[Python-Dev] an alternative to embedding policy in PEP 418 (was: PEP 418: Add monotonic clock)

[Oleg Broytman]

On Thu, Apr 05, 2012 at 11:45:06PM +0900, Stephen J. Turnbull wrote:
> On Thu, Apr 5, 2012 at 10:34 PM, Oleg Broytman <phd at phdru.name> wrote:
> >   Why doesn't open() return None for a non-existing file? or
> > socket.gethostbyname() for a non-existing name?
> 
> That's not an answer to my question, because those calls have very
> important use cases where the user knows the object exists (and in
> fact in some cases open() will create it for him), so that failure to
> exist is indeed a (user) error (such as a misspelling).  I find it
> hard to imagine use cases where "file = open(thisfile) or
> open(thatfile)" makes sense.  Not even for the case where thisfile ==
> 'script.pyc' and thatfile == 'script.py'.

   Counterexamples - any configuration file: a program looks for its config
at $HOME and not finding it there looks in /etc. So
    config = open('~/.someprogram.config') or open('/etc/someprogram/config')
would make sense. The absence of any of these files is not an error at
all - the program just starts with default configuration. So if the
resulting config in the code above would be None - it's still would be
ok. But Python doesn't allow that.
   Some configuration files are constructed by combining a number of
user-defined and system-defined files. E.g., the mailcap database. It
should be something like
    combined_database = [db for db in (
        open('/etc/mailcap'),
        open('/usr/etc/mailcap'),
        open('/usr/local/etc/mailcap'),
        open('~/.mailcap'),
    ) if db]
But no way - open() raises IOError, not return None. And I think it is
the right way. Those who want to write the code similar to the examples
above - explicitly suppress exceptions by writing wrappers.

> The point of the proposed get_clock(), OTOH, is to ask if an object
> with certain characteristics exists, and the fact that it returns the
> clock rather than True if found is a matter of practical convenience.
> Precisely because "clock = get_clock(best) or get_clock(better) or
> get_clock(acceptable)" does make sense.

Oleg.
-- 
     Oleg Broytman            http://phdru.name/            phd at phdru.name
           Programmers don't die, they just GOSUB without RETURN.