Why is dictionary.keys() a list and not a set?

[Mike Meyer]

Christoph Zwerschke <cito at online.de> writes:

> As a general note, I think it would be good to place the exact
> description in a footnote, since speaking about hashable objects,
> __hash__ and __cmp__ will certainly frighten off newbies and make it
> hard to read even for experienced users. The main text may
> lie/simplyfy a little bit.

Good point.

> However, since the Python docs are a reference and not a textbook or
> manual, I think the debt should not be payed back "later", but
> immediately in a footnote.

Ok, how about this for dictionaries/sets:

Any hashable object can be used as a dictionary key (set member). Immutable
objects, except for tuples that contain a non-hashable object, are
hashable. Python classes are normally hashable(1).

And the footnote is:

Instances of Python classes are hashable if they define a __hash__
method, or if they don't define either __cmp__ or __eq__.

        <mike
-- 
Mike Meyer <mwm at mired.org>			http://www.mired.org/home/mwm/
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