child program. args should normally
be a sequence. A string will be treated as a sequence with the string
as the only item (the program to execute).
On POSIX, with shell=True: If args is a string, it specifies the
command string to execute through the shell. If args is a sequence,
the first item specifies the command string, and any additional items
will be treated as additional shell arguments.
On Windows: the Popen class uses CreateProcess() to execute the child
program, which operates on strings. If args is a sequence, it will be
converted to a string using the list2cmdline method. Please note that
not all MS Windows applications interpret the command line the same
way: The list2cmdline is designed for applications using the same
rules as the MS C runtime.
bufsize will be supplied as the corresponding argument to the io.open()
function when creating the stdin/stdout/stderr pipe file objects:
0 means unbuffered (read & write are one system call and can return short),
1 means line buffered, any other positive value means use a buffer of
approximately that size. A negative bufsize, the default, means the system
default of io.DEFAULT_BUFFER_SIZE will be used.
stdin, stdout and stderr specify the executed programs' standard
input, standard output and standard error file handles, respectively.
Valid values are PIPE, an existing file descriptor (a positive
integer), an existing file object, and None. PIPE indicates that a
new pipe to the child should be created. With None, no redirection
will occur; the child's file handles will be inherited from the
parent. Additionally, stderr can be STDOUT, which indicates that the
stderr data from the applications should be captured into the same
file handle as for stdout.
On POSIX, if preexec_fn is set to a callable object, this object will be
called in the child process just before the child is executed. The use
of preexec_fn is not thread safe, using it in the presence of threads
could lead to a deadlock in the child process before the new executable
is executed.
If close_fds is true, all file descriptors except 0, 1 and 2 will be
closed before the child process is executed. The default for close_fds
varies by platform: Always true on POSIX. True when stdin/stdout/stderr
are None on Windows, false otherwise.
pass_fds is an optional sequence of file descriptors to keep open between the
parent and child. Providing any pass_fds implicitly sets close_fds to true.
if shell is true, the specified command will be executed through the
shell.
If cwd is not None, the current directory will be changed to cwd
before the child is executed.
On POSIX, if restore_signals is True all signals that Python sets to
SIG_IGN are restored to SIG_DFL in the child process before the exec.
Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. This
parameter does nothing on Windows.
On POSIX, if start_new_session is True, the setsid() system call will be made
in the child process prior to executing the command.
If env is not None, it defines the environment variables for the new
process.
If universal_newlines is False, the file objects stdin, stdout and stderr
are opened as binary files, and no line ending conversion is done.
If universal_newlines is True, the file objects stdout and stderr are
opened as a text file, but lines may be terminated by any of '\n',
the Unix end-of-line convention, '\r', the old Macintosh convention or
'\r\n', the Windows convention. All of these external representations
are seen as '\n' by the Python program. Also, the newlines attribute
of the file objects stdout, stdin and stderr are not updated by the
communicate() method.
In either case, the process being communicated with should start up
expecting to receive bytes on its standard input and decode them with
the same encoding they are sent in.
The startupinfo and creationflags, if given, will be passed to the
underlying CreateProcess() function. They can specify things such as
appearance of the main window and priority for the new process.
(Windows only)
This module also defines some shortcut functions:
call(*popenargs, **kwargs):
Run command with arguments. Wait for command to complete, then
return the returncode attribute.
The arguments are the same as for the Popen constructor. Example:
>>> retcode = subprocess.call(["ls", "-l"])
check_call(*popenargs, **kwargs):
Run command with arguments. Wait for command to complete. If the
exit code was zero then return, otherwise raise
CalledProcessError. The CalledProcessError object will have the
return code in the returncode attribute.
The arguments are the same as for the Popen constructor. Example:
>>> subprocess.check_call(["ls", "-l"])
0
getstatusoutput(cmd):
Return (status, output) of executing cmd in a shell.
Execute the string 'cmd' in a shell with 'check_output' and
return a 2-tuple (status, output). Universal newlines mode is used,
meaning that the result with be decoded to a string.
A trailing newline is stripped from the output.
The exit status for the command can be interpreted
according to the rules for the function 'wait'. Example:
>>> subprocess.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
>>> subprocess.getstatusoutput('cat /bin/junk')
(256, 'cat: /bin/junk: No such file or directory')
>>> subprocess.getstatusoutput('/bin/junk')
(256, 'sh: /bin/junk: not found')
getoutput(cmd):
Return output (stdout or stderr) of executing cmd in a shell.
Like getstatusoutput(), except the exit status is ignored and the return
value is a string containing the command's output. Example:
>>> subprocess.getoutput('ls /bin/ls')
'/bin/ls'
check_output(*popenargs, **kwargs):
Run command with arguments and return its output.
If the exit code was non-zero it raises a CalledProcessError. The
CalledProcessError object will have the return code in the returncode
attribute and output in the output attribute.
The arguments are the same as for the Popen constructor. Example:
>>> output = subprocess.check_output(["ls", "-l", "/dev/null"])
There is an additional optional argument, "input", allowing you to
pass a string to the subprocess's stdin. If you use this argument
you may not also use the Popen constructor's "stdin" argument.
If universal_newlines is set to True, the "input" argument must
be a string rather than bytes, and the return value will be a string.
Exceptions
----------
Exceptions raised in the child process, before the new program has
started to execute, will be re-raised in the parent. Additionally,
the exception object will have one extra attribute called
'child_traceback', which is a string containing traceback information
from the child's point of view.
The most common exception raised is OSError. This occurs, for
example, when trying to execute a non-existent file. Applications
should prepare for OSErrors.
A ValueError will be raised if Popen is called with invalid arguments.
