`__init__` 能做什么而 `__new__` 不能?
问题描述
在 Python 中,__new__
用于初始化不可变类型,__init__
通常用于初始化可变类型.如果 __init__
从语言中删除,什么不能再做(容易)?
In Python, __new__
is used to initialize immutable types and __init__
typically initializes mutable types. If __init__
were removed from the language, what could no longer be done (easily)?
例如,
class A:
def __init__(self, *, x, **kwargs):
super().__init__(**kwargs)
self.x = x
class B(A):
def __init__(self, y=2, **kwargs):
super().__init__(**kwargs)
self.y = y
可以像这样使用 __new__
重写:
Could be rewritten using __new__
like this:
class A_N:
def __new__(cls, *, x, **kwargs):
obj = super().__new__(cls, **kwargs)
obj.x = x
return obj
class B_N(A_N):
def __new__(cls, y=2, **kwargs):
obj = super().__new__(cls, **kwargs)
obj.y = y
return obj
<小时>
澄清问题范围:这不是关于如何使用 __init__
和 __new__
或它们之间有什么区别的问题.这是一个关于如果从语言中删除 __init__
会发生什么的问题.有什么会坏吗?有什么事情会变得更难或不可能吗?
Clarification for scope of question: This is not a question about how __init__
and __new__
are used or what is the difference between them. This is a question about what would happen if __init__
were removed from the language. Would anything break? Would anything become a lot harder or impossible to do?
解决方案
你可以在 __init__
中做的所有事情,也可以在 __new__
中完成.
Everything you can do in __init__
can also be done in __new__
.
那么,为什么要使用 __init__
?
因为您不必将实例存储在变量中(示例代码中的 obj
),然后再费心返回它.你可以专注于你真正想做的事 –初始化可变对象.
Then, why use __init__
?
Because you don't have to store instance in variable (obj
in your example code), and later bother returning it. You can focus on what you realy want to do – initializing mutable object.
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