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This PR updates mypy from 0.670 to 1.20.1.

Changelog

1.20

We’ve just uploaded mypy 1.20.0 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).
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](http://mypy.readthedocs.io).

Planned Changes to Defaults and Flags in Mypy 2.0

As a reminder, we are planning to enable `--local-partial-types` by default in mypy 2.0, which
will likely be the next feature release. This will often require at least minor code changes. This
option is implicitly enabled by mypy daemon, so this makes the behavior of daemon and non-daemon
modes consistent.

Note that this release improves the compatibility of `--local-partial-types` significantly to
make the switch easier (see below for more).

This can also be configured in a mypy configuration file (use `False` to disable):


local_partial_types = True


For more information, refer to the
[documentation](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-local-partial-types).

We will also enable `--strict-bytes` by default in mypy 2.0. This usually requires at most
minor code changes to adopt. For more information, refer to the
[documentation](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-strict-bytes).

Finally, `--allow-redefinition-new` will be renamed to `--allow-redefinition`. If you want
to continue using the older `--allow-redefinition` semantics which are less flexible (e.g.
limited support for conditional redefinitions), you can switch to `--allow-redefinition-old`,
which is currently supported as an alias to the legacy `--allow-redefinition` behavior.
To use `--allow-redefinition` in the upcoming mypy 2.0, you can't use `--no-local-partial-types`.
For more information, refer to the
[documentation](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-allow-redefinition-new).

Better Type Narrowing

Mypy's implementation of narrowing has been substantially reworked. Mypy will now narrow more
aggressively, more consistently, and more correctly. In particular, you are likely to notice new
narrowing behavior in equality expressions (`==`), containment expressions (`in`),
match statements, and additional expressions providing type guards.

Note that mypy (and other Python type checkers) do not model the potential for various non-local
operations to invalidate narrowing assumptions. This means mypy may conclude that some of your code
is [unreachable](https://mypy.readthedocs.io/en/stable/common_issues.html#unreachable-code) and
avoid further checking of it. The `--warn-unreachable` flag is useful for highlighting these cases.
To reset narrowing, you can insert dummy reassignments, for instance `var = var` will reset
all narrowing of `var.attr`.

Future work includes better narrowing on initial assignments, more narrowing to `Literal` types,
and better checking of unreachable code.

Contributed by Shantanu Jain.

- Rework narrowing logic for equality and identity (Shantanu, PR [20492](https://github.com/python/mypy/pull/20492))
- Refactor equality and identity narrowing for clarity (Shantanu, PR [20595](https://github.com/python/mypy/pull/20595))
- Treat NotImplemented as a singleton type (Shantanu, PR [20601](https://github.com/python/mypy/pull/20601))
- Improve narrowing logic for Enum int and str subclasses (Shantanu, PR [20609](https://github.com/python/mypy/pull/20609))
- Narrow types based on collection containment (Shantanu, PR [20602](https://github.com/python/mypy/pull/20602))
- Refactor and improve narrowing for type(x) == t checks (Shantanu, PR [20634](https://github.com/python/mypy/pull/20634))
- Narrow for type expr comparisons to type exprs (Shantanu, PR [20639](https://github.com/python/mypy/pull/20639))
- Narrowing for comparisons against `x.__class__` (Shantanu, PR [20642](https://github.com/python/mypy/pull/20642))
- Better narrowing with custom equality (Shantanu, PR [20643](https://github.com/python/mypy/pull/20643))
- Use a single pass for core narrowing logic, add comments (Shantanu, PR [20659](https://github.com/python/mypy/pull/20659))
- Narrowing for final type objects (Shantanu, PR [20661](https://github.com/python/mypy/pull/20661))
- Avoid narrowing type[T] (Shantanu, PR [20662](https://github.com/python/mypy/pull/20662))
- Avoid widening to Any for checks like `type(x) is type(y: Any)` (Shantanu, PR [20663](https://github.com/python/mypy/pull/20663))
- Preserve some lost narrowing, cleanup (Shantanu, PR [20674](https://github.com/python/mypy/pull/20674))
- Fix narrowing related code for types with overloaded `__new__` (Shantanu, PR [20676](https://github.com/python/mypy/pull/20676))
- Fix isinstance with unions of tuples (Shantanu, PR [20677](https://github.com/python/mypy/pull/20677))
- Fix regression to chained containment (Shantanu, PR [20688](https://github.com/python/mypy/pull/20688))
- Improve else handling with custom equality (Shantanu, PR [20692](https://github.com/python/mypy/pull/20692))
- Better model runtime in isinstance and type checks (Shantanu, PR [20675](https://github.com/python/mypy/pull/20675))
- Use --warn-unreachable and --strict-equality in more tests (Shantanu, PR [20707](https://github.com/python/mypy/pull/20707))
- Model exact narrowing with type(x) checks (Shantanu, PR [20703](https://github.com/python/mypy/pull/20703))
- Short term fix for bytes narrowing (Shantanu, PR [20704](https://github.com/python/mypy/pull/20704))
- Preserve narrowing in unreachable code (Shantanu, PR [20710](https://github.com/python/mypy/pull/20710))
- Fix bug when narrowing union containing custom eq against custom eq (Shantanu, PR [20754](https://github.com/python/mypy/pull/20754))
- Fix narrowing for unions (Shantanu, PR [20728](https://github.com/python/mypy/pull/20728))
- Unsoundly narrow away from None with custom eq (Shantanu, PR [20756](https://github.com/python/mypy/pull/20756))
- Improve narrowing with numeric types (Shantanu, PR [20727](https://github.com/python/mypy/pull/20727))
- Fix narrowing with final type objects (Shantanu, PR [20743](https://github.com/python/mypy/pull/20743))
- Further improve match statement narrowing against unions (Shantanu, PR [20744](https://github.com/python/mypy/pull/20744))
- Avoid narrowing to NewType (Shantanu, PR [20766](https://github.com/python/mypy/pull/20766))
- Better match narrowing for irrefutable sequence patterns (Shantanu, PR [20782](https://github.com/python/mypy/pull/20782))
- Remove prohibit_none_typevar_overlap (Shantanu, PR [20864](https://github.com/python/mypy/pull/20864))
- Fix match statement narrowing reachability for tuples (Shantanu, PR [20896](https://github.com/python/mypy/pull/20896))
- Better handling of generics when narrowing (Shantanu, PR [20863](https://github.com/python/mypy/pull/20863))
- Better match narrowing for type objects (Shantanu, PR [20872](https://github.com/python/mypy/pull/20872))
- Narrow Callable generic return types (Shantanu, PR [20868](https://github.com/python/mypy/pull/20868))
- Better match narrowing for unions of type objects (Shantanu, PR [20905](https://github.com/python/mypy/pull/20905))
- Improve reachability in narrowing logic (Shantanu, PR [20660](https://github.com/python/mypy/pull/20660))
- Better match narrowing for irrefutable mapping patterns (Shantanu, PR [20906](https://github.com/python/mypy/pull/20906))
- Fix match statement semantic reachability (Shantanu, PR [20968](https://github.com/python/mypy/pull/20968))
- Add some additional narrowing test cases (Shantanu, PR [20598](https://github.com/python/mypy/pull/20598))
- Move tests to check-narrowing , improve them slightly (Shantanu, PR [20637](https://github.com/python/mypy/pull/20637))
- Add more tests for narrowing logic (Shantanu, PR [20672](https://github.com/python/mypy/pull/20672))
- More testing related improvements and updates (Shantanu, PR [20709](https://github.com/python/mypy/pull/20709))
- Add --warn-unreachable to more tests (Shantanu, PR [20977](https://github.com/python/mypy/pull/20977))

Drop Support for Python 3.9

Mypy no longer supports running with Python 3.9, which has reached end of life.
When running mypy with Python 3.10+, it is still possible to type check code
that needs to support Python 3.9 with the `--python-version 3.9` argument.
Support for this will be dropped in the first half of 2026!

Contributed by Marc Mueller (PR [20156](https://github.com/python/mypy/pull/20156)).

Mypyc Accelerated Mypy Wheels for ARM Windows and Free Threading

For best performance, mypy can be compiled to C extension modules using mypyc. This makes
mypy 3-5x faster than when interpreted with pure Python. We now build and upload mypyc
accelerated mypy wheels for `win_arm64` and `cp314t-...` to PyPI, making it easy for Windows
users on ARM and those using the free theading builds for Python 3.14 to realise this speedup
-- just `pip install` the latest mypy.

