Question

class object_restrict(object):
    _count = 0

    def __new__(cls):
        if cls._count > 5:
            raise TypeError("Too many keys created")

        cls._count += 1
        print "object created"

    def __init__(self):
        pass

k = object_restrict()
k1 = object_restrict()
k2 = object_restrict()
k3 = object_restrict()
k4 = object_restrict()
k5 = object_restrict()

我似乎对如何在Python中限制类的对象数量有一些疑问。我被要求编写一个程序，我应该把条件放在我们只能创建一个类的5个实例的情况下，如果我们尝试创建超过5个，它应该引发异常。

正如我们在Python中所知，__new__是在需要创建实例时调用的方法。我试着写一些代码，但它没有用。

当我运行此代码时，它运行了6次。请有人在这指导我吗？我也试过检查谷歌，但没有得到任何正确的代码。

Answer 1

正如Wilfred指出的那样_count检查或变量初始化存在问题。但我想指出另一个问题，即你没有返回对象的实例。如果你有这个：

class A(object):

    _count = 0 

    def __new__(cls):
        if cls._count > 5:
            raise Exception('Too many instances were created')
        cls._count += 1

    def __init__(self):
        pass

asd = A() 
print asd

将输出：

$ None

你应该返回对象的实例，实际上是__new__责任：

class A(object):

    _count = 0 

    def __new__(cls):
        if cls._count >= 5:  # @Wilfred fix
            raise Exception('Too many instances were created')
        cls._count += 1
        return super(A, cls).__new__(cls)  # returning the instance

    def __init__(self):
        pass

asd = A() 
print asd

输出：

$ <__main__.A object at 0x7f5ddad7db10>

Answer 2

class object_restrict(object):
    _count = 0

    def __new__(cls):
        cls._count += 1
        if cls._count > 5:
            raise TypeError("Too many keys created")

        print cls._count, "object created"

    def __init__(self):
        pass

k = object_restrict()
k1 = object_restrict()
k2 = object_restrict()
k3 = object_restrict()
k4 = object_restrict()
k5 = object_restrict()

Answer 3

您将_count变量初始化为0。

首先致电：_count = 0

第六个电话：_count = 5，所以他可以创建你的对象。

初始化_count = 1或将您的if条件更新为cls._count >= 5:

Answer 4

允许实例化最多5个对象的类（Python 3）：

class RestrictClass(object):
    __count = 0
    def __new__(cls):
        if cls.__count>=5:
            raise TypeError("Already instantiated")
        cls.__count+=1
        return super(RestrictClass,cls).__new__(cls)

    def __init__(self):
        pass

    def showNumberOfObjects(self):
        print("Number of objects are now: "+str(self.__count))


k1 = RestrictClass()
k1.showNumberOfObjects()
k2 = RestrictClass()
k2.showNumberOfObjects()
k3 = RestrictClass()
k3.showNumberOfObjects()
k4 = RestrictClass()
k4.showNumberOfObjects()
k5 = RestrictClass()
k5.showNumberOfObjects()
k6 = RestrictClass()
k6.showNumberOfObjects()

输出：

Number of objects are now: 1
Number of objects are now: 2
Number of objects are now: 3
Number of objects are now: 4
Number of objects are now: 5
Traceback (most recent call last):
  File "/media/arsho/Documents/pyPrac/object.py", line 27, in <module>
    k6 = RestrictClass()
  File "/media/arsho/Documents/pyPrac/object.py", line 5, in __new__
    raise TypeError("Already instantiated")
TypeError: Already instantiated

用于限制Python中对象创建的程序

4 个答案: