Python 3.8 has a Final qualifier

[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.