Exceptions defined within this module inherit from SubprocessError.
check_call() and check_output() will raise CalledProcessError if the
called process returns a non-zero return code. TimeoutExpired
be raised if a timeout was specified and expired.
Security
--------
Unlike some other popen functions, this implementation will never call
/bin/sh implicitly. This means that all characters, including shell
metacharacters, can safely be passed to child processes.
Popen objects
=============
Instances of the Popen class have the following methods:
poll()
Check if child process has terminated. Returns returncode
attribute.
wait()
Wait for child process to terminate. Returns returncode attribute.
communicate(input=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 the Popen instance was constructed with universal_newlines
set to True, the input argument should be a string and will be encoded
using the preferred system encoding (see locale.getpreferredencoding);
if universal_newlines is False, the input argument should be a
byte string.
communicate() returns a tuple (stdout, stderr).
Note: The data read is buffered in memory, so do not use this
method if the data size is large or unlimited.
The following attributes are also available:
stdin
If the stdin argument is PIPE, this attribute is a file object
that provides input to the child process. Otherwise, it is None.
stdout
If the stdout argument is PIPE, this attribute is a file object
that provides output from the child process. Otherwise, it is
None.
stderr
If the stderr argument is PIPE, this attribute is file object that
provides error output from the child process. Otherwise, it is
None.
pid
The process ID of the child process.
returncode
The child return code. 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).
Replacing older functions with the subprocess module
====================================================
In this section, "a ==> b" means that b can be used as a replacement
for a.
Note: All functions in this section fail (more or less) silently if
the executed program cannot be found; this module raises an OSError
exception.
In the following examples, we assume that the subprocess module is
imported with "from subprocess import *".
Replacing /bin/sh shell backquote
---------------------------------
output=`mycmd myarg`
==>
output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
Replacing shell pipe line
-------------------------
output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]
Replacing os.system()
---------------------
sts = os.system("mycmd" + " myarg")
==>
p = Popen("mycmd" + " myarg", shell=True)
pid, sts = os.waitpid(p.pid, 0)
Note:
* Calling the program through the shell is usually not required.
* It's easier to look at the returncode attribute than the
exitstatus.
A more real-world 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 os.spawn*
-------------------
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"})
input/output/error pipes, and obtain their return codes.
For a complete description of this module see the Python documentation.
Main API
========
run(...): Runs a command, waits for it to complete, then returns a
CompletedProcess instance.
Popen(...): A class for flexibly executing a command in a new process
Constants
---------
DEVNULL: Special value that indicates that os.devnull should be used
PIPE: Special value that indicates a pipe should be created
STDOUT: Special value that indicates that stderr should go to stdout
Older API
=========
call(...): Runs a command, waits for it to complete, then returns
the return code.
check_call(...): Same as call() but raises CalledProcessError()
if return code is not 0
check_output(...): Same as check_call() but returns the contents of
stdout instead of a return code
getoutput(...): Runs a command in the shell, waits for it to complete,
then returns the output
getstatusoutput(...): Runs a command in the shell, waits for it to complete,
then returns a (status, output) tuple
"""
import sys
......@@ -372,10 +58,11 @@ class SubprocessError(Exception): pass
class CalledProcessError(SubprocessError):
"""This exception is raised when a process run by check_call() or
check_output() returns a non-zero exit status.
The exit status will be stored in the returncode attribute;
check_output() will also store the output in the output attribute.
"""Raised when run() is called with check=True and the process
returns a non-zero exit status.
Attributes:
cmd, returncode, stdout, stderr, output
"""
def __init__(self, returncode, cmd, output=None, stderr=None):
self.returncode = returncode
......@@ -401,6 +88,9 @@ class CalledProcessError(SubprocessError):
class TimeoutExpired(SubprocessError):
"""This exception is raised when the timeout expires while waiting for a
child process.
Attributes:
cmd, output, stdout, stderr, timeout
"""
def __init__(self, cmd, timeout, output=None, stderr=None):
self.cmd = cmd
......@@ -828,7 +518,46 @@ _PLATFORM_DEFAULT_CLOSE_FDS = object()
class Popen(object):
""" Execute a child program in a new process.
For a complete description of the arguments see the Python documentation.
Arguments:
args: A string, or a sequence of program arguments.
bufsize: supplied as the buffering argument to the open() function when
creating the stdin/stdout/stderr pipe file objects
executable: A replacement program to execute.
stdin, stdout and stderr: These specify the executed programs' standard
input, standard output and standard error file handles, respectively.
preexec_fn: (POSIX only) An object to be called in the child process
just before the child is executed.
close_fds: Controls closing or inheriting of file descriptors.
shell: If true, the command will be executed through the shell.
cwd: Sets the current directory before the child is executed.
env: Defines the environment variables for the new process.
universal_newlines: If true, use universal line endings for file
objects stdin, stdout and stderr.
startupinfo and creationflags (Windows only)
restore_signals (POSIX only)
start_new_session (POSIX only)
pass_fds (POSIX only)
Attributes:
stdin, stdout, stderr, pid, returncode
"""
_child_created = False # Set here since __del__ checks it
def __init__(self, args, bufsize=-1, executable=None,
......@@ -1079,6 +808,8 @@ class Popen(object):
def poll(self):
"""Check if child process has terminated. Set and return returncode
attribute."""
return self._internal_poll()
......
Misc/ACKS
View file @ ca170d50
......@@ -999,6 +999,7 @@ Julien Miotte
Andrii V. Mishkovskyi
Dom Mitchell
Dustin J. Mitchell
Tim Mitchell
Zubin Mithra
Florian Mladitsch
Doug Moen
......
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment
GitLab Nexedi Edition | About GitLab | About Nexedi | 沪ICP备2021021310号-2 | 沪ICP备2021021310号-7