Java singletonMap in Python

[88888 Dihedral]

Duncan Booth於 2012年9月25日星期二UTC+8上午1時33分31秒寫道：
> Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:
> 
> 
> 
> > On Mon, 24 Sep 2012 00:14:23 +0100, Mark Lawrence wrote:
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> > 
> 
> >> 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
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> >> 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/
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> > Collections.html
> 
> > 
> 
> > Copied from that page:
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> > 
> 
> > "static Map     singletonMap(Object key, Object value) 
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> > Returns an immutable map, mapping only the specified key to the
> 
> > specified value."
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> > 
> 
> > 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
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> > the point? Why bother storing the key:value pair in a data structure, 
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> > then look up the same data structure to get the same value every time?
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> > 
> 
> > # Pseudo-code
> 
> > d = singletonMap(key, calculate(key))
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> > # later:
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> > value = d[key]  # there's only one key this could be
> 
> > process(value)
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> > 
> 
> > 
> 
> > Why not just store the value, instead of key, value and mapping?
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> > 
> 
> > value = calculate(key)
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> > # later
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> > process(value)
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> > 
> 
> > 
> 
> > 
> 
> 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:
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> "If you have a simple select statement like "select foo from bar where id = :barId" 
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> 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.
> 
> 
> 
> -- 
> 
> Duncan Booth http://kupuguy.blogspot.com

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and also collecting  the upgrade fees from clients.