Contributed by Marc Mueller
(PR [mypy_mypyc-wheels106](https://github.com/mypyc/mypy_mypyc-wheels/pull/106),
PR [mypy_mypyc-wheels110](https://github.com/mypyc/mypy_mypyc-wheels/pull/110)).

Improved Compatibility for Local Partial Types

Compatibility between mypy's default behavior and the `--local-partial-types` flag
is now improved. This improves compatibility between mypy daemon and non-daemon modes,
since the mypy daemon requires local partial types to be enabled.

In particular, code like this now behaves consistently independent of
whether local partial types are enabled or not:

python
x = None

def foo() -> None:
 global x
 x = 1

The inferred type of 'x' is always 'int | None'.


Also, we are planning to turn local partial types on by default in mypy 2.0 (to be
released soon), and this makes the change much less disruptive. Explicitly disabling local
partial types will continue to be supported, but the support will likely be
deprecated and removed eventually, as the legacy behavior is hard to support together with
some important changes we are working on, in addition to being incompatible with the mypy
daemon.

This feature was contributed by Ivan Levkivskyi (PR [20938](https://github.com/python/mypy/pull/20938)).

Python 3.14 T-String Support (PEP 750)

Mypy now supports t-strings that were introduced in Python 3.14.

- Add support for Python 3.14 t-strings (PEP 750) (Neil Schemenauer and Brian Schubert, PR [20850](https://github.com/python/mypy/pull/20850))
- Add implicit module dependency if using t-string (Jukka Lehtosalo, PR [20900](https://github.com/python/mypy/pull/20900))

Experimental New Parser

If you install mypy using `pip install mypy[native-parser]` and run mypy with
`--native-parser`, you can experiment with a new Python parser. It is based on
the Ruff parser, and it's more efficient than the default parser. It will also enable
access to all Python syntax independent of which Python version you use to run mypy.
The new parser is still not feature-complete and has known issues.

Related changes:

- Add work-in-progress implementation of a new Python parser (Jukka Lehtosalo, PR [20856](https://github.com/python/mypy/pull/20856))
- Skip redundant analysis pass when using the native parser (Ivan Levkivskyi, PR [21015](https://github.com/python/mypy/pull/21015))
- Add t-string support to native parser (Ivan Levkivskyi, PR [21007](https://github.com/python/mypy/pull/21007))
- Handle hex bigint literals in native parser (Ivan Levkivskyi, PR [20988](https://github.com/python/mypy/pull/20988))
- Pass all relevant options to native parser (Ivan Levkivskyi, PR [20984](https://github.com/python/mypy/pull/20984))
- Support `no_type_check` with native parser (Ivan Levkivskyi, PR [20959](https://github.com/python/mypy/pull/20959))
- Fix error code handling in native parser (Ivan Levkivskyi, PR [20952](https://github.com/python/mypy/pull/20952))
- Add `ast-serialize` as an optional dependency (Ivan Levkivskyi, PR [21028](https://github.com/python/mypy/pull/21028))
- Use `native-parser` instead of `native-parse` for optional dependency (Jukka Lehtosalo, PR [21115](https://github.com/python/mypy/pull/21115))

Performance Improvements

Mypy now uses a binary cache format (fixed-format cache) by default to speed up incremental
checking. You can still use `--no-fixed-format-cache` to use the legacy JSON cache format,
but we will remove the JSON cache format in a future release. Mypy includes a tool to convert
individual fixed-format cache files (.ff) to the JSON format to make it possible to inspect
cache contents:

python
python -m mypy.exportjson <path> ...


If the SQLite cache is enabled, you will first need to convert the SQLite cache into
individual files using the [`misc/convert-cache.py`](https://github.com/python/mypy/blob/master/misc/convert-cache.py)
tool available in the mypy GitHub repository. You can also disable the SQLite
cache using `--no-sqlite-cache`.

The SQLite cache (`--sqlite-cache`) is now enabled by default. It improves mypy
performance significantly in certain environments where slow file system operations
used to be a bottleneck.

List of all performance improvements (for mypyc improvements there is a separate section below):

- Flip fixed-format cache to on by default (Ivan Levkivskyi, PR [20758](https://github.com/python/mypy/pull/20758))
- Enable `--sqlite-cache` by default (Shantanu, PR [21041](https://github.com/python/mypy/pull/21041))
- Save work on emitting ignored diagnostics (Shantanu, PR [20621](https://github.com/python/mypy/pull/20621))
- Skip logging and stats collection calls if they are no-ops (Jukka Lehtosalo, PR [20839](https://github.com/python/mypy/pull/20839))
- Speed up large incremental builds by optimizing internal state construction (Jukka Lehtosalo, PR [20838](https://github.com/python/mypy/pull/20838))
- Speed up suppressed dependencies options processing (Jukka Lehtosalo, PR [20806](https://github.com/python/mypy/pull/20806))
- Avoid path operations that need syscalls (Jukka Lehtosalo, PR [20802](https://github.com/python/mypy/pull/20802))
- Use faster algorithm for topological sort (Jukka Lehtosalo, PR [20790](https://github.com/python/mypy/pull/20790))
- Replace old topological sort (Jukka Lehtosalo, PR [20805](https://github.com/python/mypy/pull/20805))
- Fix quadratic performance in dependency graph loading for incremental builds (Jukka Lehtosalo, PR [20786](https://github.com/python/mypy/pull/20786))
- Micro-optimize transitive dependency hash calculation (Jukka Lehtosalo, PR [20798](https://github.com/python/mypy/pull/20798))
- Speed up options snapshot calculation (Jukka Lehtosalo, PR [20797](https://github.com/python/mypy/pull/20797))
- Micro-optimize read buffering, metastore `abspath`, path joining (Shantanu, PR [20810](https://github.com/python/mypy/pull/20810))
- Speed up type comparisons and hashing for literal types (Shantanu, PR [20423](https://github.com/python/mypy/pull/20423))
- Optimize overloaded signatures check (asce, PR [20378](https://github.com/python/mypy/pull/20378))
- Avoid unnecessary work when checking deferred functions (Ivan Levkivskyi, PR [20860](https://github.com/python/mypy/pull/20860))
- Improve `--allow-redefinition-new` performance for code with loops (Ivan Levkivskyi, PR [20862](https://github.com/python/mypy/pull/20862))
- Avoid `setattr`/`getattr` with fixed format cache (Ivan Levkivskyi, PR [20826](https://github.com/python/mypy/pull/20826))

Improvements to Allowing Redefinitions

Mypy now allows significantly more flexible variable redefinitions when using `--allow-redefinition-new`.
In particular, function parameters can now be redefined with a different type:

python
mypy: allow-redefinition-new, local-partial-types

def process(items: list[str]) -> None:
  Reassign parameter to a completely different type.
  Without --allow-redefinition-new, this is a type error because
  list[list[str]] is not compatible with list[str].
 items = [item.split() for item in items]
 ...


In mypy 2.0, we will update `--allow-redefinition` to mean `--allow-redefinition-new`.
This release adds `--allow-redefinition-old` as an alias of `--allow-redefinition`, which
can be used to continue using the old redefinition behavior in mypy 2.0 and later.

List of changes:

- Add `--allow-redefinition-old` as an alias of `--allow-redefinition` (Ivan Levkivskyi, PR [20764](https://github.com/python/mypy/pull/20764))
- Allow redefinitions for function arguments (Ivan Levkivskyi, PR [20853](https://github.com/python/mypy/pull/20853))
- Fix regression on redefinition in deferred loop (Ivan Levkivskyi, PR [20879](https://github.com/python/mypy/pull/20879))
- Fix loop convergence with redefinitions (Ivan Levkivskyi, PR [20865](https://github.com/python/mypy/pull/20865))
- Make sure new redefinition semantics only apply to inferred variables (Ivan Levkivskyi, PR [20909](https://github.com/python/mypy/pull/20909))
- Fix union edge case in function argument redefinition (Ivan Levkivskyi, PR [20908](https://github.com/python/mypy/pull/20908))
- Show an error when old and new redefinition are enabled in a file (Ivan Levkivskyi, PR [20920](https://github.com/python/mypy/pull/20920))
- `--allow-redefinition-new` is no longer experimental (Jukka Lehtosalo, PR [21110](https://github.com/python/mypy/pull/21110))
- Fix type inference for nested union types (Ivan Levkivskyi, PR [20912](https://github.com/python/mypy/pull/20912))
- Fix type inference regression for multiple variables in loops (Ivan Levkivskyi, PR [20892](https://github.com/python/mypy/pull/20892))
- Improve type inference for empty collections in conditional contexts (Ivan Levkivskyi, PR [20851](https://github.com/python/mypy/pull/20851))

