Chapter 18 Inheritance 继承

The language feature most often associated with object-oriented programming is inheritance. Inheritance is the ability to define a new class that is a modified version of an existing class. In this chapter I demonstrate inheritance using classes that represent playing cards, decks of cards, and poker hands.

面向对象编程最常被人提到的语言功能就是继承了。继承就是基于一个已有的类进行修改来定义一个新的类。在本章我会用一些例子来演示继承,这些例子会用到一些类来表示扑克牌,成副的纸牌和扑克牌型。

If you don’t play poker, you can read about it at Here, but you don’t have to; I’ll tell you what you need to know for the exercises.

如果你没玩过扑克,你可以读一下这里的介绍,不过也没必要;因为我等会会把练习中涉及到的相关内容给你解释明白的。

Code examples from this chapter are available from Here.

本章的代码样例可以在这里下载。

18.1 Card objects 纸牌对象

There are fifty-two cards in a deck, each of which belongs to one of four suits and one of thirteen ranks. The suits are Spades, Hearts, Diamonds, and Clubs (in descending order in bridge). The ranks are Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, and King. Depending on the game that you are playing, an Ace may be higher than King or lower than 2.

牌桌上面一共有52张扑克牌,每一张都属于四种花色之一,并且是十三张牌之一。花色为黑桃,红心,方块,梅花(在桥牌中按照降序排列)。排列顺序为 A,2,3,4,5,6,7,8,9,10,J,Q,K。根据具体玩的游戏的不同,A 可以比 K 大,也可以比2还小。

If we want to define a new object to represent a playing card, it is obvious what the attributes should be: rank and suit. It is not as obvious what type the attributes should be. One possibility is to use strings containing words like 'Spade' for suits and 'Queen' for ranks. One problem with this implementation is that it would not be easy to compare cards to see which had a higher rank or suit.

如果咱们要定义一个新的对象来表示一张牌,很明显就需要两个属性了:点数以及花色。但这两个属性应该是什么类型呢,就不那么明显了。一种思路是用字符串,就比如用『黑桃』来表示花色,『Q』来表示点数。不过这个实现方法不怎么方便,不好去比较纸牌的点数大小以及花色。

An alternative is to use integers to encode the ranks and suits. In this context, “encode” means that we are going to define a mapping between numbers and suits, or between numbers and ranks. This kind of encoding is not meant to be a secret (that would be “encryption”).

另外一种思路是用整数来编码,以表示点数和花色。在这里,『编码』的意思就是我们要建立一个从数值到花色或者从数值到点数的映射。这种编码并不是为了安全的考虑(那种情况下用的词是『encryption(也是编码的意思,专用于安全领域)』)。

For example, this table shows the suits and the corresponding integer codes:

例如,下面这个表格就表示了花色与整数编码之间的映射关系:

Spades    ↦    3
Hearts    ↦    2
Diamonds    ↦    1
Clubs    ↦    0

This code makes it easy to compare cards; because higher suits map to higher numbers, we can compare suits by comparing their codes.

这样的编码就比较易于比较牌的大小;因为高花色对应着大数值,我们对比一下编码大小就能比较花色顺序。

The mapping for ranks is fairly obvious; each of the numerical ranks maps to the corresponding integer, and for face cards:

牌面大小的映射就很明显了;每一张牌都对应着相应大小的整数,对于有人像的几张映射如下所示:

Jack    ↦    11
Queen    ↦    12
King    ↦    13

I am using the ↦ symbol to make it clear that these mappings are not part of the Python program. They are part of the program design, but they don’t appear explicitly in the code.

我这里用箭头符号 ↦ 来表示映射关系,但这个符号并不是 Python 所支持的。这些符号是程序设计的一部分,但最终并不以这种形式出现在代码里。

The class definition for Card looks like this:

这样实现的纸牌类的定义如下所示:

class Card:
"""Represents a standard playing card."""
    def __init__(self, suit=0, rank=2):
        self.suit = suit
        self.rank = rank

As usual, the init method takes an optional parameter for each attribute. The default card is the 2 of Clubs. To create a Card, you call Card with the suit and rank of the card you want.

一如既往,init 方法可以为每一个属性接收一个可选参数来初始化。默认的牌面为梅花2.

