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Interfaces: From Protocols to ABCs

An abstract class represents an interface. —— Bjarne Stroustrup Creator of C++

[TOC]

11，12，13 章都在讨论 Python 的接口和继承，比较晦涩，就算看懂了，到了用的时候也忘了，所以这几章暂时不学了，等用到的时候再看。

Protocols / Interface

Python 有两种接口（协议），一种是隐式的：魔法方法协议（鸭子类型），一种是显式的：ABCs。

Python 可以和 Java 一样，实现多继承（Java 实现多个接口）。

隐式：魔法方法协议

只要某一个类型走起路来像一直鸭子，叫声也像一只鸭子，那么这个类型就是鸭子的子类，即使这个类型并没有显式的继承自鸭子类。例如：

class Struggle:
    def __len__(self): return 23
    
isinstance(Struggle(), abc.Sized)
True

只要实现了 __len__ 的魔法方法，那么其就是 abc.sized 的子类，即使该类并没有显式的进行继承。这也从侧面反映出，isinstance 和 issubclass 这两个个方法在 Python 中是一个比较鸡肋的存在。

实际上，Python 很多的协议都是根据魔法方法来体现的。我实现了某个魔法方法，就相当于继承了某个类，拥有了该类的特性。

Monkey-Patching 猴子补丁

Monkey-Patching 用于在运行时修改类或者模块，其并不会改变源码。

Demo:

对于一个自定义的 Sequence，只有实现了 __setitem__() 方法，才能实现 shuffle，有时候我们在源码中可能会忘记加这一方法，在交互式 Console 中执行时，我们意识到了需要实现该方法时，我们可以不需要改动源码，通过动态加入该方法的方式，完成补丁。

>>> from random import shuffle
>>> from frenchdeck import FrenchDeck
>>> deck = FrenchDeck()
>>> shuffle(deck)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File ".../python3.3/random.py", line 265, in shuffle
x[i], x[j] = x[j], x[i]
TypeError: 'FrenchDeck' object does not support item assignment

>>> def set_card(deck, position, card): # 动态的创建出一个 function
... 	deck._cards[position] = card
...
>>> FrenchDeck.__setitem__ = set_card # 将补丁赋值给该类
>>> shuffle(deck)
>>> deck[:5]

Interfaces and protocols in Python culture

protected and private attributes

By definition, protected and private attributes are not part of an interface, even if “protected” is merely a naming convention (the single leading underscore) and private attributes are easily accessed.

Python 对 Interface 的理解

Python 的接口，其侧重点在：像不像（鸭子类型），而 Java 的接口，则遵循严格的协议。

A useful complementary definition of interface is: the subset of an object’s public methods that enable it to play a specific role in the system. That’s what is implied when the Python documentation mentions “a file-like object” or “an iterable”, without specifying a class. An interface seen as a set of methods to fulfill a role is what Smalltalkers called a procotol, and the term spread to other dynamic language communities. Protocols are independent of inheritance. A class may implement several protocols, enabling its instances to fulfill several roles.

Protocols are interfaces, but because they are informal — defined only by documentation and conventions — protocols cannot be enforced like formal interfaces can (we’ll see how ABCs enforce interface conformance later in this chapter). A protocol may be partially implemented in a particular class, and that’s OK. I point this out to emphasize that “X-like object”, “X protocol” and “X interface” are synonyms in the minds of Pythonistas.

显式：ABCs

当 ABCs(Abstract Base Classes) 被引入 Python（Python2.6） 的时候，Python 已经 15 岁了。

Python 内置的 ABCs 及使用

Python 内置的 ABCs 主要分布在两个 module 中：

  • collections.abc
    • collections.abc.Sequence
    • collections.abc.Size
    • ……
  • numbers
    • Complex
    • Real
    • Integral
    • ……

collections.abc

from collections.abc import Sequence

numbers

import numbers
isinstance(3, numbers.Real) # True
isinstance(3.3, numbers.Real) # True
isinstance(True, numbers.Real) # True
isinstance(3, numbers.Integral) # True

如何定义自己的 ABCs 并使用

继承 abc.ABC，然后抽象函数用装饰器：@abstractmethod 来装饰

策略设计模式的例子：

from abc import ABC, abstractmethod

class Promotion(ABC): # The Strategy: an abstract base class，抽象基类
    """促销基类，定下契约"""
    @abstractmethod
    def discount(self, order):
        """折扣"""
        pass


class FidelityPromotion(Promotion): # concrete strategy
    """5% discount for customers with 1000 or more fidelity points"""
    def discount(self, order):
        return 0 if order.customer.fidelity < 1000 else order.total() * .05


class BulkItemPromotion(Promotion): # concrete strategy
    """10% discount for each LineItem with 20 or more units"""
    def discount(self, order):
        discount = 0
        for item in order.cart:
            if item.quantity >= 20:
                discount += item.total() * .1 # Python 的小数可以不写 0
        return discount

注意：abc.ABC 是 Python3.4 以后才引入的，Python3.4 之前要创建 ABCs，应该使用 metaclass