PEP: 397 Title: Python launcher for Windows Version: $Revision$ Last-Modified: $Date$ Author: Mark Hammond Status: Draft Type: Standards Track Content-Type: text/plain 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. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: