python – 这个变量`instantiated`的范围是什么

发布时间：2020-12-20 13:52:06 所属栏目：Python 来源：网络整理

导读：我在 http://norvig.com/python-iaq.html遇到了Peter Norvig的单身人士课程 def singleton(object,instantiated=[]): "Raise an exception if an object of this class has been instantiated before." assert object.__class__ not in instantiated, "%s i

我在 http://norvig.com/python-iaq.html遇到了Peter Norvig的单身人士课程

def singleton(object,instantiated=[]):
    "Raise an exception if an object of this class has been instantiated before."
    assert object.__class__ not in instantiated,
        "%s is a Singleton class but is already instantiated" % object.__class__
    instantiated.append(object.__class__)

class YourClass:
    "A singleton class to do something ..."
    def __init__(self,args):
       singleton(self)
       ...

我的问题是,如果我们创建两个YourClass实例,在第二次,为什么实例化不是一个空列表？实例化的范围是什么？

谢谢.

解决方法

从 docs开始：

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list,dictionary,or instances of most classes. It accumulates the arguments passed to it on subsequent calls.

实例化的值绑定到函数定义,并且在定义单例时仅初始化一次.

因此,每次调用该函数时,您只有一个相同列表的副本：

def test(x,instantiated=[]):
    instantiated.append(x)
    print instantiated

>>> test(3)
[3]
>>> test(5)
[3,5]
>>> test(6)
[3,5,6]

它与：

>>> lst = []

>>> def test(x,instantiated):
...     instantiated.append(x)
...     print instantiated

>>> test(3,lst)
[3]
>>> test(5,lst)
[3,5]
>>> test(6,6]

如果要在后续函数调用之间隔离实例化,则应将其定义为局部变量：

def test(x,instantiated=None):
    if instantiated is None:
        instantiated = []
    instantiated.append(x)
    print instantiated

>>> test(3)
[3]
>>> test(5)
[5]
>>> test(6)
[6]

（编辑：李大同）

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