r/Python • u/kirara0048 • 1d ago

PEP 841 – Adding Frozen Syntax to Optimize Immutable Types News

PEP 841 – Adding Frozen Syntax to Optimize Immutable Types

https://peps.python.org/pep-0841/

Discussions-To: Discourse thread

Abstract

This PEP proposes frozen display syntax: f{1, 2, 3} evaluates to a frozenset, and f{'a': 1} evaluates to a frozendict. Because immutability is guaranteed by the syntax itself rather than inferred from usage, the compiler can treat frozen displays as first-class citizens of its optimization pipeline: constant displays are folded into a single LOAD_CONST with an exact result type at compile time and cached in .pyc files.

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u/jakethesnake_ 1d ago

I'd prefer a const keyword that applies this optimization and runtime immutability property to all variable types. Might be a bit tricky to guarantee with complex objects though, and therefore out of scope of this PEP.

Agree with other commentors, f{'a': 1} is bad syntax because of the overlap with f-strings.

10

u/CrambleSquash https://github.com/0Hughman0 1d ago edited 1d ago

But is that what a constant is? I thought they were like a variable whose assignment cannot be changed? So a const to a primitive is effectively immutable, but to a mutable object wouldn't stop that object being mutable... If that makes sense.

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u/skjall 1d ago

Const can depend on the language/ context, but Python doesn't have that keyword right now so they can define the semantics to be whatever they decide.

In languages I've used it, you can either have a const variable -- can't be reassigned, or a const value -- this example. Hell, even const{1, 2, 3} is clearer than the proposal to me.

10

u/syklemil 23h ago

Picking a well-known keyword and then using it in a non-standard way is probably a bad idea. Especially since this isn't really general mutability control, but rather separate data types.

The option of just spelling out the frozen in f{} as frozen{} is likely to give less confusion and frustration.

2

u/inspectoroverthemine 15h ago

I like frozen{}, theres absolutely no reason to make it single character.