笨方法学Python（41-45）

习题41、物以类聚

#!/usr/bin/python
# -*- coding:utf-8 -*-

class TheThing(object):

    def __init__(self):
        self.number = 0

    def some_function(self):
        print "I got called."

    def add_me_up(self, more):
        self.number += more
        return self.number

# two different things      
a = TheThing()
b = TheThing()

a.some_function()
b.some_function()

print a.add_me_up(20)
print b.add_me_up(30)

print a.number
print b.number

class TheMultiplier(object):

    def __init__(self, base):
        self.base = base

    def do_it(self, m):
        return m * self.base

x = TheMultiplier(a.number)
print x.do_it(b.number)

你看到参数里的self了吧？你知道它是什么东西吗？对了，它就是 Python 创建的额外的一个参数，有了它你才能实现a.some_function() 这种用法，这时它就会把\ 前者翻译成some_function(a)执行下去。为什么用self呢？因为你的函数并不知道你的这个“实例”是来自叫 TheThing 或者别的名字的class。所以你只要使用一个通用的名字 self 。这样你写出来的函数就会在任何情况下都能正常工作。

习题42、对象、类、以及从属关系

#!/usr/bin/python
# -*- coding:utf-8 -*-

#重写41，不采用class

class Animal(object):
    pass


class Dog(Animal):

    def __init__(self, name):
        self.name = name


class Cat(Animal):

    def __init__(self, name):
        self.name = name


class Person(object):

    def __init__(self, name):
        self.name = name
        self.pet = None


class Employee(Person):

    def __init__(self, name, salary):
        super(Employee, self).__init__(name)
        self.salary = salary


class Fish(object):
    pass

class Salmon(Fish):
    pass

class Halibut(Fish):
    pass

rover = Dog("Rover")

satan = Cat("Satan")

mary = Person("Mary")

小结

1. 这句self.pet = None有什么用？
   确保类的self.pet属性被设置为None。
2. super(Employee, self).__init__(name) 是做什么用的？
   这样你可以可靠地将父类的__init__方法运行起来。搜索“python super”，看看它的优缺点。

---

习题43、

cities['_find'] = find_city
city_found = cities['_find'](cities, state)

第二行代码可以分解成如下步骤：

1. Python 看到 city_found =于是知道了需要创建一个变量。
2. 然后它读到 cities，然后知道了它是一个字典
3. 然后看到了 ['_find'] ，于是 Python 就从索引找到了字典cities中对应的位置，并且获取了该位置的内容。
4. ['_find']这个位置的内容是我们的函数find_city，所以 Python 就知道了这里表示一个函数，于是当它碰到(就开始了函数调用。
5. cities, state这两个参数将被传递到函数find_city中，然后这个函数就被运行了。
6. find_city接着从cities中寻找states，并且返回它找到的内容，如果什么都没找到，就返回一个信息说它什么都没找到。
7. Python find_city接受返回的信息，最后将该信息赋值给一开始的city_found这个变量。

import random
from urllib import urlopen
import sys

WORD_URL = "http://learncodethehardway.org/words.txt"
WORDS = []

PHRASES = {
    "class ###(###):":
      "Make a class named ### that is-a ###.",
    "class ###(object):\n\tdef __init__(self, ***)" :
      "class ### has-a __init__ that takes self and *** parameters.",
    "class ###(object):\n\tdef ***(self, @@@)":
      "class ### has-a function named *** that takes self and @@@ parameters.",
    "*** = ###()":
      "Set *** to an instance of class ###.",
    "***.***(@@@)":
      "From *** get the *** function, and call it with parameters self, @@@.",
    "***.*** = '***'":
      "From *** get the *** attribute and set it to '***'."
}

# do they want to drill phrases first
PHRASE_FIRST = False
if len(sys.argv) == 2 and sys.argv[1] == "english":
    PHRASE_FIRST = True

# load up the words from the website
for word in urlopen(WORD_URL).readlines():
    WORDS.append(word.strip())


def convert(snippet, phrase):
    class_names = [w.capitalize() for w in
                   random.sample(WORDS, snippet.count("###"))]
    other_names = random.sample(WORDS, snippet.count("***"))
    results = []
    param_names = []

    for i in range(0, snippet.count("@@@")):
        param_count = random.randint(1,3)
        param_names.append(', '.join(random.sample(WORDS, param_count)))

    for sentence in snippet, phrase:
        result = sentence[:]

        # fake class names
        for word in class_names:
            result = result.replace("###", word, 1)

        # fake other names
        for word in other_names:
            result = result.replace("***", word, 1)

        # fake parameter lists
        for word in param_names:
            result = result.replace("@@@", word, 1)

        results.append(result)

    return results


# keep going until they hit CTRL-D
try:
    while True:
        snippets = PHRASES.keys()
        random.shuffle(snippets)

        for snippet in snippets:
            phrase = PHRASES[snippet]
            question, answer = convert(snippet, phrase)
            if PHRASE_FIRST:
                question, answer = answer, question

            print question

            raw_input("> ")
            print "ANSWER:  %s\n\n" % answer
except EOFError:
    print "\nBye"

---

习题44、继承(Inheritance) VS 合成(Composition)

父类和子类有三种交互方式：

1. 子类上的动作完全等同于父类上的动作
2. 子类上的动作完全改写了父类上的动作
3. 子类上的动作部分变更了父类上的动作

隐式继承

#!/usr/bin/python
# -*- coding:utf-8 -*-

#隐式继承 Implicit Inheritance
class Parent(object):

    def implicit(self):
        print "PARENT implicit()"

class Child(Parent):
    pass

dad = Parent()
son = Child()

dad.implicit()
son.implicit()

显示继承

#!/usr/bin/python
# -*- coding:utf-8 -*-

#显式覆写
class Parent(object):

    def override(self):
        print "PARENT override()"

class Child(Parent):

    def override(self):
        print "CHILD override()"

dad = Parent()
son = Child()

dad.override()
son.override()

在运行前或运行后覆写

#!/usr/bin/python
# -*- coding:utf-8 -*-

综合

#!/usr/bin/python
# -*- coding:utf-8 -*-

class Parent(object):

    def override(self):
        print "PARENT override()"

    def implicit(self):
        print "PARENT implicit()"

    def altered(self):
        print "PARENT altered()"

class Child(Parent):

    def override(self):
        print "CHILD override()"

    def altered(self):
        print "CHILD, BEFOR PARENT altered()"
        super(Child, self).altered() 
        print "CHILD, AFTER PARENT altered()"

dad = Parent()
son = Child()

dad.implicit()
son.implicit()

dad.override()
son.override()

dad.altered()
son.altered()

习题45、你来制作一个游戏

函数的风格

1. 1.

类的风格

1. 1.

代码风格

1. 1.

好的注释