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Mypy 1.19

We’ve just uploaded mypy 1.19.0 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.

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)
• Speed up type aliases (Ivan Levkivskyi, PR 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)
• Add tool to convert binary cache files to JSON (Jukka Lehtosalo, PR 20071)
• Use more efficient serialization format for long integers in cache files (Jukka Lehtosalo, PR 20151)
• More robust packing of floats in fixed-format cache (Ivan Levkivskyi, PR 20150)
• Use self-descriptive cache with type tags (Ivan Levkivskyi, PR 20137)
• Use fixed format for cache metas (Ivan Levkivskyi, PR 20088)
• Make metas more compact; fix indirect suppression (Ivan Levkivskyi, PR 20075)
• Use dedicated tags for most common cached instances (Ivan Levkivskyi, PR 19762)

... (truncated)

• 412c19a Bump version to 1.19.1
• 20aea0a Update changelog for 1.19.1 (#20414)
• 2b23b50 Serialize raw errors in cache metas (#20372)
• f60f90f Fail on PyPy in main instead of setup.py (#20389)
• 58d485b Fail with an explicit error on PyPy (#20384)
• a4b31a2 Allow types.NoneType in match cases (#20383)
• 8a6eff4 [mypyc] fix generator regression with empty tuple (#20371)
• 70eceea Fix noncommutative joins with bounded TypeVars (#20345)
• 3890fc4 Fix crash involving Unpack-ed TypeVarTuple (#20323)
• c93d917 Fix crash on star import of redefinition (#20333)
• Additional commits viewable in compare view

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