要建立一张纸牌,可以用你想要的花色和牌值调用 Card。

queen_of_diamonds = Card(1, 12)

18.2 Class attributes 类的属性

In order to print Card objects in a way that people can easily read, we need a mapping from the integer codes to the corresponding ranks and suits. A natural way to do that is with lists of strings. We assign these lists to class attributes:

想要以易于被人理解的方式来用 print 打印输出纸牌对象,我们就得建立一个从整形编码到对应的牌值和花色的映射。最自然的方法莫过于用字符串列表来实现。咱们可以先把这些列表赋值到类的属性中去:

# inside class Card:
suit_names = ['Clubs', 'Diamonds', 'Hearts', 'Spades']
rank_names = [None, 'Ace', '2', '3', '4', '5', '6', '7','8', '9', '10', 'Jack', 'Queen', 'King']
def __str__(self):
    return '%s of %s' % (Card.rank_names[self.rank],Card.suit_names[self.suit])

Variables like suit_names and rank_names, which are defined inside a class but outside of any method, are called class attributes because they are associated with the class object Card.

suit_names 和 rank_names 这样的变量,都是在类内定义,但不在任何方法之内,这就叫做类的属性,因为它们属于类 Card。

This term distinguishes them from variables like suit and rank, which are called instance attributes because they are associated with a particular instance.

这种形式就把类的属性与变量 suit 和 rank 区分开来,后面这两个变量叫做实例属性,因为这两个属性取决于具体的实例。

Both kinds of attribute are accessed using dot notation. For example, in__str__, self is a Card object, and self.rank is its rank. Similarly, Card is a class object, and Card.rank_names is a list of strings associated with the class.

这些属性都可以用点号来读取。比如,在__str__方法中,self 是一个 Card 对象,而 self.rank 就是该对象的 rank 变量。同理,Card 是一个class 对象,而 Card.rank_names 就是属于该类的一个字符串列表。

Every card has its own suit and rank, but there is only one copy of suit_names and rank_names.

没一张牌都有自己的花色和牌值,但都只有唯一的一套 suit_names 和 rank_names。

Putting it all together, the expression Card.rank_names[self.rank] means “use the attribute rank from the object self as an index into the list rank_names from the class Card, and select the appropriate string.”

放到一起,这个表达式Card.rank_names[self.rank]的意思就是『用对象 self 的 rank 属性作为一个索引,从类 Card 中的rank_names 列表中选择该索引位置的字符串。』

The first element of rank_names is None because there is no card with rank zero. By including None as a place-keeper, we get a mapping with the nice property that the index 2 maps to the string '2', and so on. To avoid this tweak, we could have used a dictionary instead of a list.

rank_names 的一个元素是None空,因为没有牌值为0的纸牌。包含 None 在内作为一个替位符,整个映射就很简明,索引2的位置对应着就是字符串「2」,其他牌值依此类推。要是觉得这样太别扭,咱们还可以用字典来替代列表。

With the methods we have so far, we can create and print cards:

目前已经有了这些方法了,咱们就可以创建和打印输出纸牌了:

>>> card1 = Card(2, 11)
>>> card1 = Card(2, 11)
>>> print(card1)
>>> print(card1)
Jack of Hearts

Figure 18.1: Object diagram. Figure 18.1: Object diagram.


Figure 18.1 is a diagram of the Card class object and one Card instance. Card is a class object; its type is type. card1 is an instance of Card, so its type is Card. To save space, I didn’t draw the contents of suit_names and rank_names.

图18.1是一个 Card 类对象以及一个 Card 实例的图解。Card 是一个类对象(就是类的一个实例);它的类型是type。card1是 Card 的一个实例,所以它的类型是 Card。为了节省空间,我没有画出 suit_names 和 rank_names 的内容。

18.3 Comparing cards 对比牌值

For built-in types, there are relational operators (<, >, ==, etc.) that compare values and determine when one is greater than, less than, or equal to another. For programmer-defined types, we can override the behavior of the built-in operators by providing a method named __lt__, which stands for “less than”.

对于内置类型,直接就可以用关系运算符(<, >, ==,等等)比较两个值来判断二者的大小以及是否相等。对与用户自定义类型,咱们就要覆盖掉内置运算符的行为,这就需要提供一个名为__lt__的方法,这个lt 就是『less than』的缩写,意思是『小于』。

__lt__ takes two parameters, self and other, and True if self is strictly less than other. The correct ordering for cards is not obvious. For example, which is better, the 3 of Clubs or the 2 of Diamonds? One has a higher rank, but the other has a higher suit. In order to compare cards, you have to decide whether rank or suit is more important.

__lt__接收两个参数,一个是self,一个是另外一个对象,如果 self 严格小于另外一个对象,就返回真。

纸牌的牌值大小排列并不是很简单。比如,梅花3和方块2哪个更大呢?一个的牌值更高,但另一个的花色更高。所以要进行比较的话,你就得确定牌值和花色哪个更重要。

The answer might depend on what game you are playing, but to keep things simple, we’ll make the arbitrary choice that suit is more important, so all of the Spades outrank all of the Diamonds, and so on.

