Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!eternal-september.org!feeder.eternal-september.org!feeds.phibee-telecom.net!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
MIME-Version: 1.0
In-Reply-To: <KYednbizIP5rAlnJnZ2dnUVZ7sidnZ2d@brightview.co.uk>
References: <FeCdnXdsTpunvlnJnZ2dnUVZ8judnZ2d@brightview.co.uk> <mailman.18106.1422141738.18130.python-list@python.org> <5ZKdnd6rbdZ3sVnJnZ2dnUVZ8mCdnZ2d@brightview.co.uk> <mailman.18109.1422143020.18130.python-list@python.org> <K8KdncuoKPL1q1nJnZ2dnUVZ8h2dnZ2d@brightview.co.uk> <mailman.18116.1422145722.18130.python-list@python.org> <KYednbizIP5rAlnJnZ2dnUVZ7sidnZ2d@brightview.co.uk>
Date: Sun, 25 Jan 2015 19:07:30 +1100
Subject: Re: Delegation in Python
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
Content-Type: text/plain; charset=UTF-8
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.18130.1422173260.18130.python-list@python.org>
Lines: 47
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:84560

On Sun, Jan 25, 2015 at 6:49 PM, Brian Gladman <noone@nowhere.net> wrote:
> Thanks, a part of this was a wish to understand how to map what I can do
> in other languages into Python.  I felt that it might just be possible
> in Python to avoid having to wrap all the methods of the base class in
> the derived class.  But it seems that __getattr__ etc are not quite as
> magic as I hoped.

They do exactly what they're documented to, nothing more and nothing
less :) It's certainly possible to use them to hook into missing
attributes, for instance:

>>> class RF:
    def __init__(self, *args, **kw):
        self._frac = Fraction(*args, **kw)
    def __getattr__(self, attr):
        return getattr(self._frac, attr)
    def __repr__(self):
        return "RF(%d, %d)" % (self._frac.numerator, self._frac.denominator)
    def is_integer(self):
        return self._frac.denominator==1

>>> RF(1,4).numerator
1

But it doesn't work for everything:
>>> RF(1,4)*2
Traceback (most recent call last):
  File "<pyshell#37>", line 1, in <module>
    RF(1,4)*2
TypeError: unsupported operand type(s) for *: 'RF' and 'int'

The only solution would be to go through every operation that you care
about, and manually hook them. Something like this:

    def __mul__(self, other):
        result = self._frac * other
        if isinstance(result, Fraction):
            return RF(result.numerator, result.denominator)
        return result

>>> RF(1,4)*2
RF(1, 2)

And do that for every other operation, method, etc. Tedious, but can
be effective if you need it.

ChrisA