Change the Flyweight pattern to Pythonic (?) (1)

A memo that interprets the article taught by the person next to the side.

Flyweight pattern faithful to GoF

One factory is defined for one target. The code below is pretty much the same as the original article, with changes to give named arguments.

python

class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

class HogeFactory(object):
    def  __init__(self):
        self._instances = {}

    def get_instance(self, args1, kwargs1):
        if ((args1), (kwargs1)) not in self._instances:
            self._instances[((args1), (kwargs1))] = Hoge(args1, kwargs1=kwargs1)
        return self._instances[((args1), (kwargs1))]


class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

class PiyoFactory(object):
    def __init__(self):
        self._instances = {}

    def get_instance(self, args1, kwargs1='test1', kwargs2='test2'):
        if ((args1), (kwargs1, kwargs2)) not in self._instances:
            self._instances[((args1), (kwargs1, kwargs2))] = Piyo(args1, kwargs1=kwargs1, kwargs2=kwargs2)
        return self._instances[((args1), (kwargs1, kwargs2))]

hogeFactory = HogeFactory()
piyoFactory = PiyoFactory()

assert hogeFactory.get_instance(1, kwargs1=2) is hogeFactory.get_instance(1, kwargs1=2)
assert hogeFactory.get_instance(1, kwargs1=2) is not hogeFactory.get_instance(1, kwargs1=3)
assert piyoFactory.get_instance('a', kwargs1='b', kwargs2='c') is piyoFactory.get_instance('a', kwargs1='b', kwargs2='c')
assert piyoFactory.get_instance('a', kwargs1='b', kwargs2='c') is not piyoFactory.get_instance('a', kwargs1='b', kwargs2='d')

Generalization of Factory with variadic arguments

The reason why one factory was needed for one target was that each target's constructor had a different signature = get_instance method had a different signature. You can use Python's variadic arguments to absorb differences in signatures.

python

class FlyweightFactory(object):
    def __init__(self, cls):
        self._instances = {}
        self._cls = cls

    def get_instance(self, *args, **kwargs):
        return self._instances.setdefault((args, tuple(kwargs.items())), self._cls(*args, **kwargs))

class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

hogeFactory = FlyweightFactory(Hoge)
piyoFactory = FlyweightFactory(Piyo)

assert hogeFactory.get_instance(1, kwargs1=2) is hogeFactory.get_instance(1, kwargs1=2)
assert hogeFactory.get_instance(1, kwargs1=2) is not hogeFactory.get_instance(1, kwargs1=3)
assert piyoFactory.get_instance('a', kwargs1='b', kwargs2='c') is piyoFactory.get_instance('a', kwargs1='b', kwargs2='c')
assert piyoFactory.get_instance('a', kwargs1='b', kwargs2='c') is not piyoFactory.get_instance('a', kwargs1='b', kwargs2='d')

Factory decorator (class)

You can write it even simpler by decorating the above FlyweightFactory.

python

class Flyweight(object):
    def __init__(self, cls):
        self._instances = {}
        self._cls = cls

    def __call__(self, *args, **kwargs):
        return self._instances.setdefault((args, tuple(kwargs.items())), self._cls(*args, **kwargs))

@Flyweight
class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

@Flyweight
class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

assert Hoge(1, kwargs1=2) is Hoge(1, kwargs1=2)
assert Hoge(1, kwargs1=2) is not Hoge(1, kwargs1=3)
assert Piyo('a', kwargs1='b', kwargs2='c') is Piyo('a', kwargs1='b', kwargs2='c')
assert Piyo('a', kwargs1='b', kwargs2='c') is not Piyo('a', kwargs1='b', kwargs2='d')

The decorator is called by taking the attached target as an argument. The code equivalent to this process can be written as follows without using a decorator. Instead of calling get_instance, there is a difference in reading the Flyweight instance as a function, but you can see that it is not so different from the previous snippet.