Incremental Checking Improvements

This release includes multiple fixes to incremental type checking:

- Invalidate cache when `--enable-incomplete-feature` changes (kaushal trivedi, PR [20849](https://github.com/python/mypy/pull/20849))
- Add back support for `warn_unused_configs` (Ivan Levkivskyi, PR [20801](https://github.com/python/mypy/pull/20801))
- Recover from corrupted fixed-format cache meta file (Jukka Lehtosalo, PR [20780](https://github.com/python/mypy/pull/20780))
- Distinguish not found versus skipped modules (Ivan Levkivskyi, PR [20812](https://github.com/python/mypy/pull/20812))
- Fix undetected submodule deletion on warm run (Ivan Levkivskyi, PR [20784](https://github.com/python/mypy/pull/20784))
- Fix staleness on changed follow-imports options (Ivan Levkivskyi, PR [20773](https://github.com/python/mypy/pull/20773))
- Verify indirect dependencies reachable on incremental run (Ivan Levkivskyi, PR [20735](https://github.com/python/mypy/pull/20735))
- Fix indirect dependencies for protocols (Ivan Levkivskyi, PR [20752](https://github.com/python/mypy/pull/20752))
- Show error locations in other modules on warm runs (Ivan Levkivskyi, PR [20635](https://github.com/python/mypy/pull/20635))
- Don't read errors from cache on silent import (Sjoerd Job Postmus, PR [20509](https://github.com/python/mypy/pull/20509))
- More robust fix for re-export of `__all__` (Ivan Levkivskyi, PR [20487](https://github.com/python/mypy/pull/20487))

Fixes to Crashes

- Fix crash on partially typed namespace package (Ivan Levkivskyi, PR [20742](https://github.com/python/mypy/pull/20742))
- Fix internal error caused by the generic type alias with an unpacked list (Kai (Kazuya Ito), PR [20689](https://github.com/python/mypy/pull/20689))
- Fix crash when missing format character (Shantanu, PR [20524](https://github.com/python/mypy/pull/20524))
- Fix crash when passing literal values as type arguments to variadic generics (Aaron Wieczorek, PR [20543](https://github.com/python/mypy/pull/20543))
- Fix crash on circular star import in incremental mode (Ivan Levkivskyi, PR [20511](https://github.com/python/mypy/pull/20511))
- Fix crash with tuple unpack inside TypeVar default (Marc Mueller, PR [20456](https://github.com/python/mypy/pull/20456))
- Fix crash on typevar with forward reference used in other module (Ivan Levkivskyi, PR [20334](https://github.com/python/mypy/pull/20334))
- Fix crash on star import of redefinition (Ivan Levkivskyi, PR [20333](https://github.com/python/mypy/pull/20333))
- Fix crash involving Unpack-ed `TypeVarTuple` (Shantanu, PR [20323](https://github.com/python/mypy/pull/20323))
- Fix crashes caused by type variable defaults in-place modifications (Stanislav Terliakov, PR [20139](https://github.com/python/mypy/pull/20139))
- Fix crash when calling `len()` with no arguments (Jukka Lehtosalo, PR [20774](https://github.com/python/mypy/pull/20774))
- Fix crash when checking `async for` inside nested comprehensions (A5rocks, PR [20540](https://github.com/python/mypy/pull/20540))
- Fix `ParamSpec` related crash (Stanislav Terliakov, PR [20119](https://github.com/python/mypy/pull/20119))

Mypyc: Faster Imports on macOS

Imports in native (compiled) modules that target other native modules that are compiled
together are now significantly faster on macOS, especially on the first run after a compiled
package has been installed. This also speeds up the first mypy run after installation/update
on macOS.

This was contributed by Jukka Lehtosalo (PR [21101](https://github.com/python/mypy/pull/21101)).

librt: Mypyc Standard Library

Mypyc now has a dedicated standard library, `librt`, to provide basic features that are optimized
for compiled code. They are faster than corresponding Python stdlib functionality. There is no
plan to replace the Python stdlib, though. We'll only include a carefully selected set of features
that help with common performance bottlenecks in compiled code.

Currently, we provide `librt.base64` that has optimized SIMD (Single Instruction, Multiple
Data) base64 encoding and decoding functions. In future mypyc releases we are planning to
add efficient data structures, string/bytes utilities, and more.

Use `python3 -m pip install librt` to make `librt` available to compiled modules. Compiled
modules don't require `librt` unless they explicitly import `librt`. If you install mypy, you
will also get a compatible version of `librt` as a dependency. We will keep `librt` backward
compatible, so you should always be able to update to a newer version of the library.

Related changes:

- Add minimal, experimental `librt.base64` module (Jukka Lehtosalo, PR [20226](https://github.com/python/mypy/pull/20226))
- Use faster base64 encode implementation in `librt.base64` (Jukka Lehtosalo, PR [20237](https://github.com/python/mypy/pull/20237))
- Add efficient `librt.base64.b64decode` (Jukka Lehtosalo, PR [20263](https://github.com/python/mypy/pull/20263))
- Enable SIMD for `librt.base64` on x86-64 (Jukka Lehtosalo, PR [20244](https://github.com/python/mypy/pull/20244))
- Add primitive for `librt.base64.b64decode` (Jukka Lehtosalo, PR [20272](https://github.com/python/mypy/pull/20272))
- Add `urlsafe_b64encode` and `urlsafe_b64decode` to `librt.base64` (Jukka Lehtosalo, PR [20274](https://github.com/python/mypy/pull/20274))
- Make `librt.base64` non-experimental (Ivan Levkivskyi, PR [20783](https://github.com/python/mypy/pull/20783))
- Support pyodide for Python 3.12 (Michael R. Crusoe, PR [20342](https://github.com/python/mypy/pull/20342))
- Support pyodide via the NEON intrinsics (Michael R. Crusoe, PR [20316](https://github.com/python/mypy/pull/20316))
- Fix `librt` compilation on platforms with OpenMP (Ivan Levkivskyi, PR [20583](https://github.com/python/mypy/pull/20583))
- Fix cross-compiling `librt` by enabling x86_64 optimizations with pragmas (James Le Cuirot, PR [20815](https://github.com/python/mypy/pull/20815))
- Use existing SIMD CPU dispatch by customizing build flags (Michael R. Crusoe, PR [20253](https://github.com/python/mypy/pull/20253))
- Document `librt` and `librt.base64` (Jukka Lehtosalo, PR [21114](https://github.com/python/mypy/pull/21114))

Mypyc: Acyclic Classes

Mypyc now supports defining acyclic native classes that don't participate in the tracing
garbage collection:

python
from mypy_extensions import mypyc_attr

mypyc_attr(acyclic=True)
class Item:
 def __init__(self, key: str, value: str) -> None:
     self.key = key
     self.value = value


Allocating and freeing instances of acyclic classes is faster than regular native class
instances, and they use less memory, but if they participate in reference cycles, there
may be memory leaks.

This was contributed by Jukka Lehtosalo (PR [20795](https://github.com/python/mypy/pull/20795)).

