Python PEP 841: Frozen syntax for immutable types

The proposed PEP 841 introduces a new frozen display syntax, f{...}, to create frozensets and frozendicts directly. This syntax offers an alternative to current methods like frozenset({...}) which involve creating mutable collections and then converting them, thereby reducing runtime lookups and allowing for compiler optimizations.

Four new AST nodes (FrozenSet, FrozenDict, FrozenSetComp, FrozenDictComp) and two new bytecode instructions, BUILD_FROZENSET and BUILD_FROZENMAP, are proposed to handle the new syntax. These changes are designed to be structurally identical to their mutable counterparts but explicitly signal immutability.

The frozen display syntax guarantees immutability at compile time, enabling the compiler to treat these constructs as first-class citizens for optimization. This includes constant folding, caching in .pyc files, and potentially zero per-execution construction cost for constant displays.

The PEP aims to increase the adoption of immutable containers like frozendict and frozenset in CPython, which is beneficial for future concurrent programming models involving free-threading and subinterpreters. This can help reduce bugs from accidental mutation and improve performance.

The proposed syntax f{ is backward compatible as it is a syntax error in current Python versions. Tooling integration is considered, with potential for linters and formatters to automatically rewrite existing frozenset and frozendict calls to the new syntax.