On May 31, 2013 6:30 PM, "Chris Barker - NOAA Federal" <chris.barker@noaa.gov> wrote:

HI Folks,

A few of us over on the pythonmac list have been hashing out how best
to support binary packages on OS-X. The binary wheel option seems very
promising.

However, there is one Mac-specific issue that does not seem to be addressed:

On OS-X, binaries can be "universal" -- what this means is that they
have more than one binary architecture embedded in a single file; the
system selects the right one at run time. Both executables and dynamic
libraries can be universal. In theory, an universal binary can
currently contain up to 4 architectures:

32+64 bit PPC and 32+64bit x86

In practice, 64 bit PPC was never broadly used, and 32bit PPC is
fading fast. Nevertheless, both 32 and 64 Intel systems are pretty
common, and who knows what there may be in the future (you never know
with Apple...)

The binary builds of Python served up on the python.org web site have
supported universal builds for years -- currently there are two
options: 32bit PPC+x86 or 32+64bit x86. It's not a single build, as it
turns out it's hard to support both up to date x86_64 and PPC with the
same compiler. In these builds, the configuration is set up so that
distutils will build universal extensions that match the architectures
included in the builds. This makes it pretty easy to build binary
packages, and/or full app distributions (py2app) that are universal as
well.

setuptools' easy_install supports binary eggs, but always choked on
universal builds -- it didn't know how to identify what matched the
system. It would be really nice if future tools could identify and
install the correct binary wheels for universal builds.

I've taken a look at PEP 427, and see this:

platform tag
E.g. 'linux_x86_64', 'any'.

That looks to me like there is a built-in assumption that a wheel is
either specific to a single architecture, or any -- so no way to
specify that it may have multiple architectures. So I propose that
platform tag allow a list of some sort:

['macosx-10_6_i386', 'macosx_10_6-x86_64']

or, I suppose to keep it simple, a single string:

'macosx_10_6_i386+macosx_10_6_x86_64'
or
'macosx_10_6_i386+x86_64'

If so, then a binary wheel could be built (and discovered) that
supported multiple architectures.

Then how would an installer ( pip? ) identify the right one? This is a
bit tricky. PEP 425 states:

"The platform tag is simply distutils.util.get_platform() with all
hyphens - and periods . replaced with underscore"

On my system right now, I have 32bitPPC + 32bit i386 universal python,
running on an intel system:

In [2]: distutils.util.get_platform()
Out[2]: 'macosx-10.3-i386'

So we may want to patch get_platform() to support universal builds -
or add anther function or something -- you may sometimes what to know
what you are running right now, and other times want to know how the
current python is built. (note that while distutils can build
universal binaries, it's not very smart about it -- all is does is
grab the compiler and linker flags from the Makefile, which include
multiple "-arch" flags.

Even if there is an easy way to query the system, that leaves the
question of what to do. If a user wants to intall a package into a
universal python, will the installer only accept a binary wheel with
all the supported architectures? or one with only the currently
running one?

Anyway, it would be nice to be able to get this stuff to "just work"
with universal binaries on the Mac.

Thoughts, ideas?

-Chris



--

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chris.Barker@noaa.gov
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