Additional Mypyc Fixes and Improvements

- Fix range loop variable off-by-one after loop exit (Vaggelis Danias, PR [21098](https://github.com/python/mypy/pull/21098))
- Fix memory leak on property setter call (Piotr Sawicki, PR [21095](https://github.com/python/mypy/pull/21095))
- Fix `ClassVar` self-references in class bodies (Vaggelis Danias, PR [21011](https://github.com/python/mypy/pull/21011))
- Fix cross-module class attribute defaults causing KeyError (Vaggelis Danias, PR [21012](https://github.com/python/mypy/pull/21012))
- Fix shadow vtable misalignment for `property` getters/setters (Vaggelis Danias, PR [21010](https://github.com/python/mypy/pull/21010))
- Fix lambda inside comprehension (Vaggelis Danias, PR [21009](https://github.com/python/mypy/pull/21009))
- Use cached ASCII characters in `CPyStr_GetItem` (Vaggelis Danias, PR [21035](https://github.com/python/mypy/pull/21035))
- Speed up int to bytes conversion (Piotr Sawicki, PR [21036](https://github.com/python/mypy/pull/21036))
- Add missing primitive documentation (Jukka Lehtosalo, PR [21037](https://github.com/python/mypy/pull/21037))
- Fix undefined behavior in generated C (Jukka Lehtosalo, PR [21094](https://github.com/python/mypy/pull/21094))
- Fix vtable construction for deep trait inheritance (Vaggelis Danias, PR [20917](https://github.com/python/mypy/pull/20917))
- Fix `__init_subclass__` running before `ClassVar` instantiations (Vaggelis Danias, PR [20916](https://github.com/python/mypy/pull/20916))
- Add support for `str.lower()` and `str.upper()` (Vaggelis Danias, PR [20948](https://github.com/python/mypy/pull/20948))
- Add `str.isdigit()` primitive (Vaggelis Danias, PR [20893](https://github.com/python/mypy/pull/20893))
- Add `str.isalnum()` primitive (Vaggelis Danias, PR [20852](https://github.com/python/mypy/pull/20852))
- Add `str.isspace()` primitive (Vaggelis Danias, PR [20842](https://github.com/python/mypy/pull/20842))
- Reduce memory usage when compiling large files (Vaggelis Danias, PR [20897](https://github.com/python/mypy/pull/20897))
- Generate error if using Python 3.14 t-string (Jukka Lehtosalo, PR [20899](https://github.com/python/mypy/pull/20899))
- Fix undefined attribute in nested coroutines (Piotr Sawicki, PR [20654](https://github.com/python/mypy/pull/20654))
- Do not emit tracebacks with negative line numbers (Piotr Sawicki, PR [20641](https://github.com/python/mypy/pull/20641))
- Fix crash on multiple nested decorated functions with same name (Piotr Sawicki, PR [20666](https://github.com/python/mypy/pull/20666))
- Fix `Final` load in unreachable branches (BobTheBuidler, PR [20617](https://github.com/python/mypy/pull/20617))
- Add new primitive for `int.to_bytes` (BobTheBuidler, PR [19674](https://github.com/python/mypy/pull/19674))
- Support constant folding in f-string to `str` conversion (BobTheBuidler, PR [19970](https://github.com/python/mypy/pull/19970))
- Implement `bytes.endswith` (esarp, PR [20447](https://github.com/python/mypy/pull/20447))
- Fix coercion from short tagged int to fixed-width int (Jukka Lehtosalo, PR [20587](https://github.com/python/mypy/pull/20587))
- Fix generation of function wrappers for decorated functions (Piotr Sawicki, PR [20584](https://github.com/python/mypy/pull/20584))
- Generate function wrappers for each callable class instance (Piotr Sawicki, PR [20575](https://github.com/python/mypy/pull/20575))
- Speed up `ord(str[n])` by inlining (Jukka Lehtosalo, PR [20578](https://github.com/python/mypy/pull/20578))
- Add inline primitives for `bytes.__getitem__` (Jukka Lehtosalo, PR [20552](https://github.com/python/mypy/pull/20552))
- Add primitive type for `bytearray` (Jukka Lehtosalo, PR [20551](https://github.com/python/mypy/pull/20551))
- Fix and clean up `bytearray` support in primitives (Jukka Lehtosalo, PR [20550](https://github.com/python/mypy/pull/20550))
- Enable `--strict-bytes` by default in mypyc (and require it) (Jukka Lehtosalo, PR [20548](https://github.com/python/mypy/pull/20548))
- Fix exception reraising when awaiting a future (Piotr Sawicki, PR [20547](https://github.com/python/mypy/pull/20547))
- Add inline primitives for BytesBuilder get item and set item (Jukka Lehtosalo, PR [20546](https://github.com/python/mypy/pull/20546))
- Optimize loops over `enumerate`, `map`, `zip`, `range`, and other builtins with known lengths (BobTheBuidler, PR [19927](https://github.com/python/mypy/pull/19927))
- Raise `ValueError` if int too big for native int type (Jukka Lehtosalo, PR [20385](https://github.com/python/mypy/pull/20385))
- Fix generator regression with empty tuple (BobTheBuidler, PR [20371](https://github.com/python/mypy/pull/20371))
- Improve constant folding for `len()` of string literals and Final values (BobTheBuidler, PR [20074](https://github.com/python/mypy/pull/20074))
- Add primitive for `bytes.startswith` (esarp, PR [20387](https://github.com/python/mypy/pull/20387))
- Fix calling async methods through vectorcall (Piotr Sawicki, PR [20393](https://github.com/python/mypy/pull/20393))
- Extend loop optimization to use constant folding for determining sequence lengths (BobTheBuidler, PR [19930](https://github.com/python/mypy/pull/19930))
- Wrap async functions with function-like type (Piotr Sawicki, PR [20260](https://github.com/python/mypy/pull/20260))
- Add a primitive for bytes `translate` method (Jukka Lehtosalo, PR [20305](https://github.com/python/mypy/pull/20305))
- Add primitives for bytes and str multiply (Jukka Lehtosalo, PR [20303](https://github.com/python/mypy/pull/20303))
- Match int arguments to primitives with native int parameters (Jukka Lehtosalo, PR [20299](https://github.com/python/mypy/pull/20299))
- Allow disabling extra flags with `MYPYC_NO_EXTRA_FLAGS` environment variable (James Hilliard, PR [20507](https://github.com/python/mypy/pull/20507))
- Fix unsupported imports for type annotations (Lukas Geiger, PR [20390](https://github.com/python/mypy/pull/20390))
- Fix unaligned memory access in librt internal helper function (Gregor Riepl, PR [20474](https://github.com/python/mypy/pull/20474))
- Fix build issue (James Hilliard, PR [20510](https://github.com/python/mypy/pull/20510))

Removed Flags `--force-uppercase-builtins` and `--force-union-syntax`

The `--force-uppercase-builtins` flag was deprecated and has been a no-op since mypy 1.17.0.
Since mypy has dropped support for Python 3.9, the `--force-union-syntax` flag is no longer
necessary.

Contributed by Marc Mueller (PR [20410](https://github.com/python/mypy/pull/20410))
and (PR [20405](https://github.com/python/mypy/pull/20405)).

Stubgen Improvements

- Fix mis-parsing of double colon ("::") (Jeremy Nimmer, PR [20285](https://github.com/python/mypy/pull/20285))

Stubtest Improvements

- Attempt to resolve decorators from their type (Shantanu, PR [20867](https://github.com/python/mypy/pull/20867))
- Fix crash on instances with redefined `__class__` (sobolevn, PR [20926](https://github.com/python/mypy/pull/20926))
- Improve checking of positional-only parameters in dunder methods (Brian Schubert, PR [19593](https://github.com/python/mypy/pull/19593))
- Check `Final` variables with literal values against runtime (Vikash Kumar, PR [20858](https://github.com/python/mypy/pull/20858))
- Fix duplicate errors with invalid line numbers (Joren Hammudoglu, PR [20417](https://github.com/python/mypy/pull/20417))
- Ignore `__conditional_annotations__` (Joren Hammudoglu, PR [20392](https://github.com/python/mypy/pull/20392))
- Transparent `type_check_only` types (Joren Hammudoglu, PR [20352](https://github.com/python/mypy/pull/20352))
- Check runtime availability of private types not marked `type_check_only` (Brian Schubert, PR [19574](https://github.com/python/mypy/pull/19574))

Documentation Updates

- Document semantics of function argument redefinition (Ivan Levkivskyi, PR [20910](https://github.com/python/mypy/pull/20910))
- Update common issues: document how the type ignore must come first (wyattscarpenter, PR [20886](https://github.com/python/mypy/pull/20886))
- Update "type inference and annotations" (Kai (Kazuya Ito), PR [20619](https://github.com/python/mypy/pull/20619))
- Document unreachability handling of `return NotImplemented` (wyattscarpenter, PR [20561](https://github.com/python/mypy/pull/20561))

