Mypy Release Notes

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Mypy 1.20

We’ve just uploaded mypy 1.20.0 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.

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.

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.

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.

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),