Java singletonMap in Python

[Steven D'Aprano]

On Mon, 24 Sep 2012 00:14:23 +0100, Mark Lawrence wrote:

> Purely for fun I've been porting some code to Python and came across the
> singletonMap[1].  I'm aware that there are loads of recipes on the web
> for both singletons e.g.[2] and immutable dictionaries e.g.[3].  I was
> wondering how to combine any of the recipes to produce the best
> implementation, where to me best means cleanest and hence most
> maintainable.  I then managed to muddy the waters for myself by
> recalling the Alex Martelli Borg pattern[4].  Possibly or even probably
> the latter is irrelevant, but I'm still curious to know how you'd code
> this beast.
> 
> First prize for the best solution is a night out with me, no guesses
> what the second prize is :)
> 
> [1]http://docs.oracle.com/javase/1.4.2/docs/api/java/util/
Collections.html

Copied from that page:

"static Map	singletonMap(Object key, Object value) 
Returns an immutable map, mapping only the specified key to the specified 
value."

I don't see the point of this. It takes a single key, with a single 
value, and is immutable so you can't change it or add new keys. What's 
the point? Why bother storing the key:value pair in a data structure, 
then look up the same data structure to get the same value every time?

# Pseudo-code
d = singletonMap(key, calculate(key))
# later:
value = d[key]  # there's only one key this could be
process(value)


Why not just store the value, instead of key, value and mapping?

value = calculate(key)
# later
process(value)



-- 
Steven