Changes to Messages

- Simpler/cleaner `reveal_type()` (Ivan Levkivskyi, PR [20929](https://github.com/python/mypy/pull/20929))
- Use "parameter" instead of "argument" in overload error (Kai (Kazuya Ito), PR [20994](https://github.com/python/mypy/pull/20994))
- Use "parameter" instead of "argument" in unpacking error (Kai (Kazuya Ito), PR [20683](https://github.com/python/mypy/pull/20683))
- Use "type arguments" instead of "type parameters" for bare generics (Kai (Kazuya Ito), PR [20494](https://github.com/python/mypy/pull/20494))
- Update "arguments" to "parameters" (Kai (Kazuya Ito), PR [20711](https://github.com/python/mypy/pull/20711))
- Use "parameter" instead of "argument" in default value error messages (Kai (Kazuya Ito), PR [20964](https://github.com/python/mypy/pull/20964))
- Update error message for parameter overlap in TypedDict (Kai (Kazuya Ito), PR [20956](https://github.com/python/mypy/pull/20956))
- Correct "Duplicate argument" error messages (Kai (Kazuya Ito), PR [20957](https://github.com/python/mypy/pull/20957))
- Use standard formatting for note on unexpected keyword (Ivan Levkivskyi, PR [20808](https://github.com/python/mypy/pull/20808))
- Fix edge cases in pretty formatting (Ivan Levkivskyi, PR [20809](https://github.com/python/mypy/pull/20809))
- Add function definition notes for missing named argument errors (Kevin Kannammalil, PR [20794](https://github.com/python/mypy/pull/20794))
- Make overloaded constructors consistent in error messages (Ivan Levkivskyi, PR [20483](https://github.com/python/mypy/pull/20483))
- Improve error message for invalid Python package (Shantanu, PR [20482](https://github.com/python/mypy/pull/20482))
- Emit end line/column in JSON format for span tracking (Adam Turner, PR [20734](https://github.com/python/mypy/pull/20734))
- Wrap callable in union syntax (Marc Mueller, PR [20406](https://github.com/python/mypy/pull/20406))

Other Notable Fixes and Improvements

- Unify handling of self attributes in daemon (Ivan Levkivskyi, PR [21025](https://github.com/python/mypy/pull/21025))
- Improve support for `hasattr()` (Ivan Levkivskyi, PR [20914](https://github.com/python/mypy/pull/20914))
- Warn when `disjoint_base` is used on protocols or TypedDicts (Brian Schubert, PR [21029](https://github.com/python/mypy/pull/21029))
- Model tuple type aliases better (Shantanu, PR [20967](https://github.com/python/mypy/pull/20967))
- Fix incorrect type inference for container literals inside loops (Ivan Levkivskyi, PR [20875](https://github.com/python/mypy/pull/20875))
- Fix edge case in type comparison for type variables with narrowed bounds (Ivan Levkivskyi, PR [20874](https://github.com/python/mypy/pull/20874))
- Prohibit access via class for instance-only attributes (Ivan Levkivskyi, PR [20855](https://github.com/python/mypy/pull/20855))
- Update JSON export tool to support binary meta cache files (Jukka Lehtosalo, PR [20096](https://github.com/python/mypy/pull/20096))
- Better handling for uncaught top-level exceptions (Ivan Levkivskyi, PR [20749](https://github.com/python/mypy/pull/20749))
- Attrs field level `kw_only=False` overrides `kw_only=True` at class level (getzze, PR [20949](https://github.com/python/mypy/pull/20949))
- Fix `__slots__` issue with deferred base (Shantanu, PR [20573](https://github.com/python/mypy/pull/20573))
- Fix deferral logic for special types (Ivan Levkivskyi, PR [20678](https://github.com/python/mypy/pull/20678))
- Fix false negatives in walrus versus inference fallback logic (Ivan Levkivskyi, PR [20622](https://github.com/python/mypy/pull/20622))
- Fix enum value inference with user-defined data type mixin (E. M. Bray, PR [16320](https://github.com/python/mypy/pull/16320))
- Fix incorrect classification of class attributes versus enum members (Stanislav Terliakov, PR [19687](https://github.com/python/mypy/pull/19687))
- Improve type inference when finding common types for callable types (Shantanu, PR [18406](https://github.com/python/mypy/pull/18406))
- Remove special case for Union type context (Shantanu, PR [20610](https://github.com/python/mypy/pull/20610))
- Evaluate argument expressions in runtime evaluation order (Shantanu, PR [20491](https://github.com/python/mypy/pull/20491))
- Allow empty type applications for `ParamSpec` (A5rocks, PR [20572](https://github.com/python/mypy/pull/20572))
- Make `X[()]` count as a type application (A5rocks, PR [20568](https://github.com/python/mypy/pull/20568))
- Accept `value` as keyword argument in `TypeAliasType` (Ali Hamdan, PR [20556](https://github.com/python/mypy/pull/20556))
- Avoid treating `pass` and `...` as no-op for reachability (Shantanu, PR [20488](https://github.com/python/mypy/pull/20488))
- Fix specialization leak in generic `TypedDict.update()` (Aaron Wieczorek, PR [20517](https://github.com/python/mypy/pull/20517))
- Fix false positive redundant expression warnings with `isinstance()` on `Any | Protocol` unions (Randolf Scholz, PR [20450](https://github.com/python/mypy/pull/20450))
- Fix type checking of class-scope imports accessed via `self` or `cls` (bzoracler, PR [20480](https://github.com/python/mypy/pull/20480))
- Prevent synthetic intersections from leaking to module public interfaces (bzoracler, PR [20459](https://github.com/python/mypy/pull/20459))
- Fix package build failure when the compiler types are not defined (Steven Pitman, PR [20429](https://github.com/python/mypy/pull/20429))
- Fix `--strict-equality` for iteratively visited code (Christoph Tyralla, PR [19635](https://github.com/python/mypy/pull/19635))
- Allow literals as kwargs dict keys (Shantanu, PR [20416](https://github.com/python/mypy/pull/20416))
- Error for invalid varargs and varkwargs to `Any` call (Shantanu, PR [20324](https://github.com/python/mypy/pull/20324))
- Fix type inference for binary operators with tuple subclasses (Randolf Scholz, PR [19046](https://github.com/python/mypy/pull/19046))
- Improve type inference for ternary expressions with literals and collections (Randolf Scholz, PR [19563](https://github.com/python/mypy/pull/19563))
- Do not treat match value patterns as `isinstance` checks (Stanislav Terliakov, PR [20146](https://github.com/python/mypy/pull/20146))
- Fix noncommutative joins with bounded TypeVars (Shantanu, PR [20345](https://github.com/python/mypy/pull/20345))
- Fix error location reporting for `**rest` patterns in match statements (Marc Mueller, PR [20407](https://github.com/python/mypy/pull/20407))
- Fix matching against union of tuples (Saul Shanabrook, PR [19600](https://github.com/python/mypy/pull/19600))
- Allow `types.NoneType` in match cases (A5rocks, PR [20383](https://github.com/python/mypy/pull/20383))
- Treat `Literal["xyz"]` as iterable (Shantanu, PR [20347](https://github.com/python/mypy/pull/20347))
- Treat functions that return `None` as returning `None` (Shantanu, PR [20350](https://github.com/python/mypy/pull/20350))
- Fix str unpack type propagation, use dedicated error code (Shantanu, PR [20325](https://github.com/python/mypy/pull/20325))
- Unpack unions inside tuples in except handlers (asce, PR [17762](https://github.com/python/mypy/pull/17762))
- Ignore empty error codes from `type: ignore` (Donghoon Nam, PR [20052](https://github.com/python/mypy/pull/20052))
- Make `NoneType` annotation error use a new error code (wyattscarpenter, PR [20222](https://github.com/python/mypy/pull/20222))
- Fail with an explicit error on PyPy (Ivan Levkivskyi, PR [20384](https://github.com/python/mypy/pull/20384))
- Add hidden `--overwrite-union-syntax` option (Marc Mueller, PR [20332](https://github.com/python/mypy/pull/20332))
- Fix `possibly-undefined` false-positive with nesting (grayjk, PR [20276](https://github.com/python/mypy/pull/20276))
- Fix spurious `possibly-undefined` errors in for-else with `break` (Łukasz Langa, PR [19696](https://github.com/python/mypy/pull/19696))
- Avoid false `possibly-undefined` errors due to omitted unrequired else statements (Christoph Tyralla, PR [20149](https://github.com/python/mypy/pull/20149))
- Fix generator expression behavior for `reveal_type` (michaelm-openai, PR [20594](https://github.com/python/mypy/pull/20594))
- Fix error on instance property and init-only variable with the same name in a dataclass (Roberto Fernández Iglesias, PR [17219](https://github.com/python/mypy/pull/17219))
- Improve error messages when `__module__` or `__qualname__` are used as type annotations (A5rocks, PR [20288](https://github.com/python/mypy/pull/20288))
- Check for multiple type var tuples for PEP 695 (A5rocks, PR [20289](https://github.com/python/mypy/pull/20289))
- Improve interaction between `--local-partial-types` and hashability (Shantanu, PR [20719](https://github.com/python/mypy/pull/20719))
- Stop looking for `.gitignore` at top level of working tree (Colin Watson, PR [20775](https://github.com/python/mypy/pull/20775))
- Try fixing Cygwin build (Ivan Levkivskyi, PR [20830](https://github.com/python/mypy/pull/20830))
- Fix daemon dependencies in `diff-cache.py` tool (Jukka Lehtosalo, PR [20837](https://github.com/python/mypy/pull/20837))
- Support fixed-format cache in `diff-cache.py` tool (Jukka Lehtosalo, PR [20827](https://github.com/python/mypy/pull/20827))
- Update `convert-cache.py` tool to work with fixed-format caches (Ivan Levkivskyi, PR [20761](https://github.com/python/mypy/pull/20761))
- Write errors to a separate cache file (Ivan Levkivskyi, PR [21022](https://github.com/python/mypy/pull/21022))
- Write ignored lines to cache meta (Ivan Levkivskyi, PR [20747](https://github.com/python/mypy/pull/20747))
- Serialize raw errors in cache metas (Ivan Levkivskyi, PR [20372](https://github.com/python/mypy/pull/20372))
- Include `misc/{diff-cache,apply-cache-diff}.py` in sdist (Michael R. Crusoe, PR [21096](https://github.com/python/mypy/pull/21096))

