[Python-Dev] versioned .so files for Python 3.2 (original) (raw)

Barry Warsaw barry at python.org
Thu Jul 15 01:59:55 CEST 2010

On Jun 25, 2010, at 08:35 AM, Nick Coghlan wrote:

I like the idea, but I think summarising the rest of this discussion in its own (relatively short) PEP would be good (there are a few things that are tricky - exact versioning scheme, PEP 384 forward compatibility, impact on distutils, articulating the benefits for distro packaging, etc).

The first draft of PEP 3149 is ready for review.

http://www.python.org/dev/peps/pep-3149/

Plain text attached here for your convenience. Comments, suggestions as always are welcome. Thanks to everyone who participated in the original discussion.

-Barry

PEP: 3149 Title: ABI version tagged .so files Version: Revision:81577Revision: 81577 Revision:81577 Last-Modified: Date:2010−05−2719:54:25−0400(Thu,27May2010)Date: 2010-05-27 19:54:25 -0400 (Thu, 27 May 2010) Date:2010−05−2719:54:25−0400(Thu,27May2010) Author: Barry Warsaw <barry at python.org> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 2010-07-09 Python-Version: 3.2 Post-History: 2010-07-14 Resolution: TBD

Abstract

PEP 3147 [1]_ described an extension to Python's import machinery that improved the sharing of Python source code, by allowing more than one byte compilation file (.pyc) to be co-located with each source file.

This PEP defines an adjunct feature which allows the co-location of extension module files (.so) in a similar manner. This optional, build-time feature will enable downstream distributions of Python to more easily provide more than one Python major version at a time.

Background

PEP 3147 defined the file system layout for a pure-Python package, where multiple versions of Python are available on the system. For example, where the alpha package containing source modules one.py and two.py exist on a system with Python 3.2 and 3.3, the post-byte compilation file system layout would be::

alpha/
    __pycache__/
        __init__.cpython-32.pyc
        __init__.cpython-33.pyc
        one.cpython-32.pyc
        one.cpython-33.pyc
        two.cpython-32.pyc
        two.cpython-33.pyc
    __init__.py
    one.py
    two.py

For packages with extension modules, a similar differentiation is needed for the module's .so files. Extension modules compiled for different Python major versions are incompatible with each other due to changes in the ABI. While PEP 384 [2]_ defines a stable ABI, it will minimize, but not eliminate extension module incompatibilities between Python major versions. Thus a mechanism for discriminating extension module file names is proposed.

Rationale

Linux distributions such as Ubuntu [3]_ and Debian [4]_ provide more than one Python version at the same time to their users. For example, Ubuntu 9.10 Karmic Koala users can install Python 2.5, 2.6, and 3.1, with Python 2.6 being the default.

In order to share as much as possible between the available Python versions, these distributions install third party (i.e. non-standard library) packages into /usr/share/pyshared and symlink to them from /usr/lib/pythonX.Y/dist-packages. The symlinks exist because in a pre-PEP 3147 world (i.e < Python 3.2), the .pyc files resulting from byte compilation by the various installed Pythons will name collide with each other. For Python versions >= 3.2, all pure-Python packages can be shared, because the .pyc files will no longer cause file system naming conflicts. Eliminating these symlinks makes for a simpler, more robust Python distribution.

A similar situation arises with shared library extensions. Because extension modules are typically named foo.so for a foo extension module, these would also name collide if foo was provided for more than one Python version. There are several approaches that could be taken to avoid this, which will be explored below, but this PEP proposes a fairly simple compile-time option to allow extension modules to live in the same file system directory and avoid any name collisions.

Proposal

A new configure option is added for building Python, called --with-so-abi-tag. This takes as an argument a unique, but arbitrary string, e.g.::

./configure --with-so-abi-tag=cpython-32

This string is passed into the Makefile and affects two aspects of the Python build. First, it is compiled into Python/dynload_shlib.c where it defines some additional .so file names to search for when importing extension modules. Second, it modifies the Makefile's $SO variable, which in turn controls the distutils module's default filename when compiling extension modules.

When --with-so-abi-tag is not given to configure nothing changes in the way the Python executable is built, or acts. Thus, this configure switch is completely optional and has no effect if not used.

What this allows is for distributions that want to distinguish among extension modules built for different versions of Python, but shared in the same file system path, to arrange for .so names that are unique and non-colliding.

