这个变量“实例化”的范围是什么

Question

我在这里看到了 Peter Norvig 的单例实现 class http://norvig.com/python-iaq.html

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，如果我们第二次创建两个 YourClass 实例，为什么 instantiated 不是空列表？ instantiated的范围是什么？

谢谢。

Answer 1

来自docs：

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.

instantiated 的值绑定到函数定义并且只在您定义singleton 时初始化一次。

因此，每次调用该函数时，您将只有一份相同列表的副本：

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

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

等同于：

>>> lst = []

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

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

如果您希望 instantiated 在后续函数调用之间被隔离，您应该将其定义为 local variable:

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

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

这个变量“实例化”的范围是什么

What is the scope of this variable `instantiated`

python

singleton

class