Typeshed updates

Please see [git log](https://github.com/python/typeshed/commits/main?after=f8f0794d0fe249c06dc9f31a004d85be6cca6ced+0&branch=main&path=stdlib) for full list of standard library typeshed stub changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:
- A5rocks
- Aaron Wieczorek
- Adam Turner
- Ali Hamdan
- asce
- BobTheBuidler
- Brent Westbrook
- Brian Schubert
- bzoracler
- Chris Burroughs
- Christoph Tyralla
- Colin Watson
- Donghoon Nam
- E. M. Bray
- Emma Smith
- Ethan Sarp
- George Ogden
- getzze
- grayjk
- Gregor Riepl
- Ivan Levkivskyi
- James Hilliard
- James Le Cuirot
- Jeremy Nimmer
- Joren Hammudoglu
- Kai (Kazuya Ito)
- kaushal trivedi
- Kevin Kannammalil
- Lukas Geiger
- Łukasz Langa
- Marc Mueller
- Michael R. Crusoe
- michaelm-openai
- Neil Schemenauer
- Piotr Sawicki
- Randolf Scholz
- Roberto Fernández Iglesias
- Saul Shanabrook
- Shantanu Jain
- Sjoerd Job Postmus
- sobolevn
- Stanislav Terliakov
- Steven Pitman
- Vaggelis Danias
- Vikash Kumar
- wyattscarpenter

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

1.19.1

- Fix noncommutative joins with bounded TypeVars (Shantanu, PR [20345](https://github.com/python/mypy/pull/20345))
- Respect output format for cached runs by serializing raw errors in cache metas (Ivan Levkivskyi, PR [20372](https://github.com/python/mypy/pull/20372))
- Allow `types.NoneType` in match cases (A5rocks, PR [20383](https://github.com/python/mypy/pull/20383))
- Fix mypyc generator regression with empty tuple (BobTheBuidler, PR [20371](https://github.com/python/mypy/pull/20371))
- Fix crash involving Unpack-ed TypeVarTuple (Shantanu, PR [20323](https://github.com/python/mypy/pull/20323))
- Fix crash on star import of redefinition (Ivan Levkivskyi, PR [20333](https://github.com/python/mypy/pull/20333))
- Fix crash on typevar with forward ref used in other module (Ivan Levkivskyi, PR [20334](https://github.com/python/mypy/pull/20334))
- Fail with an explicit error on PyPy (Ivan Levkivskyi, PR [20389](https://github.com/python/mypy/pull/20389))

Acknowledgements

Thanks to all mypy contributors who contributed to this release:
- A5rocks
- BobTheBuidler
- bzoracler
- Chainfire
- Christoph Tyralla
- David Foster
- Frank Dana
- Guo Ci
- iap
- Ivan Levkivskyi
- James Hilton-Balfe
- jhance
- Joren Hammudoglu
- Jukka Lehtosalo
- KarelKenens
- Kevin Kannammalil
- Marc Mueller
- Michael Carlstrom
- Michael J. Sullivan
- Piotr Sawicki
- Randolf Scholz
- Shantanu
- Sigve Sebastian Farstad
- sobolevn
- Stanislav Terliakov
- Stephen Morton
- Theodore Ando
- Thiago J. Barbalho
- wyattscarpenter

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

1.19

We’ve just uploaded mypy 1.19.0 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).
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](http://mypy.readthedocs.io).

Python 3.9 Support Ending Soon

This is the last mypy feature release that supports Python 3.9, which reached
end of life in October 2025.

Performance Improvements
- Switch to a more dynamic SCC processing logic (Ivan Levkivskyi, PR [20053](https://github.com/python/mypy/pull/20053))
- Speed up type aliases (Ivan Levkivskyi, PR [19810](https://github.com/python/mypy/pull/19810))

Fixed‑Format Cache Improvements

Mypy uses a cache by default to speed up incremental runs by reusing partial results
from earlier runs. Mypy 1.18 added a new binary fixed-format cache representation as
an experimental feature. The feature is no longer experimental, and we are planning
to enable it by default in a future mypy release (possibly 1.20), since it's faster
and uses less space than the original, JSON-based cache format. Use
`--fixed-format-cache` to enable the fixed-format cache.

Mypy now has an extra dependency on the `librt` PyPI package, as it's needed for
cache serialization and deserialization.

Mypy ships with a tool to convert fixed-format cache files to the old JSON format.
Example of how to use this:

$ python -m mypy.exportjson .mypy_cache/.../my_module.data.ff


This way existing use cases that parse JSON cache files can be supported when using
the new format, though an extra conversion step is needed.

This release includes these improvements:

- Force-discard cache if cache format changed (Ivan Levkivskyi, PR [20152](https://github.com/python/mypy/pull/20152))
- Add tool to convert binary cache files to JSON (Jukka Lehtosalo, PR [20071](https://github.com/python/mypy/pull/20071))
- Use more efficient serialization format for long integers in cache files (Jukka Lehtosalo, PR [20151](https://github.com/python/mypy/pull/20151))
- More robust packing of floats in fixed-format cache (Ivan Levkivskyi, PR [20150](https://github.com/python/mypy/pull/20150))
- Use self-descriptive cache with type tags (Ivan Levkivskyi, PR [20137](https://github.com/python/mypy/pull/20137))
- Use fixed format for cache metas (Ivan Levkivskyi, PR [20088](https://github.com/python/mypy/pull/20088))
- Make metas more compact; fix indirect suppression (Ivan Levkivskyi, PR [20075](https://github.com/python/mypy/pull/20075))
- Use dedicated tags for most common cached instances (Ivan Levkivskyi, PR [19762](https://github.com/python/mypy/pull/19762))

PEP 747: Annotating Type Forms

Mypy now recognizes `TypeForm[T]` as a type and implements
[PEP 747](https://peps.python.org/pep-0747/). The feature is still experimental,
and it's disabled by default. Use `--enable-incomplete-feature=TypeForm` to
enable type forms. A type form object captures the type information provided by a
runtime type expression. Example:

python
from typing_extensions import TypeForm

def trycast[T](typx: TypeForm[T], value: object) -> T | None: ...

def example(o: object) -> None:
  'int | str' below is an expression that represents a type.
  Unlike type[T], TypeForm[T] can be used with all kinds of types,
  including union types.
 x = trycast(int | str, o)
 if x is not None:
      Type of 'x' is 'int | str' here
     ...


This feature was contributed by David Foster (PR [19596](https://github.com/python/mypy/pull/19596)).

