Monday, 13 July 2026

Mypy 2.3 Released

We've just uploaded mypy 2.3.0 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, performance improvements and bug fixes. You can install it as follows: 

python3 -m pip install -U mypy

You can read the full documentation for this release on Read the Docs.

The Upcoming Switch to the New Native Parser

We are planning to enable the new native parser (--native-parser) by default soon. We recommend that you test the native parser in your projects and report any issues in the mypy issue tracker.

Mypyc Free-threading Memory Safety

Free-threaded Python builds that don't have the GIL require additional synchronization primitives or lock-free algorithms to ensure memory safety when there are race conditions (for example, when a thread reads a list item while another thread writes the same list item concurrently). This release greatly improves memory safety of free threading.

List operations are now memory-safe on free threaded Python builds, even in the presence of race conditions. This has some performance cost. For list-heavy workloads, using librt.vecs.vec instead of list is often significantly faster, but note that vec is not (and likely won't be) fully memory safe, and the user is expected to avoid race conditions. The newly introduced librt.threading.Lock helps with this. Using variable-length tuples can also be more efficient than lists, since tuples are immutable and don't require expensive synchronization to ensure memory safety.

Instance attribute access is also (mostly) memory safe now on free-threaded builds in the presence of race conditions. We are planning to fix the remaining unsafe cases in a future release.

Full list of changes:

  • Make attribute access memory safe on free-threaded builds (Jukka Lehtosalo, PR 21705)
  • Fix unsafe borrowing of instance attributes with free-threading (Jukka Lehtosalo, PR 21688)
  • Make list get/set item more memory safe on free-threaded builds (Jukka Lehtosalo, PR 21683)
  • Don't borrow list items on free-threaded builds (Jukka Lehtosalo, PR 21679)
  • Make multiple assignment from list memory-safe on free-threaded builds (Jukka Lehtosalo, PR 21684)
  • Make for loop over list memory-safe on free-threaded builds (Jukka Lehtosalo, PR 21686)
  • Fix memory safety of list.count on free-threaded builds (Jukka Lehtosalo, PR 21680)
  • Make vec creation from list memory safe on free-threaded builds (Jukka Lehtosalo, PR 21681)
### librt.threading: Fast Native Lock Type

Mypyc now supports librt.threading.Lock, which is a lock type optimized for use in compiled code. It can be 2x to 4x faster than threading.Lock.

This feature was contributed by Jukka Lehtosalo (PR 21690, PR 21697).

Mypyc: Read-only Final Instance Attributes

Instance attributes of native classes declared as Final are now read-only at runtime. This enables additional optimizations, and it's now recommended to use Final for all performance-sensitive attributes when feasible.

Related changes:

  • Make instance attribute read-only at runtime if Final (Jukka Lehtosalo, PR 21666)
  • Borrow final attributes more aggressively (Jukka Lehtosalo, PR 21702)
  • Improve documentation of Final in mypyc (Jukka Lehtosalo, PR 21713)

Mypyc Documentation Updates

  • Update documentation of race conditions under free threading (Jukka Lehtosalo, PR 21726)
  • Update mypyc free threading Python compatibility docs (Jukka Lehtosalo, PR 21711)
  • Document recent additions to librt.strings, such as ispace (Jukka Lehtosalo, PR 21696)

Miscellaneous Mypyc Improvements

  • Fix reference leak when setting unboxed refcounted attributes (Tom Bannink, PR 21657)
  • Fix function wrapper memory leak (Piotr Sawicki, PR 21654)
  • Fix handling of invalid codepoint values in librt.strings (Jukka Lehtosalo, PR 21634)
  • Fix non-deterministic ordering of spilled registers (Jukka Lehtosalo, PR 21632)
  • Fix non-deterministic compiler output due to frozensets (Jukka Lehtosalo, PR 21631)

Changes to Messages

  • Fix error code of note about unbound type variable (Jukka Lehtosalo, PR 21668)

Other Notable Fixes and Improvements

  • Use PYODIDE environment variable for Emscripten cross-compilation detection (Agriya Khetarpal, PR 21714)
  • Narrow for frozendict membership check (Shantanu, PR 21709)
  • Fix custom equality handling for membership narrowing in static containers (Shantanu, PR 21706)
  • Infer Coroutine for unannotated async functions (Jingchen Ye, PR 21651)
  • Fix variance inference issues caused by dataclass replace (Shantanu, PR 21694)
  • Fix regression in dataclass narrowing for Python >= 3.13 (ygale, PR 21675)
  • Fix star import dependencies in mypy daemon (Jukka Lehtosalo, PR 21673)
  • Fix skipped imports considered stale (Piotr Sawicki, PR 21639)
  • Support .ff files with --cache-map (Jukka Lehtosalo, PR 21633)

Typeshed Updates

Please see git log for full list of standard library typeshed stub changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

  • Agriya Khetarpal
  • Ethan Sarp
  • Ivan Levkivskyi
  • Jingchen Ye
  • Jukka Lehtosalo
  • Piotr Sawicki
  • Shantanu
  • Tom Bannink
  • Viktor Szépe
  • ygale

I'd also like to thank my employer, Dropbox, for supporting mypy development.

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