For example, let's say Python 3.2 was built with::

./configure --with-so-abi-tag=cpython-32

and Python 3.3 was built with::

./configure --with-so-abi-tag=cpython-33

For an arbitrary package foo, you would see these files when the distribution package was installed::

/usr/share/pyshared/foo.cpython-32.so
/usr/share/pyshared/foo.cpython-33.so

Proven approach

The approach described here is already proven, in a sense, on Debian and Ubuntu system where different extensions are used for debug builds of Python and extension modules. Debug builds on Windows also already use a different file extension for dynamic libraries.

PEP 384

PEP 384 defines a stable ABI for extension modules. Universal adoption of PEP 384 would eliminate the need for this PEP because all extension modules would be compatible with any Python version. In practice of course, it will be impossible to achieve universal adoption. Older extensions may not be ported to PEP 384, or an extension may require Python APIs outside of PEP 384 definition. Therefore there will always be a (hopefully diminishing, but never zero) need for ABI version tagged shared libraries.

Further, it is anticipated that the stable ABI will evolve over time, meaning that existing PEP 384 compatible extension modules may be incompatible with future versions of Python. While a complete specification is reserved for PEP 384, here is a discussion of the relevant issues.

PEP 384 describes a change to PyModule_Create() where 3 is passed as the API version if the extension was complied with Py_LIMITED_API. This should be formalized into an official macro called PYTHON_ABI_VERSION to mirror PYTHON_API_VERSION. If and when the ABI changes in an incompatible way, this version number would be bumped. To facilitate sharing, Python would be extended to search for extension modules with the PYTHON_ABI_VERSION number in its name. The prefix abi is reserved for Python's use.

Thus for example, an initial implementation of PEP 384, compiled with --with-so-abi-tag=cpython-xy would search for the following file names when extension module foo is imported (in this order)::

foo.abi3.so
foo.cpython-xy.so
foo.so

The distutils [7]_ build_ext command would also have to be extended to compile to shared library files with the abi3 tag, when the module author indicates that their extension supports that version of the ABI. This could be done in a backward compatible way by adding a keyword argument to the Extension class, such as::

Extension('foo', ['foo.c'], abi=3)

Alternatives

In the initial python-dev thread [8]_ where this idea was first introduced, several alternatives were suggested. For completeness they are listed here, along with the reasons for not adopting them.

Debian and Ubuntu could simply add a version-specific directory to sys.path that would contain just the extension modules for that version of Python. Or the symlink trick eliminated in PEP 3147 could be retained for just shared libraries. This approach is rejected because it propagates the essential complexity that PEP 3147 tries to avoid, and adds yet another directory to search for all modules, even when the number of extension modules is much fewer than the total number of Python packages. It also makes for more robust management when all of a package's module files live in the same directory, because it allows systems such as dpkg to detect file conflicts between distribution packages.

Don't share packages with extension modules

It has been suggested that Python packages with extension modules not be shared among all supported Python versions on a distribution. Even with adoption of PEP 3149, extension modules will have to be compiled for every supported Python version, so perhaps sharing of such packages isn't useful anyway. Not sharing packages with extensions though is infeasible for several reasons.

If a pure-Python package is shared in one version, should it suddenly be not-shared if the next release adds an extension module for speed? Also, even though all extension shared libraries will be compiled and distributed once for every supported Python, there's a big difference between duplicating the .so files and duplicating all .py files. The extra space increases the download time for such packages, and more immediately, increases the space pressures on already constrained distribution CD-ROMs.

Reference implementation

Work on this code is tracked in a Bazaar branch on Launchpad [5]_ until it's ready for merge into Python 3.2. The work-in-progress diff can also be viewed [6]_ and is updated automatically as new changes are uploaded.

References

.. [1] PEP 3147

.. [2] PEP 384

.. [3] Ubuntu: <http://www.ubuntu.com>

.. [4] Debian: <http://www.debian.org>

.. [5] https://code.edge.launchpad.net/~barry/python/sovers

.. [6] https://code.edge.launchpad.net/~barry/python/sovers/+merge/29411

.. [7] http://docs.python.org/py3k/distutils/index.html

.. [8] http://mail.python.org/pipermail/python-dev/2010-June/100998.html

Copyright

This document has been placed in the public domain.

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