dictionary_switches / src / ChangeInterface.rst

Changing the Interface

Sometimes the problem that you're solving is as simple as "I don't have the interface that I want." Two of the patterns in Design Patterns solve this problem: Adapter takes one type and produces an interface to some other type. Façade creates an interface to a set of classes, simply to provide a more comfortable way to deal with a library or bundle of resources.

Adapter

When you've got this, and you need that, Adapter solves the problem. The only requirement is to produce a that, and there are a number of ways you can accomplish this adaptation:

# ChangeInterface/Adapter.py
# Variations on the Adapter pattern.

class WhatIHave:
    def g(self): pass
    def h(self): pass

class WhatIWant:
    def f(self): pass

class ProxyAdapter(WhatIWant):
    def __init__(self, whatIHave):
        self.whatIHave = whatIHave

    def f(self):
        # Implement behavior using
        # methods in WhatIHave:
        self.whatIHave.g()
        self.whatIHave.h()

class WhatIUse:
    def op(self, whatIWant):
        whatIWant.f()

# Approach 2: build adapter use into op():
class WhatIUse2(WhatIUse):
    def op(self, whatIHave):
        ProxyAdapter(whatIHave).f()

# Approach 3: build adapter into WhatIHave:
class WhatIHave2(WhatIHave, WhatIWant):
    def f(self):
        self.g()
        self.h()

# Approach 4: use an inner class:
class WhatIHave3(WhatIHave):
    class InnerAdapter(WhatIWant):
        def __init__(self, outer):
            self.outer = outer
        def f(self):
            self.outer.g()
            self.outer.h()

    def whatIWant(self):
        return WhatIHave3.InnerAdapter(self)

whatIUse = WhatIUse()
whatIHave = WhatIHave()
adapt= ProxyAdapter(whatIHave)
whatIUse2 = WhatIUse2()
whatIHave2 = WhatIHave2()
whatIHave3 = WhatIHave3()
whatIUse.op(adapt)
# Approach 2:
whatIUse2.op(whatIHave)
# Approach 3:
whatIUse.op(whatIHave2)
# Approach 4:
whatIUse.op(whatIHave3.whatIWant())

I'm taking liberties with the term "proxy" here, because in Design Patterns they assert that a proxy must have an identical interface with the object that it is a surrogate for. However, if you have the two words together: "proxy adapter," it is perhaps more reasonable.

Façade

A general principle that I apply when I'm casting about trying to mold requirements into a first-cut object is "If something is ugly, hide it inside an object." This is basically what Façade accomplishes. If you have a rather confusing collection of classes and interactions that the client programmer doesn't really need to see, then you can create an interface that is useful for the client programmer and that only presents what's necessary.

Façade is often implemented as singleton abstract factory. Of course, you can easily get this effect by creating a class containing static factory methods:

# ChangeInterface/Facade.py
class A:
    def __init__(self, x): pass
class B:
    def __init__(self, x): pass
class C:
    def __init__(self, x): pass

# Other classes that aren't exposed by the
# facade go here ...

class Facade:
    def makeA(x): return A(x)
    makeA = staticmethod(makeA)
    def makeB(x): return B(x)
    makeB = staticmethod(makeB)
    def makeC(x): return C(x)
    makeC = staticmethod(makeC)

# The client programmer gets the objects
# by calling the static methods:
a = Facade.makeA(1);
b = Facade.makeB(1);
c = Facade.makeC(1.0);

[rewrite this section using research from Larman's book]

Example for Facade (?): my "nicer" version of the XML library.

Exercises

  1. Create an adapter class that automatically loads a two-dimensional array of objects into a dictionary as key-value pairs.
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