The Mypy Blog: Mypy 1.17 Released

We’ve just uploaded mypy 1.17 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features 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.

Optionally Check That Match Is Exhaustive

Mypy can now optionally generate an error if a match statement does not match exhaustively, without having to use assert_never(...). Enable this by using --enable-error-code exhaustive-match.

Example:

# mypy: enable-error-code=exhaustive-match

import enum

class Color(enum.Enum):
    RED = 1
    BLUE = 2

def show_color(val: Color) -> None:
    # error: Unhandled case for values of type "Literal[Color.BLUE]"
    match val:
        case Color.RED:
            print("red")

This feature was contributed by Donal Burns (PR 19144).

Further Improvements to Attribute Resolution

This release includes additional improvements to how attribute types and kinds are resolved. These fix many bugs and overall improve consistency.

Handle corner case: protocol/class variable/descriptor (Ivan Levkivskyi, PR 19277)
Fix a few inconsistencies in protocol/type object interactions (Ivan Levkivskyi, PR 19267)
Refactor/unify access to static attributes (Ivan Levkivskyi, PR 19254)
Remove inconsistencies in operator handling (Ivan Levkivskyi, PR 19250)
Make protocol subtyping more consistent (Ivan Levkivskyi, PR 18943)

Fixes to Nondeterministic Type Checking

Previous mypy versions could infer different types for certain expressions across different runs (typically depending on which order certain types were processed, and this order was nondeterministic). This release includes fixes to several such issues.

Fix nondeterministic type checking by making join with explicit Protocol and type promotion commute (Shantanu, PR 18402)
Fix nondeterministic type checking caused by nonassociative of None joins (Shantanu, PR 19158)
Fix nondeterministic type checking caused by nonassociativity of joins (Shantanu, PR 19147)
Fix nondeterministic type checking by making join between type and TypeVar commute (Shantanu, PR 19149)

Remove Support for Targeting Python 3.8

Mypy now requires --python-version 3.9 or greater. Support for targeting Python 3.8 is fully removed now. Since 3.8 is an unsupported version, mypy will default to the oldest supported version (currently 3.9) if you still try to target 3.8.

This change is necessary because typeshed stopped supporting Python 3.8 after it reached its End of Life in October 2024.

Contributed by Marc Mueller (PR 19157, PR 19162).

Initial Support for Python 3.14

Mypy is now tested on 3.14 and mypyc works with 3.14.0b3 and later. Binary wheels compiled with mypyc for mypy itself will be available for 3.14 some time after 3.14.0rc1 has been released.

Note that not all features are supported just yet.

Contributed by Marc Mueller (PR 19164)

Deprecated Flag: `--force-uppercase-builtins`

Mypy only supports Python 3.9+. The --force-uppercase-builtins flag is now deprecated as unnecessary, and a no-op. It will be removed in a future version.

Contributed by Marc Mueller (PR 19176)

Mypyc: Improvements to Generators and Async Functions

This release includes both performance improvements and bug fixes related to generators and async functions (these share many implementation details).

Fix exception swallowing in async try/finally blocks with await (Chainfire, PR 19353)
Fix AttributeError in async try/finally with mixed return paths (Chainfire, PR 19361)
Make generated generator helper method internal (Jukka Lehtosalo, PR 19268)
Free coroutine after await encounters StopIteration (Jukka Lehtosalo, PR 19231)
Use non-tagged integer for generator label (Jukka Lehtosalo, PR 19218)
Merge generator and environment classes in simple cases (Jukka Lehtosalo, PR 19207)

Mypyc: Partial, Unsafe Support for Free Threading

Mypyc has minimal, quite memory-unsafe support for the free threaded builds of 3.14. It is also only lightly tested. Bug reports and experience reports are welcome!

Here are some of the major limitations:

Free threading only works when compiling a single module at a time.
If there is concurrent access to an object while another thread is mutating the same

object, it's possible to encounter segfaults and memory corruption.

There are no efficient native primitives for thread synthronization, though the

regular threading module can be used.

Some workloads don't scale well to multiple threads for no clear reason.

Related PRs:

Enable partial, unsafe support for free-threading (Jukka Lehtosalo, PR 19167)
Fix incref/decref on free-threaded builds (Jukka Lehtosalo, PR 19127)

Other Mypyc Fixes and Improvements

Derive .c file name from full module name if using multi_file (Jukka Lehtosalo, PR 19278)
Support overriding the group name used in output files (Jukka Lehtosalo, PR 19272)
Add note about using non-native class to subclass built-in types (Jukka Lehtosalo, PR 19236)
Make some generated classes implicitly final (Jukka Lehtosalo, PR 19235)
Don't simplify module prefixes if using separate compilation (Jukka Lehtosalo, PR 19206)

