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first cut at pep 397: Python launcher for Windows

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PEP: 397
Title: Python launcher for Windows
Version: $Revision$
Last-Modified: $Date$
Author: Mark Hammond <mhammond@skippinet.com.au>
Status: Draft
Type: Standards Track or Informational ?????
Content-Type: text/x-rst
Created: 15-Mar-2011
Abstract
This PEP describes a Python launcher for the Windows platform. A
Python launcher is a single executable which uses a number of
heuristics to locate a Python executable and launch it with a
specified command line.
Rationale
Windows provides "file associations" so an executable can be associated
with an extension, allowing for scripts to be executed directly in some
contexts (eg., double-clicking the file in Windows Explorer.) Until now,
a strategy of "last installed Python wins" has been used and while not
ideal, has generally been workable due to the conservative changes in
Python 2.x releases. As Python 3.x scripts are often syntactically
incompatible with Python 2.x scripts, a different strategy must be used
to allow files with a '.py' extension to use a different executable based
on the Python version the script targets. This will be done by borrowing
the existing practices of another operating system - scripts will be able
to nominate the version of Python they need by way of a "shebang" line, as
described below.
Unix-like operating systems (referred to simply as "Unix" in this
PEP) allow scripts to be executed as if they were executable images
by examining the script for a "shebang" line which specifies the
actual executable to be used to run the script. This is described in
detail in the evecve(2) man page [1] and while user documentation will
be created for this feature, for the purposes of this PEP that man
page describes a valid shebang line.
Additionally, these operating systems provide symbolic-links to
Python executables in well-known directories. For example, many
systems will have a link /usr/bin/python which references a
particular version of Python installed under the operating-system.
These symbolic links allow Python to be executed without regard for
where Python it actually installed on the machine (eg., without
requiring the path where Python is actually installed to be
referenced in the shebang line or in the PATH.) PEP 394 'The "python"
command on Unix-Like Systems' [2] describes additional conventions
for more fine-grained specification of a particular Python version.
These 2 facilities combined allow for a portable and somewhat
predictable way of both starting Python interactively and for allowing
Python scripts to execute. This PEP describes an implementation of a
launcher which can offer the same benefits for Python on the Windows
platform and therefore allows the launcher to be the executable
associated with '.py' files to support multiple Python versions
concurrently.
While this PEP offers the ability to use a shebang line which should
work on both Windows and Unix, this is not the primary motivation for
this PEP - the primary motivation is to allow a specific version to be
specified without inventing new syntax or conventions to describe it.
An overview of the launcher.
This PEP outlines the general functionality and key guidelines of a
launcher for Windows. It is accompanied by reference implementation,
written in Python, which defines the detailed implementation. Over
time, changes to the implementation may be desired - if the changes
adhere to the guidelines in this PEP and have been made following
the standard Python development model this PEP need not change.
In other words, this PEP makes no attempt to describe in detail every
feature offered by the launcher but instead to offer guidelines the
launcher should adhere to.
The actual implementation of the launcher will be written in C and
will not link directly or indirectly with Python, but the Python
based reference implementation should always remain the canonical
description of the functionality. Any changes proposed to the
launcher functionality should first be made and agreed to in the
Python implementation, after which time the C implementation can be
changed to reflect the new reference. Any deviations between the
functionality of the reference implementation versus the C
implementation due should imply the C implementation is in error
(although some exceptions will need to be made due to the nature of the
reference implementation being in Python)
It is expected that 2 versions of the launcher will be needed - one
which is a console program and one which is a "windows" (ie., GUI)
program. These 2 launchers correspond to the 'python.exe' and
'pythonw.exe' executables which currently ship with Python. The
console launcher will be named 'py.exe' and the Windows one named
'pyw.exe'. The "windows" (ie., GUI) version of the launcher will attempt
to locate and launch pythonw.exe even if a virtual shebang line nominates
simply "python" - infact, the trailing 'w' notation will not be supported
in the virtual shebang line at all.
The launcher will be distributed with all future versions of Python
and if possible, should be installed somewhere likely to already be
on the system PATH (eg., the Windows System32) directory. If installed,
the "console" version of the launcher should be associated with .py files
and the "windows" version associated with .pyw files.