实际上这种关系还得取决于你玩的纸牌游戏中的规则,不过为了简单起见,咱们就做一个武断的选择,就让花色更重要,所以所有的黑桃都大于方块,依此类推了。

With that decided, we can write __lt__:

确定好规则了,就可以写这个__lt__方法了:

# inside class Card:
def __lt__(self, other):
# check the suits
    if self.suit < other.suit:
        return True
    if self.suit > other.suit:
        return False
# suits are the same... check ranks
    return self.rank < other.rank

You can write this more concisely using tuple comparison:

用元组对比就可以把代码写得更简洁了:

# inside class Card:
def __lt__(self, other):
    t1 = self.suit, self.rank
    t2 = other.suit, other.rank
    return t1 < t2

As an exercise, write an __lt__ method for Time objects. You can use tuple comparison, but you also might consider comparing integers.

做个练习,为 Time 对象写一个__lt__方法。可以用元组对比,不过也可以对比整数。

18.4 Decks 成副的纸牌

Now that we have Cards, the next step is to define Decks. Since a deck is made up of cards, it is natural for each Deck to contain a list of cards as an attribute.

现在咱们已经有了纸牌的类了,接下来的一不就是定义成副纸牌了。因为一副纸牌上是有各种牌,所以很自然就应该包含一个纸牌列表作为一个属性了。

The following is a class definition for Deck. The init method creates the attribute cards and generates the standard set of fifty-two cards:

下面就是一个一副纸牌类的定义。init 方法建立了一个属性 cards,然后生成了标准的五十二张牌来初始化。

class Deck:
    def __init__(self):
        self.cards = []
        for suit in range(4):
            for rank in range(1, 14):
                card = Card(suit, rank)
                self.cards.append(card)

The easiest way to populate the deck is with a nested loop. The outer loop enumerates the suits from 0 to 3. The inner loop enumerates the ranks from 1 to 13. Each iteration creates a new Card with the current suit and rank, and appends it to self.cards.

实现一副牌的最简单方法就是用网状循环了。外层循环枚举花色从0到3一共四种。内层的循环枚举从1到13的所有牌值。每一次循环都以当前的花色和牌值创建一个新的Card 对象,添加到 self.cards 列表中。

18.5 Printing the deck 输出整副纸牌

Here is a __str__ method for Deck:

下面是 Deck 类的__str__方法:

#inside class Deck:
def __str__(self):
    res = []
    for card in self.cards:
        res.append(str(card))
    return '\n'.join(res)

This method demonstrates an efficient way to accumulate a large string: building a list of strings and then using the string method join. The built-in function str invokes the __str__ method on each card and returns the string representation.

上面的方法展示了累积大字符串的一种有效方法:建立一个字符串列表,然后用字符串方法 join 实现。内置函数 str 调用每一张牌的__str__方法,然后返回该张纸牌的字符串表示。

Since we invoke join on a newline character, the cards are separated by newlines. Here’s what the result looks like:

由于我们调用 join的位置在换行符后面,这样这些纸牌就被换行符分开了。程序运行结果如下所示:

>>> deck = Deck()
>>> deck = Deck()
>>> print(deck)
>>> print(deck)
Ace of Clubs
2 of Clubs
3 of Clubs
...
10 of Spades
Jack of Spades
Queen of Spades
King of Spades

Even though the result appears on 52 lines, it is one long string that contains newlines.

虽然结果看上去是52行,但实际上只是一个包含了很多换行符的一个长字符串。

18.6 Add, remove, shuffle and sort 添加,删除,洗牌和排序

To deal cards, we would like a method that removes a card from the deck and returns it. The list method pop provides a convenient way to do that:

要处理纸牌,我们还需要一个方法来从牌堆中拿出和放入纸牌。列表的 pop 方法很适合来完成这件任务:

#inside class Deck:
def pop_card(self):
    return self.cards.pop()

Since pop removes the last card in the list, we are dealing from the bottom of the deck. To add a card, we can use the list method append:

pop 方法从列表中拿走最后一张牌,这样就是从一副牌的末尾来处理。

要添加一张牌,可以用列表的 append 方法:

#inside class Deck:
def add_card(self, card):
    self.cards.append(card)

A method like this that uses another method without doing much work is sometimes called a veneer. The metaphor comes from woodworking, where a veneer is a thin layer of good quality wood glued to the surface of a cheaper piece of wood to improve the appearance.

上面这种方法都是调用了其他的方法,而没有做什么别的事情,所以也被叫做镶板。这个比喻来自于木匠行业,镶板就是一薄层的高端木料用胶水贴到廉价木料上面,来提高视觉效果。

In this case add_card is a “thin” method that expresses a list operation in terms appropriate for decks. It improves the appearance, or interface, of the implementation.

在刚刚的例子中,add_card 就相当于那个『高端』的方法,表示的是适用于处理纸牌的列表操作。这样就提高了程序实现的可读性,或者说改善了接口。

As another example, we can write a Deck method named shuffle using the function shuffle from the random module:

再举一个例子,咱们再来给 Deck写一个洗牌的方法,用 random(随机的意思)模块的 shuffle 方法:

# inside class Deck:
def shuffle(self):
    random.shuffle(self.cards)

Don’t forget to import random. As an exercise, write a Deck method named sort that uses the list method sort to sort the cards in a Deck. sort uses the __lt__ method we defined to determine the order.