Fixes to Crashes
- Do not push partial types to the binder (Stanislav Terliakov, PR [20202](https://github.com/python/mypy/pull/20202))
- Fix crash on recursive tuple with Hashable (Ivan Levkivskyi, PR [20232](https://github.com/python/mypy/pull/20232))
- Fix crash related to decorated functions (Stanislav Terliakov, PR [20203](https://github.com/python/mypy/pull/20203))
- Do not abort constructing TypeAlias if only type parameters hold us back (Stanislav Terliakov, PR [20162](https://github.com/python/mypy/pull/20162))
- Use the fallback for `ModuleSpec` early if it can never be resolved (Stanislav Terliakov, PR [20167](https://github.com/python/mypy/pull/20167))
- Do not store deferred NamedTuple fields as redefinitions (Stanislav Terliakov, PR [20147](https://github.com/python/mypy/pull/20147))
- Discard partial types remaining after inference failure (Stanislav Terliakov, PR [20126](https://github.com/python/mypy/pull/20126))
- Fix an infinite recursion bug (Stanislav Terliakov, PR [20127](https://github.com/python/mypy/pull/20127))
- Fix IsADirectoryError for namespace packages when using --linecoverage-report (wyattscarpenter, PR [20109](https://github.com/python/mypy/pull/20109))
- Fix an internal error when creating cobertura output for namespace package (wyattscarpenter, PR [20112](https://github.com/python/mypy/pull/20112))
- Allow type parameters reusing the name missing from current module (Stanislav Terliakov, PR [20081](https://github.com/python/mypy/pull/20081))
- Prevent TypeGuardedType leak from narrowing declared type as part of type variable bound (Stanislav Terliakov, PR [20046](https://github.com/python/mypy/pull/20046))
- Fix crash on invalid unpack in base class (Ivan Levkivskyi, PR [19962](https://github.com/python/mypy/pull/19962))
- Traverse ParamSpec prefix where we should (Ivan Levkivskyi, PR [19800](https://github.com/python/mypy/pull/19800))
- Fix daemon crash related to imports (Ivan Levkivskyi, PR [20271](https://github.com/python/mypy/pull/20271))

Mypyc: Support for `__getattr__`, `__setattr__`, and `__delattr__`

Mypyc now has partial support for `__getattr__`, `__setattr__` and
`__delattr__` methods in native classes.

Note that native attributes are not stored using `__dict__`. Setting attributes
directly while bypassing `__setattr__` is possible by using
`super().__setattr__(...)` or `object.__setattr__(...)`, but not via `__dict__`.

Example:
python
class Demo:
 _data: dict[str, str]

 def __init__(self) -> None:
      Initialize data dict without calling our __setattr__
     super().__setattr__("_data", {})

 def __setattr__(self, name: str, value: str) -> None:
     print(f"Setting {name} = {value!r}")

     if name == "_data":
         raise AttributeError("'_data' cannot be set")

     self._data[name] = value

 def __getattr__(self, name: str) -> str:
     print(f"Getting {name}")

     try:
         return self._data[name]
     except KeyError:
         raise AttributeError(name)

d = Demo()
d.x = "hello"
d.y = "world"

print(d.x)
print(d.y)


Related PRs:
- Generate `__setattr__` wrapper (Piotr Sawicki, PR [19937](https://github.com/python/mypy/pull/19937))
- Generate `__getattr__` wrapper (Piotr Sawicki, PR [19909](https://github.com/python/mypy/pull/19909))
- Support deleting attributes in `__setattr__` wrapper (Piotr Sawicki, PR [19997](https://github.com/python/mypy/pull/19997))

Miscellaneous Mypyc Improvements
- Fix `__new__` in native classes with inheritance (Piotr Sawicki, PR [20302](https://github.com/python/mypy/pull/20302))
- Fix crash on `super` in generator (Ivan Levkivskyi, PR [20291](https://github.com/python/mypy/pull/20291))
- Fix calling base class async method using `super()` (Jukka Lehtosalo, PR [20254](https://github.com/python/mypy/pull/20254))
- Fix async or generator methods in traits (Jukka Lehtosalo, PR [20246](https://github.com/python/mypy/pull/20246))
- Optimize equality check with string literals (BobTheBuidler, PR [19883](https://github.com/python/mypy/pull/19883))
- Fix inheritance of async defs (Jukka Lehtosalo, PR [20044](https://github.com/python/mypy/pull/20044))
- Reject invalid `mypyc_attr` args (BobTheBuidler, PR [19963](https://github.com/python/mypy/pull/19963))
- Optimize `isinstance` with tuple of primitive types (BobTheBuidler, PR [19949](https://github.com/python/mypy/pull/19949))
- Optimize away first index check in for loops if length > 1 (BobTheBuidler, PR [19933](https://github.com/python/mypy/pull/19933))
- Fix broken exception/cancellation handling in async def (Jukka Lehtosalo, PR [19951](https://github.com/python/mypy/pull/19951))
- Transform `object.__new__` inside `__new__` (Piotr Sawicki, PR [19866](https://github.com/python/mypy/pull/19866))
- Fix crash with NewType and other non-class types in incremental builds (Jukka Lehtosalo, PR [19837](https://github.com/python/mypy/pull/19837))
- Optimize container creation from expressions with length known at compile time (BobTheBuidler, PR [19503](https://github.com/python/mypy/pull/19503))
- Allow per-class free list to be used with inheritance (Jukka Lehtosalo, PR [19790](https://github.com/python/mypy/pull/19790))
- Fix object finalization (Marc Mueller, PR [19749](https://github.com/python/mypy/pull/19749))
- Allow defining a single-item free "list" for a native class (Jukka Lehtosalo, PR [19785](https://github.com/python/mypy/pull/19785))
- Speed up unary "not" (Jukka Lehtosalo, PR [19774](https://github.com/python/mypy/pull/19774))

Stubtest Improvements
- Check `_value_` for ellipsis-valued stub enum members (Stanislav Terliakov, PR [19760](https://github.com/python/mypy/pull/19760))
- Include function name in overload assertion messages (Joren Hammudoglu, PR [20063](https://github.com/python/mypy/pull/20063))
- Fix special case in analyzing function signature (iap, PR [19822](https://github.com/python/mypy/pull/19822))
- Improve `allowlist` docs with better example (sobolevn, PR [20007](https://github.com/python/mypy/pull/20007))

Documentation Updates
- Update duck type compatibility: mention strict-bytes and mypy 2.0 (wyattscarpenter, PR [20121](https://github.com/python/mypy/pull/20121))
- Document `--enable-incomplete-feature TypeForm` (wyattscarpenter, PR [20173](https://github.com/python/mypy/pull/20173))
- Change the inline TypedDict example (wyattscarpenter, PR [20172](https://github.com/python/mypy/pull/20172))
- Replace `List` with built‑in `list` (PEP 585) (Thiago J. Barbalho, PR [20000](https://github.com/python/mypy/pull/20000))
- Improve junit documentation (wyattscarpenter, PR [19867](https://github.com/python/mypy/pull/19867))

Other Notable Fixes and Improvements
- Fix annotated with function as type keyword list parameter (KarelKenens, PR [20094](https://github.com/python/mypy/pull/20094))
- Fix errors for raise NotImplemented (Shantanu, PR [20168](https://github.com/python/mypy/pull/20168))
- Don't let help formatter line-wrap URLs (Frank Dana, PR [19825](https://github.com/python/mypy/pull/19825))
- Do not cache fast container types inside lambdas (Stanislav Terliakov, PR [20166](https://github.com/python/mypy/pull/20166))
- Respect force-union-syntax flag in error hint (Marc Mueller, PR [20165](https://github.com/python/mypy/pull/20165))
- Fix type checking of dict type aliases (Shantanu, PR [20170](https://github.com/python/mypy/pull/20170))
- Use pretty callable formatting more often for callable expressions (Theodore Ando, PR [20128](https://github.com/python/mypy/pull/20128))
- Use dummy concrete type instead of `Any` when checking protocol variance (bzoracler, PR [20110](https://github.com/python/mypy/pull/20110))
- PEP 696: Fix swapping TypeVars with defaults (Randolf Scholz, PR [19449](https://github.com/python/mypy/pull/19449))
- Fix narrowing of class pattern with union type (Randolf Scholz, PR [19517](https://github.com/python/mypy/pull/19517))
- Do not emit unreachable warnings for lines that return `NotImplemented` (Christoph Tyralla, PR [20083](https://github.com/python/mypy/pull/20083))
- Fix matching against `typing.Callable` and `Protocol` types (Randolf Scholz, PR [19471](https://github.com/python/mypy/pull/19471))
- Make `--pretty` work better on multi-line issues (A5rocks, PR [20056](https://github.com/python/mypy/pull/20056))
- More precise return types for `TypedDict.get` (Randolf Scholz, PR [19897](https://github.com/python/mypy/pull/19897))
- Prevent false unreachable warnings for `final` instances that occur when strict optional checking is disabled (Christoph Tyralla, PR [20045](https://github.com/python/mypy/pull/20045))
- Check class references to catch non-existent classes in match cases (A5rocks, PR [20042](https://github.com/python/mypy/pull/20042))
- Do not sort unused error codes in unused error codes warning (wyattscarpenter, PR [20036](https://github.com/python/mypy/pull/20036))
- Fix `[name-defined]` false positive in `class A[X, Y=X]:` case (sobolevn, PR [20021](https://github.com/python/mypy/pull/20021))
- Filter SyntaxWarnings during AST parsing (Marc Mueller, PR [20023](https://github.com/python/mypy/pull/20023))
- Make untyped decorator its own error code (wyattscarpenter, PR [19911](https://github.com/python/mypy/pull/19911))
- Support error codes from plugins in options (Sigve Sebastian Farstad, PR [19719](https://github.com/python/mypy/pull/19719))
- Allow returning Literals in `__new__` (James Hilton-Balfe, PR [15687](https://github.com/python/mypy/pull/15687))
- Inverse interface freshness logic (Ivan Levkivskyi, PR [19809](https://github.com/python/mypy/pull/19809))
- Do not report exhaustive-match after deferral (Stanislav Terliakov, PR [19804](https://github.com/python/mypy/pull/19804))
- Make `untyped_calls_exclude` invalidate cache (Ivan Levkivskyi, PR [19801](https://github.com/python/mypy/pull/19801))
- Add await to empty context hack (Stanislav Terliakov, PR [19777](https://github.com/python/mypy/pull/19777))
- Consider non-empty enums assignable to Self (Stanislav Terliakov, PR [19779](https://github.com/python/mypy/pull/19779))

