class Foo(float):
...
C = Foo(1.23)
given a class/obj definition like this, is there any way for me to make C read-only? I want it to raise an error if C = ... is ever called after the object C is initialized.
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Owen Mehta-Jones
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1Possible duplicate of [How do I create a constant in Python?](https://stackoverflow.com/questions/2682745/how-do-i-create-a-constant-in-python) – fferri Nov 04 '19 at 21:24
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What do you mean by "immutable"? Do you mean changing the state of the object referred to by the name `C`? Or do you mean reassigning the name `C` to refer to a different value? – Code-Apprentice Nov 04 '19 at 21:24
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@Code-Apprentice reassigning the name C – Owen Mehta-Jones Nov 04 '19 at 21:25
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1No, there is no way to prevent reassigning a name in Python. – Code-Apprentice Nov 04 '19 at 21:25
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@CaitLANJenner not quite, that is talking about changing the attributes in the object at C, I want it so that the name C cannot be reassigned – Owen Mehta-Jones Nov 04 '19 at 21:26
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"Immutable" is the wrong word for that. I think the right one is "read-only". – wjandrea Nov 04 '19 at 21:27
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@wjandrea yes, will update question to match. – Owen Mehta-Jones Nov 04 '19 at 21:29
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Python does not have immutable variables. – juanpa.arrivillaga Nov 04 '19 at 21:32
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What you are asking for is akin to Java's `final` qualifier. This is different than "immutable". – Code-Apprentice Nov 04 '19 at 21:48
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Quick note, `C` is a name, not an object. This is an instance of `Foo`, not `C`. This may seem like splitting hairs, but it's an important distinction in python. The *value* `Foo(1.28)` is referred to by name `C`, and can be referred to by many other names. `a = b = c = Foo(1.28)` still produces only one instance of `Foo` here – C.Nivs Nov 04 '19 at 21:52
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@C.Nivs so then is there any way for python to recognize that the name ```C``` has changed what it is pointing at? – Owen Mehta-Jones Nov 04 '19 at 22:18
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@OwenMehta-Jones python doesn't provide any way for you to accomplish what you desire here. – juanpa.arrivillaga Nov 04 '19 at 22:58
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It's impossible, but you might find my answer to [Can I prevent modifying an object in Python?](https://stackoverflow.com/questions/3711657/can-i-prevent-modifying-an-object-in-python) useful. – martineau Nov 04 '19 at 23:06
2 Answers
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It's not possible. You can make the Foo object immutable but you can't make variables with references to it immutable. The object doesn't own the variable or have any control over it.
John Kugelman
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No.
No matter what you assign to C, C itself is not an instance of whatever class you create. C is a variable. Operations that operate on the variable, like C = something_new, don't even look at your object except to decrement the refcount. You can define how operations on your object behave, but you can't redefine the semantics of the variable itself.
user2357112
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So could I make it so that when the refcount is decremented, it produces an error? – Owen Mehta-Jones Nov 04 '19 at 21:28
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@OwenMehta-Jones you cannot without hacking the interpreter itself. – juanpa.arrivillaga Nov 04 '19 at 21:32
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1@OwenMehta-Jones: Refcounts go up and down all the time. Even if you could raise an error when your object's refcount goes down, doing so would render your objects completely unusable. It'd be like a doomsday device that goes off whenever someone stops looking at it (or blinks while looking at it). – user2357112 Nov 04 '19 at 21:35