一定别忘了导入 random 模块。

做个练习吧,写一个名为 sort 的方法给 Deck,使用列表的sort 方法来给 Deck 中的牌进行排序。sort 方法要用到我们之前写过的 __lt__ 方法来确定顺序。

18.7 Inheritance 继承

Inheritance is the ability to define a new class that is a modified version of an existing class. As an example, let’s say we want a class to represent a “hand”, that is, the cards held by one player. A hand is similar to a deck: both are made up of a collection of cards, and both require operations like adding and removing cards.

继承就是基于已有的类进行修改来获取新类的能力。举个例子,比方说我们需要一个表示『一手牌』的类,这个就是指一个牌手手中拿着的牌。『一手牌』和『一副牌』有些相似:都是由一系列的纸牌组成的,也都要有添加和移除纸牌的运算。

A hand is also different from a deck; there are operations we want for hands that don’t make sense for a deck. For example, in poker we might compare two hands to see which one wins. In bridge, we might compute a score for a hand in order to make a bid.

『一手牌』还和『一副牌』有所区别;对于手中的牌有一些运算并不适用于整副的牌。比如说,在扑克游戏中,我们可能需要对比两手牌来看看哪一副胜利。在桥牌里面,还可能需要对手中的牌进行计分以决胜负。

This relationship between classes—similar, but different—lends itself to inheritance. To define a new class that inherits from an existing class, you put the name of the existing class in parentheses:

类之间这种相似又有区别的关系,就适合用继承来实现了。要继承一个已有的类来定义新类,就要把已有类的名字放到括号中,如下所示:

class Hand(Deck):
"""Represents a hand of playing cards."""

This definition indicates that Hand inherits from Deck; that means we can use methods like pop_card and add_card for Hands as well as Decks.

上面这样的定义就表示了 Hand 继承了 Deck;也就意味着我们可以在 Hands 中使用 Decks 中的那些方法,比如 pop_card 以及 add_card 等等。

When a new class inherits from an existing one, the existing one is called the parent and the new class is called the child.

当一个新类继承了一个已有的类时,这个已有的类就叫做基类,新定义的类叫做子类。

In this example, Hand inherits __init__ from Deck, but it doesn’t really do what we want: instead of populating the hand with 52 new cards, the init method for Hands should initialize cards with an empty list. If we provide an init method in the Hand class, it overrides the one in the Deckclass:

在本章的这个例子中,Hand 类从 Deck 类继承了__init__方法,但这个方法和我们的需求还不一样:Hand类的 init 方法应该用一个空列表来初始化手中的牌,而不是像 Deck 类中那样用一整副52张牌。

# inside class Hand:
def __init__(self, label=''):
    self.cards = []
    self.label = label

When you create a Hand, Python invokes this init method, not the one in Deck.

像上面这样改写一下之后,这样再建立一个 Hand 类的时候,Python 就会调用这个自定义的 init 方法,而不是 Deck 当中的。

>>> hand = Hand('new hand')
>>> hand = Hand('new hand')
>>> hand.cards []
>>> hand.cards []
>>> hand.label
>>> hand.label
'new hand'

The other methods are inherited from Deck, so we can use pop_card and add_card to deal a card:

其他方法都从 Deck 类中继承了过来,所以我们就可以直接用 pop_card 和 add_card 方法来处理纸牌了:

>>> deck = Deck()
>>> deck = Deck()
>>> card = deck.pop_card()
>>> card = deck.pop_card()
>>> hand.add_card(card)
>>> hand.add_card(card)
>>> print(hand)
>>> print(hand)
King of Spades

A natural next step is to encapsulate this code in a method called move_cards:

接下来很自然地,我们把这段名为 move_cards 的方法放进去:

#inside class Deck:
def move_cards(self, hand, num):
    for i in range(num):
        hand.add_card(self.pop_card())

move_cards takes two arguments, a Hand object and the number of cards to deal. It modifies both self and hand, and returns None.

move_cards 方法接收两个参数,一个 Hand 对象,以及一个要处理的纸牌数量。该方法会修改 self 和 hand。返回为空。

In some games, cards are moved from one hand to another, or from a hand back to the deck. You can use move_cards for any of these operations: self can be either a Deck or a Hand, and hand, despite the name, can also be a Deck.