Typeshed updates

Please see [git log](https://github.com/python/typeshed/commits/main?after=ebce8d766b41fbf4d83cf47c1297563a9508ff60+0&branch=main&path=stdlib) for full list of standard library typeshed stub changes.

1.18.2

- Fix crash on recursive alias (Ivan Levkivskyi, PR [19845](https://github.com/python/mypy/pull/19845))
- Add additional guidance for stubtest errors when runtime is `object.__init__` (Stephen Morton, PR [19733](https://github.com/python/mypy/pull/19733))
- Fix handling of None values in f-string expressions in mypyc (BobTheBuidler, PR [19846](https://github.com/python/mypy/pull/19846))

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

- Ali Hamdan
- Anthony Sottile
- BobTheBuidler
- Brian Schubert
- Chainfire
- Charlie Denton
- Christoph Tyralla
- CoolCat467
- Daniel Hnyk
- Emily
- Emma Smith
- Ethan Sarp
- Ivan Levkivskyi
- Jahongir Qurbonov
- Jelle Zijlstra
- Joren Hammudoglu
- Jukka Lehtosalo
- Marc Mueller
- Omer Hadari
- Piotr Sawicki
- PrinceNaroliya
- Randolf Scholz
- Robsdedude
- Saul Shanabrook
- Shantanu
- Stanislav Terliakov
- Stephen Morton
- wyattscarpenter

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

1.18.1

compared to 1.17 when type checking mypy itself. In extreme cases, the improvement
can be 10x or higher. The list below is an overview of the various mypy optimizations.
Many mypyc improvements (discussed in a separate section below) also improve performance.

Type caching optimizations have a small risk of causing regressions. When
reporting issues with unexpected inferred types, please also check if
`--disable-expression-cache` will work around the issue, as it turns off some of
these optimizations.

- Improve self check performance by 1.8% (Jukka Lehtosalo, PR [19768](https://github.com/python/mypy/pull/19768), [19769](https://github.com/python/mypy/pull/19769), [19770](https://github.com/python/mypy/pull/19770))
- Optimize fixed-format deserialization (Ivan Levkivskyi, PR [19765](https://github.com/python/mypy/pull/19765))
- Use macros to optimize fixed-format deserialization (Ivan Levkivskyi, PR [19757](https://github.com/python/mypy/pull/19757))
- Two additional micro‑optimizations (Ivan Levkivskyi, PR [19627](https://github.com/python/mypy/pull/19627))
- Another set of micro‑optimizations (Ivan Levkivskyi, PR [19633](https://github.com/python/mypy/pull/19633))
- Cache common types (Ivan Levkivskyi, PR [19621](https://github.com/python/mypy/pull/19621))
- Skip more method bodies in third‑party libraries for speed (Ivan Levkivskyi, PR [19586](https://github.com/python/mypy/pull/19586))
- Simplify the representation of callable types (Ivan Levkivskyi, PR [19580](https://github.com/python/mypy/pull/19580))
- Add cache for types of some expressions (Ivan Levkivskyi, PR [19505](https://github.com/python/mypy/pull/19505))
- Use cache for dictionary expressions (Ivan Levkivskyi, PR [19536](https://github.com/python/mypy/pull/19536))
- Use cache for binary operations (Ivan Levkivskyi, PR [19523](https://github.com/python/mypy/pull/19523))
- Cache types of type objects (Ivan Levkivskyi, PR [19514](https://github.com/python/mypy/pull/19514))
- Avoid duplicate work when checking boolean operations (Ivan Levkivskyi, PR [19515](https://github.com/python/mypy/pull/19515))
- Optimize generic inference passes (Ivan Levkivskyi, PR [19501](https://github.com/python/mypy/pull/19501))
- Speed up the default plugin (Jukka Lehtosalo, PRs [19385](https://github.com/python/mypy/pull/19385) and [19462](https://github.com/python/mypy/pull/19462))
- Remove nested imports from the default plugin (Ivan Levkivskyi, PR [19388](https://github.com/python/mypy/pull/19388))
- Micro‑optimize type expansion (Jukka Lehtosalo, PR [19461](https://github.com/python/mypy/pull/19461))
- Micro‑optimize type indirection (Jukka Lehtosalo, PR [19460](https://github.com/python/mypy/pull/19460))
- Micro‑optimize the plugin framework (Jukka Lehtosalo, PR [19464](https://github.com/python/mypy/pull/19464))
- Avoid temporary set creation in subtype checking (Jukka Lehtosalo, PR [19463](https://github.com/python/mypy/pull/19463))
- Subtype checking micro‑optimization (Jukka Lehtosalo, PR [19384](https://github.com/python/mypy/pull/19384))
- Return early where possible in subtype check (Stanislav Terliakov, PR [19400](https://github.com/python/mypy/pull/19400))
- Deduplicate some types before joining (Stanislav Terliakov, PR [19409](https://github.com/python/mypy/pull/19409))
- Speed up type checking by caching argument inference context (Jukka Lehtosalo, PR [19323](https://github.com/python/mypy/pull/19323))
- Optimize binding method self argument type and deprecation checks (Ivan Levkivskyi, PR [19556](https://github.com/python/mypy/pull/19556))
- Keep trivial instance types/aliases during expansion (Ivan Levkivskyi, PR [19543](https://github.com/python/mypy/pull/19543))

Fixed‑Format Cache (Experimental)

Mypy now supports a new cache format used for faster incremental builds. It makes
incremental builds up to twice as fast. The feature is experimental and
currently only supported when using a compiled version of mypy. Use `--fixed-format-cache`
to enable the new format, or `fixed_format_cache = True` in a configuration file.

We plan to enable this by default in a future mypy release, and we'll eventually
deprecate and remove support for the original JSON-based format.

Unlike the JSON-based cache format, the new binary format is currently
not easy to parse and inspect by mypy users. We are planning to provide a tool to
convert fixed-format cache files to JSON, but details of

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

* **Chores**
* Updated development tooling dependencies to latest stable versions for enhanced code quality analysis.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info
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Plan: Pro

Run ID: 1179bf59-0e53-4f9a-b459-40f62ddb42ce

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Reviewing files that changed from the base of the PR and between d377ad0 and 9effcf6.

📒 Files selected for processing (1)
  • src/requirements_dev.txt
📝 Walkthrough

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Updated the mypy development dependency version from 0.670 to 1.20.1 in the requirements file. No other changes to the dependency list.

Changes

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Development Dependency Update
src/requirements_dev.txt		 Upgraded mypy from version 0.670 to 1.20.1.

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🎯 1 (Trivial) | ⏱️ ~3 minutes

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🐰 Hop hop, the types now gleam so bright,
mypy's upgraded to version of might!
From ancient 0.670 to 1.20.1,
Type-checking adventures, now they're begun! ✨

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Closing this in favor of #567

@pyup-bot pyup-bot closed this Apr 21, 2026
@salimfadhley
salimfadhley deleted the pyup-update-mypy-0.670-to-1.20.1 branch April 21, 2026 19:15
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