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To: python-list@python.org
From: Mark Lawrence <breamoreboy@yahoo.co.uk>
Subject: Re: Java singletonMap in Python
Date: Mon, 24 Sep 2012 20:28:14 +0100
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On 24/09/2012 18:33, Duncan Booth wrote:
> Steven D'Aprano <steve+comp.lang.python@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.