在有的游戏中,纸牌需要从一手牌拿出去放到另外一手牌中去,或者从手中拿出去放到牌堆里面。这就亏用 move_cards 来实现这些操作:第一个变量 self 可以是一副牌也可以是一手牌,第二个变量虽然名字是 hand,实际上也可以是一个 Deck 对象。

Inheritance is a useful feature. Some programs that would be repetitive without inheritance can be written more elegantly with it. Inheritance can facilitate code reuse, since you can customize the behavior of parent classes without having to modify them. In some cases, the inheritance structure reflects the natural structure of the problem, which makes the design easier to understand.

继承是一个很有用的功能。有的程序如果不用继承的话就会有很多重复代码,用继承来写出来就会更简洁很多了。继承有助于代码重用,因为你可以对基类的行为进行定制而不用去修改基类本身。在某些情况下,继承的结构也反映了要解决的问题中的自然关系,这就让程序设计更易于理解。

On the other hand, inheritance can make programs difficult to read. When a method is invoked, it is sometimes not clear where to find its definition. The relevant code may be spread across several modules. Also, many of the things that can be done using inheritance can be done as well or better without it.

然而继承也容易降低程序可读性。当调用一个方法的时候,有时候不容易找到该方法的定义位置。相关的代码可能跨了好几个模块。此外,很多事情可以用继承来实现,但不用继承也能做到同样效果,甚至做得更好。

18.8 Class diagrams 类图

So far we have seen stack diagrams, which show the state of a program, and object diagrams, which show the attributes of an object and their values. These diagrams represent a snapshot in the execution of a program, so they change as the program runs.

目前为止,我们见过栈图了,栈图是展示一个程序的状态的,我们还见过对象图了,表示的是一个对象中的各个属性及其值。这些图都是对一个程序运行中某个瞬间的反映,因此随着程序运行而产生变化。

They are also highly detailed; for some purposes, too detailed. A class diagram is a more abstract representation of the structure of a program. Instead of showing individual objects, it shows classes and the relationships between them.

这些图解还都非常详细;有的时候就都过于繁琐冗余了。而类图则是对一个程序结构的更抽象的表示。类图并不会表现出各个独立的对象,而是会表现出程序中的各个类以及它们之间的关系。

There are several kinds of relationship between classes:

类之间有很多种关系,大概如下所示:

• Objects in one class might contain references to objects in another class. For example, each Rectangle contains a reference to a Point, and each Deck contains references to many Cards. This kind of relationship is called HAS-A, as in, “a Rectangle has a Point.”

一个类中的对象可能包含了另一个类对象的引用。例如,每一个 Rectangle (矩形)对象都包含了对 Point(点)的引用,而每一个 Deck (成副的牌)对象都包含了对很多个 Card (纸牌)对象的引用。这种关系也叫做『含有』,就好比是说,『一个矩形中含有一个点。』

• One class might inherit from another. This relationship is called IS-A, as in, “a Hand is a kind of a Deck.”

一类可能继承了其他的类。这种关系也可以叫做『是一个』,比如说,『一手牌就是一种牌的组合。』

• One class might depend on another in the sense that objects in one class take objects in the second class as parameters, or use objects in the second class as part of a computation. This kind of relationship is called a dependency.

一种类可能要依赖其他类,比如一个类中的对象用另外一个类中的对象作为参数,或者用做计算中的某一部分。这种关系就叫做『依赖』。

A class diagram is a graphical representation of these relationships. For example, Figure 18.2 shows the relationships between Card, Deck and Hand.

类图就是对这些关系的一个图形化的表示。比如,在途18.2中,就展示了 Card,Deck 以及 Hand 三个类的关系。


Figure 18.2: Class diagram. Figure 18.2: Class diagram.


The arrow with a hollow triangle head represents an IS-A relationship; in this case it indicates that Hand inherits from Deck.

有空心三角形的箭头表示了『是一个』的关系;在这里意思就是 Hand 继承了 Deck。

The standard arrow head represents a HAS-A relationship; in this case a Deck has references to Card objects.

另一个箭头表示了『有一个』的关系;在这里的意思是 Deck 当中有若干对 Card 对象的引用。

The star (*) near the arrow head is a multiplicity; it indicates how many Cards a Deck has. A multiplicity can be a simple number, like 52, a range, like 5..7 or a star, which indicates that a Deck can have any number of Cards.

箭头处有个小星号*;这里可以表明一个 Deck 中含有的 Card的个数。可以标出个数,比如52,或者是范围,比如5..7或者一个星号,这就意味着一个 Deck 中可以含有任意个数的 Card。

There are no dependencies in this diagram. They would normally be shown with a dashed arrow. Or if there are a lot of dependencies, they are sometimes omitted.

这个图解中没有出现依赖关系。这种关系一般用虚线箭头来表示。或者当依赖关系很多的时候,有时候就都忽略掉了。

A more detailed diagram might show that a Deck actually contains a list of Cards, but built-in types like list and dict are usually not included in class diagrams.

