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I'm piqued by one thing in this article. Using an object as a dictionary key. What's the use for that? I don't think it's ever occurred to me to do that.

cakebrewery 9y ago
Avoids the extra step of hashing the object to make the key by simply passing the object itself.
talltimtom 9y ago
Anything hashable can be a dict key, so if you have some mapping between one thing that’s hashable and other things, you can use a dict directly rather than having to come up with an index and keep track of that association.
chubot 9y ago
I'm not sure what part of the article you refer to, but: - If the objects are value types (as dataclasses are, since they respect equality): it makes sense to use them as dict keys, just as it makes sense to use tuples as dict keys. Changing a tuple to a namedtuple is natural. - If the objects have reference semantics rather than value semantics, you can still use them as dict keys in Python. The default __hash__ is the id/address of the object (the default __eq__ uses id() too). I don't particularly like this style but I have seen it used. It feels error-prone to me and not "algebraic".
dagw 9y ago
Using an object as a dictionary key. What's the use for that? I've used tuples has dict keys several times, so I don't see why I conceivably wouldn't use a Data Class containing the same data as a dict key.
comnetxr 9y ago
memoization is a common use case. If you are doing an expensive computation that gives the same answer for the the same inputs, you can build a lookup table for cases you have already encountered to avoid recomputing. you can use an object to encapsulate the inputs (or the inputs might be objects already).
UncleEntity 9y ago
Dynamic dispatching comes to mind. d = {Foo : do_something_with_foo, Bar : do_something_with_bar} d.get(type(x), default_function)(x) Probably some more efficient way but honestly I've used this before instead of writing a whole if elif chain. --edit-- Makes python's lack of a switch operator sometimes less painful...
- ianamartin 9y ago
  Oh, that makes sense. Not sure why this question rubbed people the wrong way. I was genuinely curious about when you would use an object as a key. I guess my uses for dictionaries are pretty vanilla. But that’s a kind of thing I do all the time. I set up dictionaries where the key is a possible value from some operation or query, and the value is the function I want to perform on that value. This extends that concept. Thanks!

cakebrewery 9y ago

Avoids the extra step of hashing the object to make the key by simply passing the object itself.

talltimtom 9y ago

Anything hashable can be a dict key, so if you have some mapping between one thing that’s hashable and other things, you can use a dict directly rather than having to come up with an index and keep track of that association.

chubot 9y ago

I'm not sure what part of the article you refer to, but: - If the objects are value types (as dataclasses are, since they respect equality): it makes sense to use them as dict keys, just as it makes sense to use tuples as dict keys. Changing a tuple to a namedtuple is natural. - If the objects have reference semantics rather than value semantics, you can still use them as dict keys in Python. The default __hash__ is the id/address of the object (the default __eq__ uses id() too). I don't particularly like this style but I have seen it used. It feels error-prone to me and not "algebraic".

dagw 9y ago

Using an object as a dictionary key. What's the use for that? I've used tuples has dict keys several times, so I don't see why I conceivably wouldn't use a Data Class containing the same data as a dict key.

comnetxr 9y ago

memoization is a common use case. If you are doing an expensive computation that gives the same answer for the the same inputs, you can build a lookup table for cases you have already encountered to avoid recomputing. you can use an object to encapsulate the inputs (or the inputs might be objects already).

UncleEntity 9y ago

Dynamic dispatching comes to mind. d = {Foo : do_something_with_foo, Bar : do_something_with_bar} d.get(type(x), default_function)(x) Probably some more efficient way but honestly I've used this before instead of writing a whole if elif chain. --edit-- Makes python's lack of a switch operator sometimes less painful...

ianamartin 9y ago
Oh, that makes sense. Not sure why this question rubbed people the wrong way. I was genuinely curious about when you would use an object as a key. I guess my uses for dictionaries are pretty vanilla. But that’s a kind of thing I do all the time. I set up dictionaries where the key is a possible value from some operation or query, and the value is the function I want to perform on that value. This extends that concept. Thanks!