The launcher will not be tied to a specific version of Python - eg., a
launcher distributed with Python 3.3 should be capable of locating and
executing any Python 2.x and Python 3.x version. Future versions of the
launcher should remain backwards compatible with older versions, so later
versions of Python can install an updated version of the launcher without
impacting how the previously installed version of the launcher is used.
The launcher will offer some conveniences for Python developers working
interactively - for example, starting the launcher with no command-line
arguments will launch the default Python with no command-line arguments.
Further, command-line arguments will be supported to allow a specific
Python version to be launched interactively - however, these conveniences
will not detract from the primary purpose of launching scripts and will
be easy to avoid if desired.
Guidelines for a Python launcher.
The Python launcher described in this PEP will intentionally be
constrained to the use-cases described in the Rationale section
above. It will not attempt to be a general purpose script launcher
or shebang processor.
The launcher should support for format of shebang lines as described
in [1], including all restrictions listed.
The launcher should support shebang lines commonly found on Unix.
For example, a shebang line of '!# /usr/bin/python' should work even
though there is unlikely to be an executable in the relative Windows
directory "\usr\bin". This means that many scripts can use a single
shebang line and be likely to work on both Unix and Windows without
modification.
The launcher will support fully-qualified paths to executables.
While this will make the script inherently non-portable, it is a
feature offered by Unix and would be useful for Windows users in
some cases.
The launcher should be capable of supporting implementations other
than CPython, such as jython and IronPython. In particular, if there
are common symbolic links used on Unix to specify such an
implementation (such as "/usr/bin/jython"), the launcher need not
support such references on Windows. However, if there is strong desire
from the community to support these other implementations in a Windows
specific way, any such proposals will be judged on their merit. Even
without specialized support for these alternate implementations, the
ability to specify the fully-qualified path to an executable could be
used as a fallback.
On Unix, the user can control which specific version of Python is used
by adjusting the links in /usr/bin to point to the desired version. As
the launcher on Windows will not use Windows links, environment variables
will be used to override the semantics for determining exactly what
version of Python will be used. For example, while a shebang line of
"/usr/bin/python2" will automatically locate a Python 2.x implementation,
an environment variable can override exactly which Python 2.x
implementation will be chosen. Similarly for "/usr/bin/python" and
"/usr/bin/python3".
While the guidelines above are general, the launcher should make
concessions for any Windows specific issues which increase the
usefulness of the launcher on Windows. In other words, the
guidelines above will not prevent Windows specific functionality
which makes the launcher significantly more useful for Windows
users.
Shebang line parsing
If the first command-line argument does not start with a dash ('-')
character, an attempt will be made to open that argument as a file
and parsed for a shebang line according to the rules in [1]. Once
parsed, the command will be examined to see if it is a "virtual
command". A virtual command starts with either of the strings
'/usr/bin/python', '/usr/bin/env python' or 'python' - while the
latter will not work on Unix, it will be offered as a convenience
for Windows users not familiar with the Unix conventions and with no
desire to have the shebang line work on Unix.
Commands which are virtual are not treated as containing a file-system
path, but instead are treated as references to an installed Python.
Optionally, the virtual command may include a specific version, such
as '/usr/bin/python2' or '/usr/bin/python3.2'. If only a "major version"
qualifier is found, the launcher will enumerate the installed Python
versions and use the latest minor release found for the major version,
which is likely, although not guaranteed, to be the most recently
installed version in that family. If major and minor version qualifiers
are found, that specific version will be located and used.
If a virtual command has no version specifier, or if no shebang line
is found, the launcher will attempt to locate a Python 2.x
installation and use the latest found. If none are found, it will
attempt to use the latest Python 3.x implementation found.
The use of 'virtual' shebang lines will be encouraged as this should
allow for portable shebang lines to be specified which work on
multiple operating systems.
If the shebang line is not a virtual one as described above, it is
assumed to be a path to an executable - presumably a Python
executable, but this will not be checked. No version qualifiers are
parsed - if a specific version is required, then the path should
reflect that. Relative path specifications will be considered
relative to the directory of the launcher. Thus, a shebang line of
'#! python' will look for a python executable in the same directory
as the launcher.
Non-virtual shebang lines should be discouraged as they make the
script specific to a specific Windows installation. However, they
are supported for maximum flexibility.