(ref. http://docs.python.jp/3.4/reference/datamodel.html#object.call)

python

class Flyweight(object):
    def __init__(self, cls):
        self._instances = {}
        self._cls = cls

    def __call__(self, *args, **kwargs):
        return self._instances.setdefault((args, tuple(kwargs.items())), self._cls(*args, **kwargs))

class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

# Hoge,Take Piyo as an argument,Create a Flyweight instance.
Hoge = Flyweight(Hoge)
Piyo = Flyweight(Piyo)

assert Hoge(1, kwargs1=2) is Hoge(1, kwargs1=2)
assert Hoge(1, kwargs1=2) is not Hoge(1, kwargs1=3)
assert Piyo('a', kwargs1='b', kwargs2='c') is Piyo('a', kwargs1='b', kwargs2='c')
assert Piyo('a', kwargs1='b', kwargs2='c') is not Piyo('a', kwargs1='b', kwargs2='d')

Factory decorator (closure)

Furthermore, the implementation using the following closures does not have to be a class as long as it is callable.

python

def flyweight(cls):
    instances = {}
    return lambda *args, **kwargs: instances.setdefault((args, tuple(kwargs.items())), cls(*args, **kwargs))

@flyweight
class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

@flyweight
class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

assert Hoge(1, kwargs1=2) is Hoge(1, kwargs1=2)
assert Hoge(1, kwargs1=2) is not Hoge(1, kwargs1=3)
assert Piyo('a', kwargs1='b', kwargs2='c') is Piyo('a', kwargs1='b', kwargs2='c')
assert Piyo('a', kwargs1='b', kwargs2='c') is not Piyo('a', kwargs1='b', kwargs2='d')

The equivalent code without a decorator looks like this:

python

def flyweight(cls):
    instances = {}
    return lambda *args, **kwargs: instances.setdefault((args, tuple(kwargs.items())), cls(*args, **kwargs))

class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

Hoge = flyweight(Hoge)
Piyo = flyweight(Piyo)

assert Hoge(1, kwargs1=2) is Hoge(1, kwargs1=2)
assert Hoge(1, kwargs1=2) is not Hoge(1, kwargs1=3)
assert Piyo('a', kwargs1='b', kwargs2='c') is Piyo('a', kwargs1='b', kwargs2='c')
assert Piyo('a', kwargs1='b', kwargs2='c') is not Piyo('a', kwargs1='b', kwargs2='d')

This method is very smart, but it has the disadvantage that you can't create a subclass of the Hoge / Piyo class. This is because the Hoge / Piyo is no longer a class, but a function object, as you can see in the code that doesn't use decorators. Therefore, the following code will result in an error.

python

def flyweight(cls):
    instances = {}
    return lambda *args, **kwargs: instances.setdefault((args, tuple(kwargs.items())), cls(*args, **kwargs))

@flyweight
class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

@flyweight
class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

#Hoge is no longer a class and cannot be inherited
class Hoge2(Hoge):
    def __init__(self, args1, kwargs1='test1'):
        super().__init__(args1, kwargs1=kwargs1)

In the sense that the decorator makes Hoge / Piyo no longer a class, it doesn't change when the decorator is represented by a class, but there is an instance of the Flyweight class that replaces the Hoge / Piyo class. It is the miso that holds the original class. Therefore, it can be subclassed with the following code.

python

class Flyweight(object):
    def __init__(self, cls):
        self._instances = {}
        self._cls = cls

    def __call__(self, *args, **kwargs):
        return self._instances.setdefault((args, tuple(kwargs.items())), self._cls(*args, **kwargs))

@Flyweight
class Hoge(object):
    def __init__(self, args1, kwargs1='test1'):
        self.args1 = args1
        self.kwargs1 = kwargs1

@Flyweight
class Piyo(object):
    def __init__(self, args1, kwargs1, kwargs2):
        self.args1 = args1
        self.kwargs1 = kwargs1
        self.kwargs2 = kwargs2

# Hoge(The entity is a Flyweight instance)Is an instance variable that holds the original class_Have cls
class Hoge2(Hoge._cls):
    def __init__(self, args1, kwargs1='test1'):
        super().__init__(args1, kwargs1=kwargs1)