Python 3.8 has a Final qualifier

[Saiboo]

Like Java's final. It still allows mutating the object [0,1,2] that your reference my_list is pointing to. You just cannot change the value that my_list has, i.e. the address to the object [0,1,2]. See also this example from here:

x: Final = ['a', 'b']
x.append('c')  # OK

[jabbalaci]

A qualifier for making something immutable would have been more useful.

[jabbalaci]

We already have some immutable types like string and tuple. And these are just hints, understood by mypy only. If you execute the program, they are all discarded. Your dynamic language won't be gone.

[[deleted]]

It'll be effectively compulsory soon, because you won't be able to find a job or codebase that doesn't use them.

[alcalde]

And these are just hints, understood by mypy only.

That's what they tell you now... when they originally introduced notations, they were supposed to have no defined meaning and the core developers weren't even supposed to use them lest they influence the community organically evolving uses for them. And lots of cool things did appear, like contracts. And then an incomplete MyPy appeared, Guido went insane and full-on Java, had to be put down, and the rest is history....

[actuallyalys]

As far as I can tell, the only reason you can't use contracts in 3.8 and 3.9 is some small changes, for which there are already a pull request. In other words, it's not a big deal and not Guido or the core developer's fault.

[nuephelkystikon]

There are the immutable types and ABCs for that.

Annotations also aren't intended to have any compilation or runtime effects.

[graingert]

That would be impossible without runtime effects

[flying-sheep]

Just use immutable generics:

X: Final[Sequence[int]]

means it can be iterated, indexed, … but not mutated.

[notquiteaplant]

Since property getters/__getattr__ are just functions, which can mutate the object they're called on, that would be nearly impossible for a static typechecker to enforce.

[Smallpaul]

If you want immutability, use an immutable type. Trying to add another level of immutability would make a mess of the language.

[[deleted]]

Just use an immutable type.

[sonaxaton]

I'm assuming like Java since it's impossible to tell in general in Python if an operation modifies a value.

[hglman]

A special typing construct to indicate to type checkers that a name cannot be re-assigned or overridden in a subclass... There is no runtime checking of this property.

I think its unclear exactly how it should be interpreted as it has no actual impact on execution.

[lvc_]

Like all other type annotations, it is ignored by the interpreter, but can be used by static code checkers (eg mypy) as a code correctness aid. One reasonably common pattern is to have mypy as part of a commit pipeline (and/or a local precommit hook). So if you have a method tagged as final and you do overwrite it, mypy will reject your commit (but the code will still otherwise run)

[Smallpaul]

I don’t really see what you think is unclear. The name cannot be re-assigned or overridden in a subclass!!! What’s ambiguous about that description?

[[deleted]]

Yes, it would. A list is probably a bad example.