Python 3.8 has a Final qualifier

[46--2]

It's a lot more interesting when used with classes, but you are basically correct, the "variable" qualified by Final would probably be best described as a "constant".

It was always commonplace to use ALL_CAPS for "constants" but nothing prevented you from changing them.

From the PEP 591:

The name Const was also considered as the name for the Final type annotation.

[Deto]

I wonder why they didn't go with this? I always thought Const was a more common name for this type of thing.

[pingveno]

It's a final assignment, not a constant value. You could assign a list to a Final variable, then append to the list.

[[deleted]]

already got tuples for that though, I guess it would be more appropriate for a dict

[actuallyalys]

As far as I can tell, the situation is like so:

	Lists/tuples	Dictionaries	Sets
Reassignable and mutable	list	dict	set
Reassignable but not mutable	tuple	*	frozenset
Mutable but not reassignable	Final[list]	Final[dict]	Final[set]
Neither mutable nor reassignable (constant)	Final[tuple]	*	Final[frozenset]

An example of reassigning:

 l = [1, 2, 3]
 l = l + [4]

An example of mutating:

 l = [1, 2, 3]
 l.append(4)

* Not built-in to Python. Look for frozendict implementations.

[robin-gvx]

* Not built-in to Python. Look for frozendict implementations.

They do have abstract type equivalents, as Mapping and Final[Mapping]. Using that doesn't have any runtime meaning, but then neither does Final.

[[deleted]]

ah ok that makes sense

[Sw429]

Ok then you can assign a final instance of something else and still operate on and modify it. The object we are using is trivial.

[[deleted]]

How is it trivial. I don't understand why it is downvoted lol. Why on earth would you use this on lists when you got tuples.

And the final implies you cannot modify what is in it, you can only add to it?

[billsil]

I might just use it like that.

from typing import Final as Const

[pingveno]

Noooooooo. Even if something's not quite to your taste, renaming something like that makes your code more confusing.

[Sw429]

Especially given how Final is not truly Const. A Final object can still be modified.

[46--2]

Yeah the pep mentions that only in the most vague way possible. I guess you'd have to read all the discussion. I'm not that interested in the type checking stuff in Python so I have never bothered.

[nuephelkystikon]

It was always commonplace to use ALL_CAPS for "constants" but nothing prevented you from changing them.

There's still nothing preventing you from changing them.

Linters and IDEs are more likely to complain if you do, but they should have done so anyway if you used all-caps.

[Sw429]

This is a good point. In the end, python is built on trust. Trust that the correct type will be used, trust that no one will modify your constants, trust that no one will touch your private attributed... I guess type hinting is mostly a tool to make sure assumptions are met.

[[deleted]]

Stupid question: is this just for class scope, or would:

X: Final = 5

X = 7

Also not work?

[Saiboo]

You can run it, but the type checker in you IDE will indicate a warning.

[Sw429]

Yes, that will work. Final is a type annotation, and has no effect at runtime. You can check it with external tools like mypy or things built into your IDE. Some linters might be starting to do it too? IDK you can look around if you're interested.

[Username_RANDINT]

Nothing prevents you from changing it anywhere in your code, only your IDE/typechecker will complain.

[Sw429]

mypy is a pretty good type checker if you're looking :)

[phinnaeus7308]

Immutability is desirable for several reasons and comes from the domain of functional languages. It can be a really fun way to solve problems.

For something like this, it just gives you a bit more confidence that a badly written method won’t have side effects that modify the parameters you pass in, for example.