On 24/09/2012 18:33, Duncan Booth wrote:
Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote:
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)
Google is your friend. Searching for "java singletonMap" gives this as
the second hit:
http://stackoverflow.com/questions/7125536/when-would-i-use-java-collections-singletonmap-method
The answers seem to be that it's for all those cases in Java where you have a
method that takes a map as an argument and you want to pass in a map with a
single
kep/value pair. In that case it lets you replace 3 lines of Java with 1.
e.g. from the comments:
"If you have a simple select statement like "select foo from bar where id =
:barId"
then you would need a parameter map with a single key-value pair, barId=123.
That's a great place to use singletonMap()"
Of course in Python you just use a dict literal in that case so it's pointless.
Thank goodness for that, I'd assumed that I'd missed something blatantly
obvious. There are two chances of something like this getting into the
standard library, zero or none. I think in a way that's a great pity as
I'm sure that the Python devs would enjoy supporting the little feller
with code such as this http://tinyurl.com/9v7d7ld :)
--
Cheers.
Mark Lawrence.
--
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