python-peps / pep-0397.txt

PEP: 397
Title: Python launcher for Windows
Version: $Revision$
Last-Modified: $Date$
Author: Mark Hammond <mhammond@skippinet.com.au>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 15-Mar-2011
Post-History: 21-July-2011, 17-May-2011, 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 an implementation [3],
    written in C, 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 launcher will come in 2 versions - 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 directly into the Windows directory
    (note that the System32 directory is not a good option as this directory
    is not on the default PATH for 32bit processes on a 64bit Windows.)  If
    the launcher can't be installed in the Windows directory, the installer
    can suggest or choose an alternative, but it will be the responsibility
    of the user to ensure this directory is on the PATH.
    
    Once 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 may 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
    must not detract from the primary purpose of launching scripts and must
    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 will be capable of supporting implementations other than
    CPython, such as jython and IronPython, but given both the absence of
    common links on Unix (such as "/usr/bin/jython") and the inability for the
    launcher to automatically locate the installation location of these
    implementations on Windows, the launcher will support this via
    customization options.  Scripts taking advantage of this will not be
    portable (as these customization options must be set to reflect the
    configuration of the machine on which the launcher is running) but this
    ability is nonetheless considered worthwhile.

    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, cutomization options
    (exposed via both environment variables and INI files) will be used to
    override the semantics for determining 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".  This is
    specified in detail later in this PEP.

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 categorized according to the following rules:

    * If the command starts with the definition of a customized command
      followed by a whitespace character (including a newline), the customized
      command will be used.  See below for a description of customized
      commands.

    * The launcher will define a set of strings which are considered Unix
      compatible commands to launch Python, such as '/usr/bin/python' etc.
      If a command matching one of these strings will be treated as a
      'virtual command' and the rules described in Python Version Qualifiers
      (below) will be used to locate the executable to use.

    * Otherwise the command is assumed to be directly ready to execute - ie.
      a fully-qualified path (or a reference to an executable on the PATH)
      optionally followed by arguments.  The contents of the string will not
      be parsed - it will be passed directly to the Windows CreateProcess
      function after appending the name of the script and the launcher
      command-line arguments.  This means that the rules used by
      CreateProcess will be used, including how relative path names and
      executable references without extensions are treated.  Notably, the
      Windows command processor will not be used, so special rules used by the
      command processor (such as automatic appending of extensions other than
      '.exe', support for batch files, etc) will not be used.

    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 and different installations of the same
    operating system.

    If the first argument can not be opened as a file or if no valid
    shebang line can be found, the launcher will act as if a shebang line of
    '#!python' was found - ie., 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 cause the launcher to display
    an appropriate message and terminate with a specific exit code.

Virtual commands in shebang lines:

    Virtual Commands are shebang lines which start with strings which would
    be expected to work on Unix platforms - examples include
    '/usr/bin/python', '/usr/bin/env python' and 'python'.  Optionally, the 
    virtual command may be suffixed with a version qualifier (see below),
    such as '/usr/bin/python2' or '/usr/bin/python3.2'.  The command executed
    is based on the rules described in Python Version Qualifiers below.

Customized Commands:

    The launcher will support the ability to define "Customized Commands" in a
    Windows .ini file (ie, a file which can be parsed by the Windows function
    GetPrivateProfileString).  A section called '[commands]' can be created 
    with key names defining the virtual command and the value specifying the
    actual command-line to be used for this virtual command.

    For example, if an INI file has the contents:

    [commands]
    vpython=c:\bin\vpython.exe -foo

    Then a shebang line of '#! vpython' in a script named 'doit.py' will 
    result in the launcher using the command-line 'c:\bin\vpython.exe -foo 
    doit.py'

    The precise details about the names, locations and search order of the
    .ini files is in the launcher documentation [4]

Python Version Qualifiers

    Some of the features described allow an optional Python version qualifier 
    to be used.

    A version qualifier starts with a major version number and can optionally
    be followed by a period ('.') and a minor version specifier.  If the minor
    qualifier is specified, it may optionally be followed by "-32" to indicate
    the 32bit implementation of that version be used.  Note that no "-64"
    qualifier is necessary as this is the default implementation (see below).

    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 specified 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 (ie, without knowing
    whether a 32 or 64bit version of Python and corresponding launcher was
    installed last).  As noted above, an optional "-32" suffix can be used
    on a version specifier to change this behaviour.

    The launcher will support various customization options to allow
    fine-grained control over which specific Python version is chosen given
    a partial or empty version qualifier - see the launcher documentation [4]
    for details.

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 cause help for the launcher being printed
    followed by help for Python itself.  The output from the launcher will
    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 dash ("-")
    followed by a version qualifier, as described above, 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) via sys.executable - currently the
    child is launched using the exact same version running the parent script.
    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.

    Another hurdle is the support for alternative Python implementations
    using the "customized commands" feature described above, where loading
    the command dynamically into a running executable is not possible.

    The final hurdle is the rules above regarding 64bit and 32bit programs -
    a 32bit launcher would be unable to load the 64bit version of Python and
    vice-versa.

    Given these considerations, the launcher will execute its command 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
    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.)  As this Windows API
    is available in Windows XP and later, this launcher will not work on 
    Windows 2000 or earlier.

References

    [1] http://linux.die.net/man/2/execve

    [2] http://www.python.org/dev/peps/pep-0394/

    [3] https://bitbucket.org/vinay.sajip/pylauncher

    [4] https://bitbucket.org/vinay.sajip/pylauncher/src/tip/Doc/launcher.rst

Copyright

    This document has been placed in the public domain.



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