更细节化的图解就可能表现出一个 Deck 中会包含一个 Card 对象组成的列表,但一般情况下类图不会包括内置类型比如列表和字典。

18.9 Debugging 调试

Inheritance can make debugging difficult because when you invoke a method on an object, it might be hard to figure out which method will be invoked.

继承可以让调试变得很夸你呢,因为你调用某个对象中的某个方法的时候,很难确定到底是调用的哪一个方法。

Suppose you are writing a function that works with Hand objects. You would like it to work with all kinds of Hands, like PokerHands, BridgeHands, etc. If you invoke a method like shuffle, you might get the one defined in Deck, but if any of the subclasses override this method, you’ll get that version instead. This behavior is usually a good thing, but it can be confusing.

假设你写一个处理 Hand 对象的函数。你可能要让该函数适用于所有类型的牌型,比如常规牌型,桥牌牌型等等。假设你要调用洗牌的方法 shuffle,你可能用的是 Deck 类当中的,不过如果子类当中有覆盖的该方法,你运行的就是子类中的方法了。这种行为一般是很有好处的,不过也容易把人弄糊涂。

Any time you are unsure about the flow of execution through your program, the simplest solution is to add print statements at the beginning of the relevant methods. If Deck.shuffle prints a message that says something like Running Deck.shuffle, then as the program runs it traces the flow of execution.

在你的程序运行的过程中,只要你对程序流程有疑问了,就可以在相关的方法头部添加print 语句来打印输出一下信息,这就是最简单的解决方法了。如果 Deck.shuffle 输出了信息比如说『在运行 Deck 的 shuffle』,那就可以根据这些信息来追踪执行流程了。

As an alternative, you could use this function, which takes an object and a method name (as a string) and returns the class that provides the definition of the method:

另外一个思路,就是用下面这个函数,该函数接收一个对象和一个方法的名字(作为字符串),然后返回提供该方法定义的类的名称。

def find_defining_class(obj, meth_name):
    for ty in type(obj).mro():
        if meth_name in ty.__dict__:
            return ty

Here’s an example:

如下所示:

>>> hand = Hand()
>>> hand = Hand()
>>> find_defining_class(hand, 'shuffle')
>>> find_defining_class(hand, 'shuffle')
<class 'Card.Deck'>

So the shuffle method for this Hand is the one in Deck. find_defining_class uses the mro method to get the list of class objects (types) that will be searched for methods. “MRO” stands for “method resolution order”, which is the sequence of classes Python searches to “resolve” a method name.

所这样就能判断这里面 Hand 中的 shuffle 方法是来自 Deck 的。

find_defining_class 用了 mro 方法来获取所有搜索方法的类对象的列表。

『MRO』的意思是『method resolution order(方法 解决方案 顺序)』,也就是 Python 搜索来找到方法名的类的序列。

Here’s a design suggestion: when you override a method, the interface of the new method should be the same as the old. It should take the same parameters, return the same type, and obey the same preconditions and postconditions. If you follow this rule, you will find that any function designed to work with an instance of a parent class, like a Deck, will also work with instances of child classes like a Hand and PokerHand.

下面是一个在设计上的建议:当你覆盖一个方法的时候,新的方法的接口最好同旧的完全一致。应该接收同样的参数,返回同样类型,并且遵循同样的前置条件和后置条件。只要你遵守这个规则,你就会发现所有之前设计来处理一个基类的函数,比如处理 Deck 的,就都可以用于子类的实例上面,比如 Hand 类或者 PokerHand 类。

If you violate this rule, which is called the “Liskov substitution principle”, your code will collapse like (sorry) a house of cards.

如果你违背了上面这个『里氏替换原则』,你的代码就可能很悲剧地崩溃,就像无数纸牌坍塌一样。

18.10 Data encapsulation 数据封装

The previous chapters demonstrate a development plan we might call “object-oriented design”. We identified objects we needed—like Point,Rectangle and Time—and defined classes to represent them. In each case there is an obvious correspondence between the object and some entity in the real world (or at least a mathematical world).

之前的章节中,我们展示了所谓『面向对象设计』的开发规划模式。在这些章节中,我们显示确定好需要的对象—比如点,矩形以及时间—然后再定义一些类去代表这些内容。在这些例子中,类的对象与现实世界(或者至少是数学世界)中的一些实体都有显著的对应关系。

But sometimes it is less obvious what objects you need and how they should interact. In that case you need a different development plan. In the same way that we discovered function interfaces by encapsulation and generalization, we can discover class interfaces by data encapsulation.

不过有时候就不太好确定具体需要什么样的对象,以及如何去实现。这时候就需要一种完全不同的开发规划模式了。之前我们对函数接口进行过封装和泛化的处理,现在也可以通过数据封装来改进类的接口。

Markov analysis, from Section 13.8, provides a good example. If you download my code from Here , you’ll see that it uses two global variables—suffix_map and prefix—that are read and written from several functions.

