类和实例属性之间有什么区别? [英] What is the difference between class and instance attributes?

问题描述

两者之间是否存在有意义的区别?

Is there any meaningful distinction between:

class A(object):
    foo = 5   # some default value

vs.

class B(object):
    def __init__(self, foo=5):
        self.foo = foo

如果要创建很多实例，这两种样式在性能或空间要求上是否有差异?阅读代码时，您是否认为这两种样式的含义明显不同?

If you're creating a lot of instances, is there any difference in performance or space requirements for the two styles? When you read the code, do you consider the meaning of the two styles to be significantly different?

推荐答案

除了性能方面的考虑之外，还有很大的语义差异.在类属性的情况下，仅引用一个对象.在实例属性设置实例中，可以有多个引用对象.例如

Beyond performance considerations, there is a significant semantic difference. In the class attribute case, there is just one object referred to. In the instance-attribute-set-at-instantiation, there can be multiple objects referred to. For instance

>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
...  def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo    
[]

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