为什么对 class 属性有三个引用？

Question

让我们考虑这段代码：

import sys
import gc

class A:
    a = "something here to print"
    def __init__(self):
        pass

a = A()
print(sys.getrefcount(A.a))  # prints 3


refs = gc.get_referents(A.a)
print(len(refs))             # prints 0

我不明白为什么打印3。第三个引用在哪里？

为什么 gc.get_referents returns 是一个空列表？

Answer 1

sys 的文档回答了您问题的第一部分：

sys.getrefcount(object): Return the reference count of the object. The count returned is generally one higher than you might expect, because it includes the (temporary) reference as an argument to getrefcount().

考虑这个问题的第二部分：

import sys
import gc


class A:
    a = "something here to print"
    def __init__(self):
        pass

a = A()
print(sys.getrefcount(A.a))     # prints 3

refs = gc.get_referents(A.a)
print(len(refs))                # prints 0

refs2 = gc.get_referrers(A.a)   # prints 2 (what you expected)
print(len(refs2))

参见两种 gc 方法的文档：

gc.get_referents(*objs): Return a list of objects directly referred to by any of the arguments.[...]

gc.get_referrers(*objs): Return the list of objects that directly refer to any of objs. [...]

为什么对 class 属性有三个引用？

Why there are three references to the class attribute?

class-attributes

python-3.x