Random notes from mg

a blog by Marius Gedminas

Marius is a Python hacker. He works for Programmers of Vilnius, a small Python/Zope 3 startup. He has a personal home page at http://gedmin.as. His email is marius@gedmin.as. He does not like spam, but is not afraid of it.

Thu, 21 May 2009

Surprising old-style class behaviour

Some anonymous Planet Python poster (at least I couldn't find the author's name on the blog) Christian Wyglendowski asks about a surprising difference between old-style and new-style classes. Since the comments on their blog are closed (which you find out only after pressing Submit), I'll answer here.

The question, slightly paraphrased: given a class

class LameContainerOld:
    def __init__(self):
        self._items = {'bar':'test'}
    def __getitem__(self, name):
        return self._items[name]
    def __getattr__(self, attr):
        return getattr(self._items, attr)

why does the 'in' operator work

>>> container = LameContainerOld()
>>> 'foo' in container
>>> 'bar' in container

when the equivalent new-style class raises a KeyError: 0 exception? Also, why does __getattr__ appear to be called to get the bound __getitem__ method of the dict?

>>> container.__getitem__
<bound method LameContainerNew.__getitem__ of {'bar': 'test'}>

What actually happens here is that LameOldContainer.__getattr__ gets called for special methods such as __contains__ and __repr__. This is why (1) the 'in' check works, and (2) it appears, at first glance, that you get the wrong __getitem__ bound method. If you pay close attention to the output, you'll see that it's the __getitem__ of LameOldContainer; it's just that repr(LameOldContainer()) gets proxied through to the dict.__repr__ when you don't expect it:

>>> container
{'bar': 'test'}

Special methods never go through __getattr__ for new-style classes, therefore neither __contains__ nor __repr__ are proxied if you make the container inherit object. If there's no __contains__ method, Python falls back to the sequence protocol and starts calling __getitem__ for numbers 0 through infinity, or until it gets an IndexError exception.

posted at 22:12 | tags: | permanent link to this entry | 2 comments
I'm that anonymous poster. :-)  Sorry about the comments being broken on my blog ... I wasn't aware of that.

That said, thanks for the great explanation.  That makes a lot of sense.
posted by Christian Wyglendowski at Fri May 22 00:26:44 2009
Thank you for another insightful bit about python.
posted by rgz at Fri May 22 04:40:19 2009

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