[Python-Dev] Proposed Mac OS X change for 2.3.5 and 2.4.1 - MACOSX_DEPLOYMENT_TARGET vs. Python, round two, fight!

[Bob Ippolito]

I've included a lot of background information here, if you just want to 
know the details
of the proposed change, skip to the bottom.

As some of you may know, Python 2.4's configure script and distutils 
has some
tricky behavior with regard to the ``MACOSX_DEPLOYMENT_TARGET`` 
environment
variable on Mac OS X 10.3 and later.  Unless otherwise specified, 
assume that I
am talking about a build environment of Mac OS X 10.3 or later.  This 
behavior
is as follows:

If ``MACOSX_DEPLOYMENT_TARGET`` is not set during configure:

1.  distutils builds modules with
     ``-F/Python/Installation/Location -framework Python`` as per usual 
2.3.x
     behavior if it is also not set
2.  If ``MACOSX_DEPLOYMENT_TARGET`` is set during a later run of 
distutils, then
     distutils complains that "10.3" mismatches the configure time 
setting of ""

This Python framework has the following misfeatures:

1.  All your extensions are dependent on the installation location of 
this
     particular Python
2.  This installation of Python 2.4 *may break the building of 
extensions* for
     any previous version of Python that uses the same distutils 
behavior.  It
     will certainly break them if they are installed to the same 
framework
     directory, but if the version of Python 2.3 is early enough, such 
as the
     stock 2.3.0, it will break that too.  Also, any future version of 
Python
     installed to the same frameworks directory *will break the building 
of
     extensions* for this Python installation!
3.  The Python framework will not be compatible with versions of Mac OS 
X
     earlier than 10.3 anyway due to changes in libSystem!

This is stupid, and it should NOT be default behavior!

If ``MACOSX_DEPLOYMENT_TARGET`` is set to "10.3" or higher during 
configure:

1.  distutils builds modules with ``-undefined dynamic_lookup``
2.  If ``MACOSX_DEPLOYMENT_TARGET`` is set to something other than 
"10.3", or
     unset, it will complain that the current setting mismatches the 
configure
     setting of "10.3"

The features:

1.  All your extensions are independent of the Python installation 
location,
     and are thus compatible with any other Python with the same major 
version.
2.  This installation of Python 2.4 will still, unavoidably, break the 
building
     of extensions for any previous version of Python using the bad 
distutils
     behavior mentioned above.  This installation is not susceptible to
     breakage, however.

The misfeatures:

1.  You need to have ``MACOSX_DEPLOYMENT_TARGET`` set during configure, 
most
     people don't know to do this.
2.  You also need to have ``MACOSX_DEPLOYMENT_TARGET`` set when using
     distutils.  Even less people know to do this.

The solution to this is to have the following behavior instead:

1.  If no ``MACOSX_DEPLOYMENT_TARGET`` is set during configure, and the 
build
     machine is Mac OS X 10.3 or later, then set it to "10.3".
2.  If no ``MACOSX_DEPLOYMENT_TARGET`` is set during distutils, but it 
WAS
     set during configure, then set it to the configure time value.
3.  Otherwise, if it is set to a LOWER value, then fail.  Checking for 
an
     exact match is bad, because the user or extension author should be
     free to build a particular extension using 10.4 specific features
     against a Python that is 10.3+ compatible.

For a build machine running Mac OS X 10.2 or earlier, it should ignore
all of this behavior as per usual.

Unless anyone has a reasonable objection to this proposed solution, then
we should make sure it goes into Python 2.4.1 and Python 2.3.5.

These threads might also be of interest:
http://mail.python.org/pipermail/pythonmac-sig/2004-November/012192.html
http://mail.python.org/pipermail/pythonmac-sig/2004-December/012237.html
http://mail.python.org/pipermail/pythonmac-sig/2004-December/012275.html

-bob