If the first argument can not be opened as a file or if no valid
shebang line can be found, a default Python interpreter will be
located and the arguments passed to that. However, if a valid
shebang line is found but the process specified by that line can not
be started, the default interpreter will not be started - the error
to create the specified child process will be considered fatal and
the launcher will display an appropriate message and terminate with
a specific exit code.
On 64bit Windows with both 32bit and 64bit implementations of the
same (major.minor) Python version installed, the 64bit version will
always be preferred. This will be true for both 32bit and 64bit
implementations of the launcher - a 32bit launcher will prefer to
execute a 64bit Python installation of the same version if
available. This is so the behavior of the launcher can be predicted
knowing only what versions are installed on the PC and without
regard to the order in which they were installed.
Command-line handling
Only the first command-line argument will be checked for a shebang line
and only if that argument does not start with a '-'.
If the only command-line argument is "-h" or "--help", the launcher will
print a small banner and command-line usage, then pass the argument to
the default Python. This will have the effect of help for the launcher
being printed followed by help for Python itself. The output from the
launcher will attempt to clearly indicate the extended help information
is coming from the launcher and not Python.
As a concession to interactively launching Python, the launcher will
support the first command-line argument optionally being a version
specifier in the form "-n[.n][:bits]" (where n is a single integer and
bits is either '32' or '64') to nominate a specific version be used. For
example, while "py.exe" may locate and launch the latest Python 2.x
implementation installed, a command-line such as "py.exe -3" could specify
the latest Python 3.x implementation be launched, while "py.exe -2.6:32"
could specify a 32bit implementation Python 2.6 be located and launched.
If a Python 2.x implementation is desired to be launched with the -3 flag,
the command-line would need to be similar to "py.exe -2 -3" (or the
specific version of Python could obviously be launched manually without
use of this launcher.) Note that this feature can not be used with shebang
processing as the file scanned for a shebang line and this argument must
both be the first argument and therefore are mutually exclusive.
All other arguments will be passed untouched to the child Python process.
Process Launching
Ideally, the launcher process would execute Python directly inside
the same process, primarily so the parent of the launcher process could
terminate the launcher and have the Python interpreter terminate. If the
launcher executes Python as a sub-process and the parent of the launcher
terminates the launcher, the Python process will be unaffected.
However, there are a number of practical problems associated with this
approach. Windows does not support the execv* family of Unix functions,
so this could only be done by the launcher dynamically loading the Python
DLL, but this would have a number of side-effects. The most serious
side effect of this is that the value of sys.executable would refer to the
launcher instead of the Python implementation. Many Python scripts use the
value of sys.executable to launch child processes, and these scripts may
fail to work as expected if the launcher is used - consider a "parent"
script with a shebang line of '#! /usr/bin/python3' which attempts to
launch a child script with no shebang line at all. Currently that
child script would be launched using the exact same version running
the parent script, but if sys.executable referred to the launcher the
child would be likely executed using a Python 2.x version and would be
likely to fail with a SyntaxError. A solution for this would need to be
found before this could be considered.
The rules mentioned above regarding 64bit and 32bit programs would also be
rendered impossible with this 'in-process' model - a 32bit launcher would
be unable to load the 64bit version of Python and vice-versa.
Given the impossibility of supporting the 64bit and 32bit requirements
above, the launcher will execute Python in a child process, remaining
alive while the child process is executing, then terminate with the same
exit code as returned by the child.
To address the concerns regarding the termination of the launcher not
killing the child, the Win32 Job API will be used to arrange so that the
child process is automatically killed when the parent is terminated
(although children of that child process will continue as is the case
now.) However, that functionality only works on Windows XP and later, so
the launcher running on the Windows 2000 platform will not have this
feature - killing the launcher on that platform will keep the child
process alive. This feature has been implemented in the reference
implementation.
Alternative implementations and distributions of Python.
While this PEP is primarily concerned with the python.org distributions of
CPython, it does not preclude future enhancements to support both
different distributions of CPython (eg. ActivePython, Enthought), nor
different implementations of Python (eg. jython, IronPython.)
Enhancements to support these alternatives should be proposed by
(community or corporate) representatives of these alternatives and
addressed via the normal Python development process and after relevant
patches have been submitted for the reference implementation.
References
[1] http://linux.die.net/man/2/execve
[2] http://www.python.org/dev/peps/pep-0394/
Copyright
This document has been placed in the public domain.
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