[Python-Dev] 2to3 porting HOWTO: setup.py question (original) (raw)

R. David Murray rdmurray at bitdance.com
Sun Jul 22 21:57:10 CEST 2012

Benjamin sent me this message separately(*) privately and I responded privately. Here is my response.

(*) or his mailer did

On Sun, 22 Jul 2012 20:22:50 +0100, Oscar Benjamin <oscar.benjamin at bristol.ac.uk> wrote:

On 22 July 2012 14:08, R. David Murray <rdmurray at bitdance.com> wrote:

> On Sun, 22 Jul 2012 11:21:38 +0300, anatoly techtonik <techtonik at gmail.com> > wrote: > > http://docs.python.org/py3k/howto/pyporting.html#during-installation > > > > What's the point in making implicit Python 3 check here: > > try: # Python 3 > > from distutils.command.buildpy import buildpy2to3 as buildpy > > except ImportError: # Python 2 > > from distutils.command.buildpy import buildpy > > > > instead of explicit check like: > > import sys > > if sys.versioninfo[0] >= 3: > > from distutils.command.buildpy import buildpy2to3 as buildpy > > It's called testing for the thing that actually matters, rather than > testing a constant with a much broader meaning. Yes, in this case the > results are the same, but IMO it is better programming practice to test > the thing that actually matters when you can.

I recently changed a setup.py from try/ImportError to an explicit sys.versioninfo check. I'm not totally sure how to reproduce this but I had a problem where I was installing into a 2.x virtualenv and it was running 2to3 during install and subsequently failing to import the 3.x code (the problem didn't occur when using the same python that generated the virtualenv). I may be wrong but I imagined that sometimes buildpy2to3 is importable on 2.x, perhaps for cross-building or something. In any case 'testing the thing that matters' means testing what version of Python you are about to install into not whether the python version supports running 2to3.

I'm not familiar with distutils, really, so you could be right about what it is important to test. I was commenting based on the code snippet presented, which just deciding which "build" object to use. If build_py_2to3 can be imported by python2 and subsequently screws up the build, then yes the logic is incorrect. But I have to defer to the packaging people on that. (I wish I had time to help with packaging because it is important, but it doesn't seem like a sensible place for me personally to spend my currently available time.)

--David

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