Multidispatching#
Multidispatching allows you to define methods and functions which should behave
differently based on arguments’ types without cluttering if-elif-else
chains
and isinstance
calls.
All you need is inside generic.multidispatch
module. See examples below to
learn how to use it to define multifunctions and multimethods.
First the basics:
>>> class Cat: pass
>>> class Dog: pass
>>> class Duck: pass
Multifunctions#
Suppose we want to define a function which behaves differently based on
arguments’ types. The naive solution is to inspect argument types with
isinstance
function calls but generic provides us with @multidispatch
decorator which can easily reduce the amount of boilerplate and provide
desired level of extensibility:
>>> from generic.multidispatch import multidispatch
>>> @multidispatch(Dog)
... def sound(o):
... print("Woof!")
>>> @sound.register(Cat)
... def cat_sound(o):
... print("Meow!")
Each separate definition of sound
function works for different argument
types, we will call each such definition a multifunction case or simply a
case. We can test if our sound
multifunction works as expected:
>>> sound(Dog())
Woof!
>>> sound(Cat())
Meow!
>>> sound(Duck())
Traceback (most recent call last):
...
TypeError: No available rule found for ...
The main advantage of using multifunctions over single function with a bunch of
isinstance
checks is extensibility – you can add more cases for other types
even in separate module:
>>> @sound.register(Duck)
... def duck_sound(o):
... print("Quack!")
When behaviour of multifunction depends on some argument we will say that this multifunction dispatches on this argument.
Multifunctions of several arguments#
You can also define multifunctions of several arguments and even decide on which of first arguments you want to dispatch. For example the following function will only dispatch on its first argument while requiring both of them:
>>> @multidispatch(Dog)
... def walk(dog, meters):
... print("Dog walks for %d meters" % meters)
But sometimes you want multifunctions to dispatch on more than one argument,
then you just have to provide several arguments to multidispatch
decorator
and to subsequent when
decorators:
>>> @multidispatch(Dog, Cat)
... def chases(dog, cat):
... return True
>>> @chases.register(Dog, Dog)
... def chases_dog_dog(dog1, dog2):
... return None
>>> @chases.register(Cat, Dog)
... def chases_cat_dog(cat, dog):
... return False
You can have any number of arguments to dispatch on but they should be all positional, keyword arguments are allowed for multifunctions only if they’re not used for dispatch.
Multimethods#
Another functionality provided by generic.multimethod
module are
multimethods. Multimethods are similar to multifunctions except they are…
methods. Technically the main and the only difference between multifunctions and
multimethods is the latter is also dispatch on self
argument.
Implementing multimethods is similar to implementing multifunctions, you just
have to decorate your methods with multimethod
decorator instead of
multidispatch
. But there’s some issue with how Python’s classes works which
forces us to use also has_multimethods
class decorator:
>>> class Vegetable: pass
>>> class Meat: pass
>>> from generic.multimethod import multimethod, has_multimethods
>>> @has_multimethods
... class Animal(object):
...
... @multimethod(Vegetable)
... def can_eat(self, food):
... return True
...
... @can_eat.register(Meat)
... def can_eat(self, food):
... return False
register rule (<class '__main__.Animal'>, <class '__main__.Vegetable'>)
register rule (<class '__main__.Animal'>, <class '__main__.Meat'>)
This would work like this:
>>> animal = Animal()
>>> animal.can_eat(Vegetable())
True
>>> animal.can_eat(Meat())
False
So far we haven’t seen any differences between multifunctions and multimethods
but as it have already been said there’s one – multimethods use self
argument for dispatch. We can see that if we would subclass our Animal
class
and override can_eat
method definition:
>>> @has_multimethods
... class Predator(Animal):
... @Animal.can_eat.register(Meat)
... def can_eat(self, food):
... return True
register rule (<class '__main__.Predator'>, <class '__main__.Meat'>)
This will override can_eat
on Predator
instances but only for the case
for Meat
argument, case for the Vegetable
is not overridden, so class
inherits it from Animal
:
>>> predator = Predator()
>>> predator.can_eat(Vegetable())
True
>>> predator.can_eat(Meat())
True
The only thing to care is you should not forget to include @has_multimethods
decorator on classes which define or override multimethods.
You can also provide a “catch-all” case for multimethod using otherwise
decorator just like in example for multifunctions.
Providing “catch-all” case#
There should be an analog to else
statement – a case which is used when no
matching case is found, we will call such case a catch-all case, here is how
you can define it using otherwise
decorator:
>>> @has_multimethods
... class Animal(object):
...
... @multimethod(Vegetable)
... def can_eat(self, food):
... return True
...
... @can_eat.register(Meat)
... def can_eat(self, food):
... return False
...
... @can_eat.otherwise
... def can_eat(self, food):
... return "?"
register rule (<class '__main__.Animal'>, <class '__main__.Vegetable'>)
register rule (<class '__main__.Animal'>, <class '__main__.Meat'>)
register rule (<class '__main__.Animal'>, <class 'object'>)
>>> Animal().can_eat(1)
'?'
You can try calling sound
with whatever argument type you wish, it will
never fall with TypeError
anymore.
API reference#
- generic.multidispatch.multidispatch(*argtypes: Optional[type]) Callable[[T], FunctionDispatcher[T]] #
Declare function as multidispatch.
This decorator takes
argtypes
argument types and replace decorated function withFunctionDispatcher
object, which is responsible for multiple dispatch feature.
- generic.multimethod.multimethod(*argtypes: Optional[type]) Callable[[T], MethodDispatcher[T]] #
Declare method as multimethod.
This decorator works exactly the same as
multidispatch()
decorator but replaces decorated method withMethodDispatcher
object instead.Should be used only for decorating methods and enclosing class should have
has_multimethods()
decorator.
- generic.multimethod.has_multimethods(cls: type[C]) type[C] #
Declare class as one that have multimethods.
Should only be used for decorating classes which have methods decorated with
multimethod()
decorator.
- class generic.multidispatch.FunctionDispatcher(argspec: FullArgSpec, params_arity: int)#
Multidispatcher for functions.
This object dispatch calls to function by its argument types. Usually it is produced by
multidispatch()
decorator.You should not manually create objects of this type.
- register(*argtypes: Optional[type]) Callable[[T], T] #
Decorator for registering new case for multidispatch.
New case will be registered for types identified by
argtypes
. The length ofargtypes
should be equal to the length ofargtypes
argument were passed correspondingmultidispatch()
call, which also indicated the number of arguments multidispatch dispatches on.
- class generic.multimethod.MethodDispatcher(argspec: FullArgSpec, params_arity: int)#
Multiple dispatch for methods.
This object dispatch call to method by its class and arguments types. Usually it is produced by
multimethod()
decorator.You should not manually create objects of this type.