Re: Java singletonMap in Python
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. -- Duncan Booth http://kupuguy.blogspot.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Java singletonMap in Python
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.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. -- Duncan Booth http://kupuguy.blogspot.com 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. -- http://mail.python.org/mailman/listinfo/python-list
Re: Java singletonMap in Python
On Sun, Sep 23, 2012 at 7:14 PM, Mark Lawrence breamore...@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
--
http://mail.python.org/mailman/listinfo/python-list
Re: Java singletonMap in Python
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. -- http://mail.python.org/mailman/listinfo/python-list
Re: Java singletonMap in Python
On 24/09/2012 20:22, Devin Jeanpierre wrote:
On Sun, Sep 23, 2012 at 7:14 PM, Mark Lawrence breamore...@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.
--
http://mail.python.org/mailman/listinfo/python-list
Java singletonMap in Python
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. -- http://mail.python.org/mailman/listinfo/python-list
Re: Java singletonMap in Python
On 24 September 2012 00:14, Mark Lawrence breamore...@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, 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. What exactly is wanted when an attempt is made to instantiate an instance? Should it raise an error or return the previously created instance? This attempt makes all calls to __new__ after the first return the same instance: def singleton(cls): instance = None class sub(cls): def __new__(cls_, *args, **kwargs): nonlocal instance if instance is None: instance = super(sub, cls_).__new__(cls_, *args, **kwargs) return instance sub.__name__ == cls.__name__ return sub @singleton class A(object): pass print(A() is A()) Oscar -- http://mail.python.org/mailman/listinfo/python-list
Re: Java singletonMap in Python
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 -- http://mail.python.org/mailman/listinfo/python-list
