different key, same value in dictionaries

[Magdoll]

Is there a cleaner way to do this example:

d = {('a','b'): 10, ('a','c'): 20, ('b','c'): 30}

The key is always a pair (x,y), but d[(x,y)] should have the same
result as d[(y,x)]. So either I would have to store both d[(x,y)] and
d[(y,x)] (unncessary extra space?), or write something like:

if x <= y: return d[(x,y)]
else: return d[(y,x)]

I'm not familiar with python enough, so I want to know whether these
are my only choices....

[Gary Herron]

Re: different key, same value in dictionaries

Magdoll wrote:

Is there a cleaner way to do this example:
>
d = {('a','b'): 10, ('a','c'): 20, ('b','c'): 30}
>
The key is always a pair (x,y), but d[(x,y)] should have the same
result as d[(y,x)]. So either I would have to store both d[(x,y)] and
d[(y,x)] (unncessary extra space?), or write something like:
>
if x <= y: return d[(x,y)]
else: return d[(y,x)]
>
I'm not familiar with python enough, so I want to know whether these
are my only choices....
>

You could use ImmutableSets as indexes. (In fact this is the whole
reason for the existence of ImmutableSets.)

You could derive your own dictionary type from the builtin dictionary
type, and map an index operation d[(x,y)] to
d[ImmutableSet(a, b)]. Then all of d[a,b], d[b,a], d[(a,b)] and d[(b,a)]
would index the same element.

See http://docs.python.org/lib/module-sets.html for details of the set
module.

Gary Herron

[Matt Nordhoff]

Re: different key, same value in dictionaries

Gary Herron wrote:

You could use ImmutableSets as indexes. (In fact this is the whole
reason for the existence of ImmutableSets.)
>
You could derive your own dictionary type from the builtin dictionary
type, and map an index operation d[(x,y)] to
d[ImmutableSet(a, b)]. Then all of d[a,b], d[b,a], d[(a,b)] and d[(b,a)]
would index the same element.
>
See http://docs.python.org/lib/module-sets.html for details of the set
module.

Since Python 2.4, sets are built-in types. Use "set" and "frozenset"
instead of "sets.Set" and "sets.Immutable Set", respectively.

If you need Python 2.3 compatibility, you can do something like this:

try:
set, frozenset
except NameError:
from sets import Set as set, ImmutableSet as frozenset
--