为什么我的实例计数器在这个Python代码中显示0？

Question

我制作了一个简单的代码来演示和理解类 - 但是当我运行它时,我的列表显示它们是空的,包含"None"值而不是用户输入的字符串作为名称.

#Static methods do not require the object to be initiated. Can be remotely accessed from outside the function .

#Counting critters and remote access.

class Orc (object):

    total = 0       

    def get_score (self):
        print "The number of orcs the orc factory has made is",Orc.total   

    def __init__ (self):
        Orc.total += 1
        name =  raw_input ("I am a critter by the name of:\n")


#Creating 10 Orcs 


list = []

for i in range (4):    list[i] = list.append(Orc.get_score(Orc()))  

print "You have created 4 Orcs!" print "The name of your first orc is",list[0] print "The name of your fourth orc is", list[3]

Answer 1

您的代码中存在一些错误.首先是你使用列表的方式.其次,在对象上调用方法的方式.错误的组合解释了为什么你最后有一个列表None.

列表名称

list = []

不要列出名单list.它已经是,列表类的名称,即在Python中你可以做my_list = []或my_list = list()具有完全相同的效果.

你想把你的列表称为有意义的东西,比如 orc_list

列表插入

for i in range (4):
    orc_list[i] = orc_list.append(...)

orc_list.append做它说的:它将一个元素附加到给定的列表.但是,它并没有返回任何东西.那么你的代码正在做什么

1. 拿一个空列表
2. 设为i0
3. 将您传递的内容插入append列表末尾
4. 插入None索引i,从而覆盖你在3中所做的事情.
5. 递增 i
6. 回到3.

你想简单地使用 orc_list.append(...)

方法调用

Orc.get_score(Orc())

我想你会被这个self论点搞糊涂了.在类中,Python将自动传递您正在处理的实例作为self参数.您不需要提供该参数.

你想写

Orc().get_score()

这会创建一个Orc对象,然后调用get_score它.Python' 为您注入' Orc您创建的实例get_score.

方法返回

我们现在已经到了

orc_list.append(Orc().get_score())

这相当于

score = Orc().get_score()
orc_list.append(score)

问题是没有return声明get_score.这意味着None当您调用该方法时,python将返回.这意味着您要附加None到列表中.

你想拥有

def get_score(self):
    print "The number of orcs the orc factory has made is", Orc.total
    return Orc.total

静态行为

如果你真的想要一个没有绑定到Orc类实例的方法,你可以使用类方法或静态方法.

在您的情况下,您不需要对类对象执行任何操作,因此您可以选择使用静态方法.

你会宣布

@staticmethod
def get_score():
    print "The number of orcs the orc factory has made is", Orc.total

然后,您将使用该方法调用 Orc.get_score()

Answer 2

要在Python中定义类方法,请使用classethoddecorator并调用第一个参数cls

class Orc(object):

    total = 0       

    @classmethod # this will make the method a class method
    def get_score (cls): # convention is then to call the 1st param 'cls'
        print "The number of orcs the orc factory has made is", cls.total   

    def __init__ (self):
        Orc.total += 1
        # use self is you want' to register a name
        # however putting a raw_input in an __init__ is NOT recommanded
        # you should pass name as a parameter 
        # and call the raw_input in the for loop
        self.name =  raw_input ("I am a critter by the name of:\n")

orcs = [] # don't call your lists 'list' because `list` is standard Python function

for i in range(4): # put this on two lines for clarity or use a comprehension list
   orcs.append(Orc())  

print "You have created 4 Orcs!" 
print "The name of your first orc is", orcs[0].name # if you don't use `name`, you will see the reference of the object

print "The name of your fourth orc is", orcs[3].name

更清洁的版本(你应该瞄准的东西):

class Orc(object):

    total = 0       

    @classmethod # 
    def get_instances_count(cls):
        """
            Return the number or orcs that have been instanciated 
        """
        # ^ Put some documentation below your method
        # these are called "docstring" and are detected by Python

        # you should return values in method rather than print
        # there are rare cases when you do want print, but when you'll
        # encounter them, you won't need me to correct your code anymore
        return cls.total   

    def __init__ (self, name):
        Orc.total += 1
        self.name = name # we get the name as a parameter


l = []

for i in range(4): # put this on two lines for clarity or use a comprehension list
   orc = Orc(raw_input("Enter a name:\n"))
   l.append(orc)  

print "You have created %s Orcs!" % Orc.get_instances_count()
print "The name of your first orc is", l[0].name #

print "The name of your fourth orc is", l[3].name

现在更多的Pythonic版本(一旦用于Python,你应该可以做的事情):

class Orc(object):

    total = 0       

    # you don't need accessors in Python: most things are public anyway
    # and you got property

    def __init__ (self, name):
        Orc.total += 1
        self.name = name # we get the name as a parameter

    def __str__(self):
      # this will be call when printing an orc
      return self.name

# list comprehension are quick and clean ways to create lists
# give a real name to your list
orcs = [Orc(raw_input("Enter a name:\n")) for i in range(4)]

# using parenthesis for `print` is a good habit to take with then incoming Python 3
print("You have created %s Orcs!" % Orc.total)
for i, orc in enumerate(orcs):
  print("Orc #%s is %s" % (i, orc))