Stubgen Improvements

Add import for types in __exit__ method signature (Alexey Makridenko, PR 19120)
Add support for including class and property docstrings (Chad Dombrova, PR 17964)
Don't generate Incomplete | None = None argument annotation (Sebastian Rittau, PR 19097)
Support several more constructs in stubgen's alias printer (Stanislav Terliakov, PR 18888)

Miscellaneous Fixes and Improvements

Combine the revealed types of multiple iteration steps in a more robust manner (Christoph Tyralla, PR 19324)
Improve the handling of "iteration dependent" errors and notes in finally clauses (Christoph Tyralla, PR 19270)
Lessen dmypy suggest path limitations for Windows machines (CoolCat467, PR 19337)
Fix type ignore comments erroneously marked as unused by dmypy (Charlie Denton, PR 15043)
Fix misspelled exhaustive-match error code (johnthagen, PR 19276)
Fix missing error context for unpacking assignment involving star expression (Brian Schubert, PR 19258)
Fix and simplify error de-duplication (Ivan Levkivskyi, PR 19247)
Disallow ClassVar in type aliases (Brian Schubert, PR 19263)
Add script that prints list of compiled files when compiling mypy (Jukka Lehtosalo, PR 19260)
Fix help message url for "None and Optional handling" section (Guy Wilson, PR 19252)
Display fully qualified name of imported base classes in errors about incompatible overrides (Mikhail Golubev, PR 19115)
Avoid false unreachable, redundant-expr, and redundant-casts warnings in loops more robustly and efficiently, and avoid multiple revealed type notes for the same line (Christoph Tyralla, PR 19118)
Fix type extraction from isinstance checks (Stanislav Terliakov, PR 19223)
Erase stray type variables in functools.partial (Stanislav Terliakov, PR 18954)
Make inferring condition value recognize the whole truth table (Stanislav Terliakov, PR 18944)
Support type aliases, NamedTuple and TypedDict in constrained TypeVar defaults (Stanislav Terliakov, PR 18884)
Move dataclass kw_only fields to the end of the signature (Stanislav Terliakov, PR 19018)
Provide a better fallback value for the python_version option (Marc Mueller, PR 19162)
Avoid spurious non-overlapping equality error with metaclass with __eq__ (Michael J. Sullivan, PR 19220)
Narrow type variable bounds (Ivan Levkivskyi, PR 19183)
Add classifier for Python 3.14 (Marc Mueller, PR 19199)
Capitalize syntax error messages (Charulata, PR 19114)
Infer constraints eagerly if actual is Any (Ivan Levkivskyi, PR 19190)
Include walrus assignments in conditional inference (Stanislav Terliakov, PR 19038)
Use PEP 604 syntax when converting types to strings (Marc Mueller, PR 19179)
Use more lower-case builtin types in error messages (Marc Mueller, PR 19177)
Fix example to use correct method of Stack (Łukasz Kwieciński, PR 19123)
Forbid .pop of Readonly NotRequired TypedDict items (Stanislav Terliakov, PR 19133)
Emit a friendlier warning on invalid exclude regex, instead of a stacktrace (wyattscarpenter, PR 19102)
Enable ANSI color codes for dmypy client in Windows (wyattscarpenter, PR 19088)
Extend special case for context-based type variable inference to unions in return position (Stanislav Terliakov, PR 18976)

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

Alexey Makridenko
Brian Schubert
Chad Dombrova
Chainfire
Charlie Denton
Charulata
Christoph Tyralla
CoolCat467
Donal Burns
Guy Wilson
Ivan Levkivskyi
johnthagen
Jukka Lehtosalo
Łukasz Kwieciński
Marc Mueller
Michael J. Sullivan
Mikhail Golubev
Sebastian Rittau
Shantanu
Stanislav Terliakov
wyattscarpenter

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

The Mypy Blog

Monday, 14 July 2025

Mypy 1.17 Released

Optionally Check That Match Is Exhaustive

Further Improvements to Attribute Resolution

Fixes to Nondeterministic Type Checking

Remove Support for Targeting Python 3.8

Initial Support for Python 3.14

Deprecated Flag: `--force-uppercase-builtins`

Mypyc: Improvements to Generators and Async Functions

Mypyc: Partial, Unsafe Support for Free Threading

Other Mypyc Fixes and Improvements

Stubgen Improvements

Miscellaneous Fixes and Improvements

Acknowledgements

Contributors

Monday, 14 July 2025

Mypy 1.17 Released

Optionally Check That Match Is Exhaustive

Further Improvements to Attribute Resolution

Fixes to Nondeterministic Type Checking

Remove Support for Targeting Python 3.8

Initial Support for Python 3.14

Deprecated Flag: --force-uppercase-builtins

Mypyc: Improvements to Generators and Async Functions

Mypyc: Partial, Unsafe Support for Free Threading

Other Mypyc Fixes and Improvements

Stubgen Improvements

Miscellaneous Fixes and Improvements

Acknowledgements

Deprecated Flag: `--force-uppercase-builtins`