其中哪些在Python中是不可变的? [英] Which of these are immutable in Python?

问题描述

我试图弄清楚以下内容在Sage中是否是不可变的(它是基于Python构建的，因此我相信如果它在python中是不可变的，我相信在大多数情况下它将在Sage中是不可变的)

I am trying to figure out whether the following are immutable in Sage (which is built on Python so I believe if it is immutable in python, I believe in most cases it will be immutable in Sage)

下面是对象e，f，g，i

Below are objects e, f, g, i

class e: pass
f = e()
g = pi # (g's "type" in Sage is symbolic expression. It's supposed to be 3.1415....etc)
i = lambda x: x*x

我认为e是一个类，这意味着它是可变的(不可变的类有意义吗?难道不是所有类都可以修改吗?).由于f是类的实例，所以我猜测它也是可变的，因为类是可变的.

I gather that e is a class which means it is mutable (Does an immutable class make sense? Can't all classes be modified?). Since f is an instance of a class, I am guessing it is also mutable since classes are mutable.

由于数字在Python中是不可变的，因此g也应该是不可变的，因为尽管它是非理性的，但g是一个数字

Since numbers are immutable in Python, g should be immutable as well since it is a number despite being irrational

最后，我是一个函数，这意味着它应该是可变的?

Finally i is a function which means it should be mutable?

我不太确定我了解不变性的概念.函数不可变意味着什么?要让一个类是不变的?

I'm not quite sure I understand that concept of immutability. What would it mean for a function to be immutable? For a class to be immutable?

推荐答案

e是可变的.例如，您可以在类上添加新方法:e.foo = lambda self,x: x.

e is mutable. You can, for instance, add a new method on the class: e.foo = lambda self,x: x.

f是可变的.例如，您可以向该类实例添加一个新字段:f.x = 99.

f is mutable. You can, for instance, add a new field to this class instance: f.x = 99.

g是不可变的.您对此无能为力.

g is immutable. You can't change anything about it.

i不是不可变的.您可以对它进行各种邪恶的事情:i.func_code = (lambda x: 123).func_code之后，i(10)将是123，而不是100.(您也可以对其进行更合理的处理.i.__doc__ = "This function returns the square of its argument."之后，您将从i.__doc__ = "This function returns the square of its argument."获得更有用的结果c9>.)

i is not immutable. You can do all sorts of evil things to it: i.func_code = (lambda x: 123).func_code after which i(10) will be 123 instead of 100. (You can also do more reasonable things to it. After i.__doc__ = "This function returns the square of its argument." you will get a more helpful result from help(i).)

如果您可以对某个对象执行某些操作以更改其将来的行为，则该对象是可变的.您不能更改10的行为.您可以更改功能对象，类，类实例或列表的行为. (但不是元组.一旦创建了元组，它就会一直存在直到存在.)

An object is mutable if there's something you can do to the object that changes its possible future behaviour. You can't change the behaviour of 10; you can change the behaviour of a function object, or a class, or a class instance, or a list. (But not a tuple. Once a tuple is made, it stays just as it is for as long as it exists.)

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