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Updates based on python-dev discussion.

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Barry Warsaw 2010-07-22 14:18:43 +00:00
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pep-3149.txt
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@ -8,7 +8,7 @@ Type: Standards Track
Content-Type: text/x-rst
Created: 2010-07-09
Python-Version: 3.2
Post-History: 2010-07-14
Post-History: 2010-07-14, 2010-07-22
Resolution: TBD
@ -49,10 +49,14 @@ compilation file system layout would be::
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.
to changes in the ABI. Different configuration/compilation options
for the same Python version can result in different ABIs
(e.g. --with-wide-unicode).
While PEP 384 [2]_ defines a stable ABI, it will minimize, but not
eliminate extension module incompatibilities between Python builds or
major versions. Thus a mechanism for discriminating extension module
file names is proposed.
Rationale
@ -77,51 +81,85 @@ 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.
than one Python version.
In addition, because different configuration/compilation options for
the same Python version can cause different ABIs to be presented to
extension modules. On POSIX systems for example, the configure
options ``--with-pydebug``, ``--with-pymalloc``, and
``--with-wide-unicode`` all change the ABI. This PEP proposes to
encode build-time options in the file name of the ``.so`` extension
module files.
PyPy [5]_ can also benefit from this PEP, allowing it to avoid name
collisions in extension modules built for its API, but with a
different `.so` tag.
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.::
The configure/compilation options chosen at Python interpreter
build-time will be encoded in the shared library file name for
extension modules. This "tag" will appear between the module base
name and the operation file system extension for shared libraries.
./configure --with-so-abi-tag=cpython-32
The following information *MUST* be included in the shared library
file name:
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.
* The Python implementation (e.g. cpython, pypy, jython, etc.)
* The interpreter's major and minor version numbers
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.
These two fields are separated by a hyphen and no dots are to appear
between the major and minor version numbers. E.g. ``cpython-32``.
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.
Python implementations *MAY* include additional flags in the file name
tag as appropriate. For example, on POSIX systems these flags will
also contribute to the file name:
For example, let's say Python 3.2 was built with::
* ``--with-pydebug`` (flag: ``d``)
* ``--with-pymalloc`` (flag: ``m``)
* ``--with-wide-unicode`` (flag: ``u``)
./configure --with-so-abi-tag=cpython-32
By default in Python 3.2, ``configure`` enables ``--with-pymalloc`` so
shared library file names would appear as ``foo.cpython-32m.so``.
When the other two flags are also enabled, the file names would be
``foo.cpython-32dmu.so``.
and Python 3.3 was built with::
(This PEP only addresses build issues on POSIX systems that use the
``configure`` script. While Windows or other platform support is not
explicitly disallowed under this PEP, platform expertise is needed in
order to evaluate, describe, and implement support on such platforms.)
./configure --with-so-abi-tag=cpython-33
The shared library file name tag is used unconditionally; it cannot be
changed. The tag and extension module suffix are available through
the ``sysconfig`` modules via the following variables::
>>> sysconfig.get_config_var('SO')
'.cpython-32mu.so'
>>> sysconfig.get_config_var('SOABI')
'cpython-32mu'
Note that ``$SOABI`` contains just the tag, while ``$SO`` includes the
platform extension for shared library files, and is the exact suffix
added to the extension module name.
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
/usr/share/pyshared/foo.cpython-32m.so
/usr/share/pyshared/foo.cpython-33m.so
Python's dynamic module loader will recognize and import shared
library extension modules with a tag that matches its build-time
options. For backward compatibility, Python will also continue to
import untagged extension modules, e.g. ``foo.so``.
This shared library tag would be used globally for all distutils-based
extension modules, regardless of where on the file system they are
built. Extension modules built by means other than distutils would
either have to calculate the tag manually, or fallback to the
non-tagged `.so` file name.
Proven approach
@ -130,26 +168,22 @@ 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.
use a different file extension for dynamic libraries, and in fact
encoded (in a different way than proposed in this PEP) the Python
major and minor version in the `.dll` file name.
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 defines a stable ABI for extension modules. In theory,
universal adoption of PEP 384 would eliminate the need for this PEP
because all extension modules could be compatible with any Python
version. In practice of course, it will be impossible to achieve
universal adoption, and as described above, different built-time flags
still affect the ABI. Thus even with a stable ABI, this PEP may still
be necessary. 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
@ -160,15 +194,15 @@ 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
Thus, an initial implementation of PEP 384, when Python is configured
with the default set of flags, would search for the following file
names when extension module `foo` is imported (in this order)::
foo.cpython-XYm.so
foo.abi3.so
foo.cpython-xy.so
foo.so
The distutils [7]_ ``build_ext`` command would also have to be
The distutils [6]_ ``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
@ -180,7 +214,7 @@ by adding a keyword argument to the ``Extension`` class, such as::
Alternatives
============
In the initial python-dev thread [8]_ where this idea was first
In the initial python-dev thread [7]_ where this idea was first
introduced, several alternatives were suggested. For completeness
they are listed here, along with the reasons for not adopting them.
@ -193,12 +227,12 @@ Debian and Ubuntu could simply add a version-specific directory to
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.
avoid, and adds potentially several additional directories to search
for all modules, even when the number of extension modules is much
fewer than the total number of Python packages. For example, builds
were made available both with and without wide unicode, with and
without pydebug, and with and without pymalloc, the total number of
directories search increases substantially.
Don't share packages with extension modules
@ -216,17 +250,17 @@ 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
The extra size 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]_
Work on this code is tracked in a Bazaar branch on Launchpad [8]_
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
can also be viewed [9]_ and is updated automatically as new changes
are uploaded.
@ -241,13 +275,15 @@ References
.. [4] Debian: <http://www.debian.org>
.. [5] https://code.edge.launchpad.net/~barry/python/sovers
.. [5] http://codespeak.net/pypy/dist/pypy/doc/
.. [6] https://code.edge.launchpad.net/~barry/python/sovers/+merge/29411
.. [6] http://docs.python.org/py3k/distutils/index.html
.. [7] http://docs.python.org/py3k/distutils/index.html
.. [7] http://mail.python.org/pipermail/python-dev/2010-June/100998.html
.. [8] http://mail.python.org/pipermail/python-dev/2010-June/100998.html
.. [8] https://code.edge.launchpad.net/~barry/python/sovers
.. [9] https://code.edge.launchpad.net/~barry/python/sovers/+merge/29411
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