Java singletonMap in Python

[Mark Lawrence]

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
[2]http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python
[3]http://code.activestate.com/recipes/498072-implementing-an-immutable-dictionary/
[4]http://code.activestate.com/recipes/66531-singleton-we-dont-need-no-stinkin-singleton-the-bo/
--
Cheers.

Mark Lawrence.

[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

[Duncan Booth]

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.

--
Duncan Booth http://kupuguy.blogspot.com

[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：

Cheers to those who are programmers that really love authoring in the high level one with tools to translate into other computer languages,
and also collecting the upgrade fees from clients.

[Devin Jeanpierre]

On Sun, Sep 23, 2012 at 7:14 PM, Mark Lawrence <bream...@yahoo.co.uk> 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

The word "singleton" usually means "thing with only one item". For
example, {a} is a singleton set containing only a, and with matrices,
any dimension of size one is called a singleton dimension, and so on.
In this case, a singleton map is a map with only one key-value pair,
such as {a:b}.

The singleton design antipattern is not relevant here.

-- Devin

[Mark Lawrence]

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.

[Mark Lawrence]

On 24/09/2012 20:22, Devin Jeanpierre wrote:

> On Sun, Sep 23, 2012 at 7:14 PM, Mark Lawrence <bream...@yahoo.co.uk> 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
>
> The word "singleton" usually means "thing with only one item". For
> example, {a} is a singleton set containing only a, and with matrices,
> any dimension of size one is called a singleton dimension, and so on.
> In this case, a singleton map is a map with only one key-value pair,
> such as {a:b}.
>
> The singleton design antipattern is not relevant here.
>
> -- Devin
>

Java thinks so otherwise there wouldn't also be the singleton which is
the spelling for singletonSet (don't ask me!!!) and a singletonList.
From the Python viewpoint I think YAGNI is perfect. I now understand
why the BDFL and others fight so hard to keep bloatware out of the
standard library.

--
Cheers.

Mark Lawrence.

[Dwight Hutto]

On Tue, Sep 25, 2012 at 12:29 AM, Dennis Lee Bieber
<wlf...@ix.netcom.com> wrote:
> On Mon, 24 Sep 2012 20:28:14 +0100, Mark Lawrence
> <bream...@yahoo.co.uk> declaimed the following in
> gmane.comp.python.general:

>
>>
>> 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 :)
>

> Oh, the chance /might/ be a bit higher... On par with winning the
> jackpot in one of those multi-state lotteries (because you'd have to
> donate the entire jackpot to Python.org to persuade them to add a "do
> nothing" package <G>)

It's called
import time
time.sleep(x)

--
Best Regards,
David Hutto
CEO: http://www.hitwebdevelopment.com