[Distutils] Best practices for optional C extensions

[Tres Seaver]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 07/13/2013 09:14 AM, Ben Darnell wrote:
> I'd like to add a C extension to speed up a small bit of code in a
> package (Tornado), but make it optional both for compatibility with
> non-cpython implementations and for ease of installation on systems
> without a C compiler available.  Ideally any user who runs "pip
> install tornado" on a system capable of compiling extensions would get
> the extensions; if this capability cannot be detected automatically
> I'd prefer the opt-out case to be the one that requires non-default
> arguments.  Are there any packages that provide a good example to
> follow for this?
> 
> PEP 426 uses "c-accelerators" as an example of an "extra", but it's
> unclear how this would work (based on the equivalent setuptools
> feature).  There doesn't appear to be a way to know what extras are
> requested at build time. If the extra required a package like cython
> then you could build the extension whenever that package is present,
> but what about hand-written extensions?  Extras are also opt-in
> instead of opt-out, so I'd have to recommend that most people use "pip
> install tornado[fast]" instead of "pip install tornado" (with
> "tornado[slow]" available as an option for limited environments).

zope.interface subclasses the 'build_ext' command so:


- ---------------------------- %< -----------------------------
from distutils.command.build_ext import build_ext
from distutils.errors import CCompilerError
from distutils.errors import DistutilsExecError
from distutils.errors import DistutilsPlatformError


class optional_build_ext(build_ext):
    """Allow the building of C extensions to fail.
    """
    def run(self):
        try:
            build_ext.run(self)
        except DistutilsPlatformError as e:
            self._unavailable(e)

    def build_extension(self, ext):
        try:
            build_ext.build_extension(self, ext)
        except (CCompilerError, DistutilsExecError) as e:
            self._unavailable(e)

    def _unavailable(self, e):
        print('*' * 80)
        print("""WARNING:

An optional code optimization (C extension) could not be compiled.

Optimizations for this package will not be available!""")
        print()
        print(e)
        print('*' * 80)
- ---------------------------- %< -----------------------------



Tres.
- -- 
===================================================================
Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)
Comment: Using GnuPG with undefined - http://www.enigmail.net/

iEYEARECAAYFAlHitjoACgkQ+gerLs4ltQ7SJQCgrhgN58g9ztFPEkFAOM49Wu4p
RpQAoLnboKietjTx3eXho1kyRvH3r2uN
=qWRK
-----END PGP SIGNATURE-----