比如马科夫分析,在13.8中出现的,就是一个很好的例子。如果你从这里下载了我的样例代码,你就会发现这里用了两个全局变量—suffix_map 以及 prefix—这两个全局变量会被多个函数读取和写入。

suffix_map = {}
prefix = ()

Because these variables are global, we can only run one analysis at a time. If we read two texts, their prefixes and suffixes would be added to the same data structures (which makes for some interesting generated text).

这些变量是全局的,因此我们每次只运行了一次分析。如果我们要读取两个文本,他们的前置和后置词汇都会被添加到同样的数据结构上面去(这样就能生成一些有趣的机器制造的文本了)。

To run multiple analyses, and keep them separate, we can encapsulate the state of each analysis in an object. Here’s what that looks like:

如果要运行多次分析,并且要对这些分析进行区分,我们可以把每次分析的状态封装到对象中。如下所示:

class Markov:
    def __init__(self):
        self.suffix_map = {}
        self.prefix = ()

Next, we transform the functions into methods. For example, here’s process_word:

接下来就是把各个函数转换成方法。例如下面就是process_word 方法:

def process_word(self, word, order=2):
    if len(self.prefix) < order:
        self.prefix += (word,)
    return
    try:
        self.suffix_map[self.prefix].append(word)
    except
        KeyError:             # if there is no entry for this prefix, make one
    self.suffix_map[self.prefix] = [word]
    self.prefix = shift(self.prefix, word)

Transforming a program like this—changing the design without changing the behavior—is another example of refactoring (see Section 4.7).

上面这种方式对程序进行的修改只是改变了设计,而不改变程序的行为,这就是重构的另一个例子(参考4.7)。

This example suggests a development plan for designing objects and methods:

这一样例展示了一种设计累的对象和方法的开发规划模式:

  1. Start by writing functions that read and write global variables (when necessary).

先开始写一些函数来读去和写入全局变量(在必要的情况下)。

  1. Once you get the program working, look for associations between global variables and the functions that use them.

一旦程序可以工作了,就检查一下全局变量与使用它们的函数之间的关系。

  1. Encapsulate related variables as attributes of an object.

把相关的变量作为类的属性封装到一起。

  1. Transform the associated functions into methods of the new class.

把相关的函数转换成新类的方法。

As an exercise, download my Markov code from Here, and follow the steps described above to encapsulate the global variables as attributes of a new class called Markov. Solution (note the capital M).

做一个练习,从这里下载我的马科夫分析代码,然后根据上面说的步骤来一步步把全局变量封装成一个名为 Markov 的新类的属性。样例代码 (一定要注意 M 是大写的哈)

18.11 Glossary 术语列表

encode: To represent one set of values using another set of values by constructing a mapping between them.

编码:通过建立映射的方式来用一系列的值来表示另外一系列的值。

class attribute: An attribute associated with a class object. Class attributes are defined inside a class definition but outside any method.

类的属性:属于某个类的对象的属性。类的属性都在类定义的内部,在类内方法之外。

instance attribute: An attribute associated with an instance of a class.

实例属性:属于某个类的实例的属性。

veneer: A method or function that provides a different interface to another function without doing much computation.

嵌板:某一方法或者函数,为另外的函数提供了不同的界面,而没有做额外运算。

inheritance: The ability to define a new class that is a modified version of a previously defined class.

继承:基于已定义过的类,进行修改来定义一个新类,这种特性就是继承。

parent class: The class from which a child class inherits.

基类:被子类继承的类。

child class: A new class created by inheriting from an existing class; also called a “subclass”.

子类:基于已有类而建立的新类;也称为『分支类』。

IS-A relationship: A relationship between a child class and its parent class.

『是一个』关系:一种子类与基类之间的关系。

HAS-A relationship: A relationship between two classes where instances of one class contain references to instances of the other.

『有一个』关系:某一个类的实例中包含其他类的实例的引用的关系。

dependency: A relationship between two classes where instances of one class use instances of the other class, but do not store them as attributes.

依赖关系:两个类之间的一种关系,一个类的实例使用了另外一个类的实例,但并未作为属性来存储。

class diagram: A diagram that shows the classes in a program and the relationships between them.

类图:一种展示程序中各个类及其之间关系的图解。

multiplicity: A notation in a class diagram that shows, for a HAS-A relationship, how many references there are to instances of another class.

多样性:类图中显示的一种记号,适用于『有一个』关系中,表示一个类当中另一个类的实例的引用的个数。

data encapsulation: A program development plan that involves a prototype using global variables and a final version that makes the global variables into instance attributes.

数据封装:一种程序开发规划方式,用全局变量做原型体,然后逐步将这些全局变量转换成实例的属性。

18.12 Exercises 练习

Exercise 1 练习1

For the following program, draw a UML class diagram that shows these classes and the relationships among them.

