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update from Peter Astrand

This commit is contained in:
David Goodger 2004-08-03 13:13:43 +00:00
parent a48fe13ca8
commit 9d455a5e43
2 changed files with 157 additions and 89 deletions
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4
pep-0000.txt
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@ -119,7 +119,7 @@ Index by Category
S 319 Python Synchronize/Asynchronize Block Pelletier
S 321 Date/Time Parsing and Formatting Kuchling
S 323 Copyable Iterators Martelli
S 324 popen5 - New POSIX process module Astrand
S 324 process - New POSIX process module Astrand
S 325 Resource-Release Support for Generators Pedroni
S 330 Python Bytecode Verification Pelletier
S 331 Locale-Independent Float/String conversions Reis
@ -347,7 +347,7 @@ Numerical Index
S 321 Date/Time Parsing and Formatting Kuchling
SF 322 Reverse Iteration Hettinger
S 323 Copyable Iterators Martelli
S 324 popen5 - New POSIX process module Astrand
S 324 process - New POSIX process module Astrand
S 325 Resource-Release Support for Generators Pedroni
SR 326 A Case for Top and Bottom Values Carlson, Reedy
SF 327 Decimal Data Type Batista

198
pep-0324.txt
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@ -1,5 +1,5 @@
PEP: 324
Title: popen5 - New POSIX process module
Title: process - New POSIX process module
Version: $Revision$
Last-Modified: $Date$
Author: Peter Astrand <astrand@lysator.liu.se>
@ -33,7 +33,7 @@ Motivation
Currently, Python has a large number of different functions for
process creation. This makes it hard for developers to choose.
The popen5 modules provides the following enhancements over
The process module provides the following enhancements over
previous functions:
- One "unified" module provides all functionality from previous
@ -56,8 +56,8 @@ Motivation
and redirect stderr, but not stdout. This is not possible with
current functions, without using temporary files.
- With popen5, it's possible to control if all open file
descriptors should be closed before the new program is
- With the process module, it's possible to control if all open
file descriptors should be closed before the new program is
executed.
- Support for connecting several subprocesses (shell "pipe").
@ -78,23 +78,23 @@ Rationale
The following points summarizes the design:
- popen5 was based on popen2, which is tried-and-tested.
- process was based on popen2, which is tried-and-tested.
- The factory functions in popen2 have been removed, because I
consider the class constructor equally easy to work with.
- popen2 contains several factory functions and classes for
different combinations of redirection. popen5, however,
contains one single class. Since popen5 supports 12 different
combinations of redirection, providing a class or function for
each of them would be cumbersome and not very intuitive. Even
with popen2, this is a readability problem. For example, many
people cannot tell the difference between popen2.popen2 and
popen2.popen4 without using the documentation.
different combinations of redirection. process, however,
contains one single class. Since the process module supports 12
different combinations of redirection, providing a class or
function for each of them would be cumbersome and not very
intuitive. Even with popen2, this is a readability problem.
For example, many people cannot tell the difference between
popen2.popen2 and popen2.popen4 without using the documentation.
- One small utility function is provided: popen5.run(). It aims
to be an enhancement over os.system(), while still very easy to
use:
- Two small utility functions are provided: process.call() and
process.callv(). These aims to be an enhancement over
os.system(), while still very easy to use:
- It does not use the Standard C function system(), which has
limitations.
@ -105,6 +105,43 @@ Rationale
- The return value is easier to work with.
The call() utility function accepts an 'args' argument, just
like the Popen class constructor. It waits for the command to
complete, then returns the returncode attribute. The
implementation is very simple:
def call(*args, **kwargs):
return Popen(*args, **kwargs).wait()
The motivation behind the call() function is simple: Starting a
process and wait for it to finish is a common task.
The callv() function is identical to call(), except that each
non-keyword argument is treated as a program argument. This
gives a slightly nicer syntax. The drawback is that callv() does
not allow specifying the program and it's arguments as a
whitespace-separated string: The entire (first) string would be
intepreted as the executable. The implementation of callv() is
also very simple:
def callv(*args, **kwargs):
return Popen(args, **kwargs).wait()
While Popen supports a wide range of options, many users have
simple needs. Many people are using os.system() today, mainly
because it provides a simple interface. Consider this example:
os.system("stty sane -F " + device)
With process.call(), this would look like:
process.call(["stty", "sane", "-F", device])
Some people feel that the list brackets are clumsy. With
callv(), they are not needed:
process.callv("stty", "sane", "-F", device)
- The "preexec" functionality makes it possible to run arbitrary
code between fork and exec. One might ask why there are special
arguments for setting the environment and current directory, but
@ -119,26 +156,38 @@ Rationale
- env and cwd are considered quite cross-platform: They make
sense even on Windows.