阅读下面的代码,画一个 UML 类图,表示出程序中的类,以及类之间的关系。

class PingPongParent:
    pass  class Ping(PingPongParent):
    def __init__(self, pong):
            self.pong = pong
class Pong(PingPongParent):
    def __init__(self, pings=None):
        if pings is None:
            self.pings = []
        else:
            self.pings = pings
    def add_ping(self, ping):
        self.pings.append(ping)
        pong = Pong()
        ping = Ping(pong)
        pong.add_ping(ping)

Exercise 2 练习2

Write a Deck method called deal_hands that takes two parameters, the number of hands and the number of cards per hand. It should create the appropriate number of Hand objects, deal the appropriate number of cards per hand, and return a list of Hands.

为 Deck 类写一个名为 deal_hands 的方法,接收两个参数,一个为牌型数量,一个为每一个牌型的纸牌数。该方法需要创建适当的牌型对象数量,处理适当的每个牌型中的纸牌数,然后返回一个牌型组成的列表。

Exercise 3 练习3

The following are the possible hands in poker, in increasing order of value and decreasing order of probability:

下面是扑克牌中可能的各个牌型,排列顺序为值的升序,出现概率的降序:

pair: two cards with the same rank

一对:两张同样牌值的牌

two pair: two pairs of cards with the same rank

双对:两对同样牌值的牌

three of a kind: three cards with the same rank

三张:三张同样牌值的牌

straight: five cards with ranks in sequence (aces can be high or low, so Ace-2-3-4-5 is a straight and so is 10-Jack-Queen-King-Ace, but Queen-King-Ace-2-3 is not.)

顺子:五张牌值连续的牌(A 可以用作开头,也可以用作结尾,所以 A-2-3-4-5是一个顺子,10-J-Q-K-A 也是一个,但 Q-K-A-2-3就不行了。)

flush: five cards with the same suit

同花:五张牌花色一致

full house: three cards with one rank, two cards with another

三带二:三张同牌值的牌,两张另外的同牌值的牌

four of a kind: four cards with the same rank

四条:四张同一牌值的牌

straight flush: five cards in sequence (as defined above) and with the same suit

同花顺:五张组成顺子并且是同一花色的牌

The goal of these exercises is to estimate the probability of drawing these various hands.

此次练习的目的就是要估计获得以上各个牌型的概率。

  1. Download the following files from Here

这个网址下载下面的文件:

:Card.py : A complete version of the Card, Deck and Hand classes in this chapter.

该文件是本章所涉及的 Card,Deck 以及 Hand 类的完整实现。

PokerHand.py : An incomplete implementation of a class that represents a poker hand, and some code that tests it.

该文件是一个不完整版本的类,表示的是一个牌型,以及一些测试代码。

  1. If you run PokerHand.py, it deals seven 7-card poker hands and checks to see if any of them contains a flush. Read this code carefully before you go on.

如果你运行 PokerHand.py,改程序会处理七个七张牌的牌型,然后检查是否其中包含一副顺子。

好好阅读一下这份代码,然后再继续后面的练习。

  1. Add methods to PokerHand.py named has_pair, has_twopair, etc. that return True or False according to whether or not the hand meets the relevant criteria. Your code should work correctly for “hands” that contain any number of cards (although 5 and 7 are the most common sizes).

在 PokerHand.py 里面增加名为has_pair, has_twopair等等方法。这些方法根据牌型中是否满足特定的组合而返回 True 或者 False。你的代码应该能适用于有任意张牌的牌型(虽然5或者7是最常见的牌数)。

  1. Write a method named classify that figures out the highest-value classification for a hand and sets the label attribute accordingly. For example, a 7-card hand might contain a flush and a pair; it should be labeled “flush”.

写一个名为 classify 的函数,判断出一副牌型中的最高值的一份,然后用来命名到标签属性。例如,一个七张牌的牌型可能包含一个顺子和一个对子;这就应该被标为『顺子』。

  1. When you are convinced that your classification methods are working, the next step is to estimate the probabilities of the various hands. Write a function in PokerHand.py that shuffles a deck of cards, divides it into hands, classifies the hands, and counts the number of times various classifications appear.

当你确定你的分类方法运转正常了,下一步就是要估计各个牌型的出现概率。在 PokerHand.py 中写一个函数来对一副牌进行洗牌,分成多个牌型,对各个牌型进行分类,然后统计不同类型出现的次数。

  1. Print a table of the classifications and their probabilities. Run your program with larger and larger numbers of hands until the output values converge to a reasonable degree of accuracy. Compare your results to the values at Here.

打印输出一个由类型和概率组成的列表。逐步用大规模的牌型来测试你的程序,直到输出的值趋向于一个比较合理的准确范围。把你的运行结果与这里的结果进行对比。

Solution 样例代码

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