- No MS Windows support is available, currently. To be able to
provide more functionality than what is already available from
the popen2 module, help from C modules is required.
Specification
This module defines one class called Popen:
class Popen(args, bufsize=0, argv0=None,
class Popen(args, bufsize=0, executable=None,
stdin=None, stdout=None, stderr=None,
preexec_fn=None, preexec_args=(), close_fds=0,
cwd=None, env=None, universal_newlines=0)
preexec_fn=None, close_fds=False,
cwd=None, env=None, universal_newlines=False,
startupinfo=None, creationflags=0):
Arguments are:
- args should be a sequence of program arguments. The program to
execute is normally the first item in the args sequence, but can
be explicitly set by using the argv0 argument. The Popen class
uses os.execvp() to execute the child program.
be explicitly set by using the executable argument.
- On UNIX: the Popen class uses os.execvp() to execute the child
program, which operates on sequences. If args is a string, it
will be converted to a sequence using the cmdline2list method.
Please note that syntax for quoting arguments is different from
a typical UNIX shell. See the documentation of the cmdline2list
method for more information.
- 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, if given, has the same meaning as the corresponding
argument to the built-in open() function: 0 means unbuffered, 1
@ -159,8 +208,7 @@ Specification
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,
with arguments preexec_args.
called in the child process just before the child is executed.
- If close_fds is true, all file descriptors except 0, 1 and 2
will be closed before the child process is executed.
@ -171,46 +219,67 @@ Specification
- If env is not None, it defines the environment variables for the
new process.
- If universal_newlines is true, the file objects fromchild and
childerr are opened as a text files, but lines may be terminated
- If universal_newlines is true, the file objects stdout and
stderr are opened as a text files, but lines may be terminated
by any of '\n', the Unix end-of-line convention, '\r', the
Macintosh convention or '\r\n', the Windows convention. All of
these external representations are seen as '\n' by the Python
program. Note: This feature is only available if Python is
built with universal newline support (the default). Also, the
newlines attribute of the file objects fromchild, tochild and
childerr are not updated by the communicate() method.
newlines attribute of the file objects stdout, stdin and stderr
are not updated by the communicate() method.
The module also defines one shortcut function:
- 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)
run(*args):
Run command with arguments. Wait for command to complete,
then return the returncode attribute. Example:
retcode = popen5.run("stty", "sane")
This module also defines two shortcut functions:
- call(*args, **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 = call(["ls", "-l"])
- callv(*args, **kwargs):
Run command with arguments. Wait for command to complete, then
return the returncode attribute.
This function is identical to call(), except that each non-keyword
argument is treated as a program argument. Example:
retcode = callv("ls", "-l")
This is equivalent to:
retcode = call(["ls", "-l"])
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.
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 childs 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 PopenException will also be raised if Popen is called with
invalid arguments.
A ValueError will be raised if Popen is called with invalid arguments.
Security
--------
popen5 will never call /bin/sh implicitly. This means that all
characters, including shell metacharacters, can safely be passed
to child processes.
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
@ -218,17 +287,11 @@ Specification
Instances of the Popen class have the following methods:
poll()
Returns -1 if child process hasn't completed yet, or its exit
status otherwise. See below for a description of how the exit
status is encoded.
Check if child process has terminated. Returns returncode
attribute.
wait()
Waits for and returns the exit status of the child process.
The exit status encodes both the return code of the process
and information about whether it exited using the exit()
system call or died due to a signal. Functions to help
interpret the status code are defined in the os module (the
W*() family of functions).
Wait for child process to terminate. Returns returncode attribute.
communicate(input=None)
Interact with process: Send data to stdin. Read data from
@ -244,30 +307,35 @@ Specification
The following attributes are also available:
fromchild
A file object that provides output from the child process.
stdin
If the stdin argument is PIPE, this attribute is a file object
that provides input to the child process. Otherwise, it is None.
tochild
A file object that provides input to the child process.
stdout
If the stdout argument is PIPE, this attribute is a file object
that provides output from the child process. Otherwise, it is
None.
childerr
A file object that provides error output from the child
process.
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 means that
the process was terminated by a signal with number
-returncode.
process hasn't terminated yet. A negative value -N indicates
that the child was terminated by signal N (UNIX only).
Open Issues
Perhaps the module should be called something like "process",
instead of "popen5".
Currently, the reference implementation requires the "win32all"
extensions when running on the Windows platform. This dependency
could probably be eliminated by providing a small "glue" module
written in C, just like the _winreg module.
Reference Implementation