The Mypy Blog: January 2021

We’ve just uploaded mypy 0.800 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, bug fixes and library stub (typeshed) updates. 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

Mypy 0.800 officially supports the recently released Python 3.9. We now provide compiled binary wheels for Python 3.9, improving type checking speed significantly.

Typing Usability Improvements (PEP 585 and PEP 604)

The necessity to repeatedly import various types and special forms from typing has been a long-term nuisance for users of static type checking and Python.

Two new Python features improve this situation and are now supported by mypy:

PEP 585 lets you use list[int] instead of List[int] (no need to import List and other generic collections from typing).
PEP 604 lets you write X | Y instead of Union[X, Y], and X | None instead of Optional[X] (no need to import Union or Optional from typing).

Note: Using list[int] requires Python 3.9 and X | Y requires Python 3.10 (alpha) in order to work at runtime. To use them on older versions of Python, use from __future__ import annotations. This allows them to be used in type annotations, but the older variants (or string literal escaping) may be required in non-annotation contexts, such as in type aliases. See the docs for more details.

Here is an example that uses the new features:

    from __future__ import annotations
    
    def fields(s: str | None) -> list[str]:
        if not s:
            return []
        else:
            return s.split(',')

These were implemented by Allan Daemon in PR 9564 and by Marc Mueller in PR 9647.

Improvements to Finding Modules

This release adds several improvements to how mypy finds Python source files to type check.

You can now pass paths to files within namespace packages on the command line, and mypy can better infer their module names. As before, use --namespace-packages to enable namespace packages.

When you use --explicit-package-bases together with --namespace-packages, mypy assumes that only the current directory and directories explicitly specified in MYPYPATH (or mypy_path in the config file) are valid package roots. This can help with situations where the module name of a file is ambiguous. For example, it may not be clear whether src/pkg/mod.py should be treated as src.pkg.mod or pkg.mod, and you can use this option to disambiguate between the two (more information in the docs).

The above improvements were implemented in PR 9742 by Shantanu.

Other related improvements (also implemented by Shantanu):

When you run mypy as mypy <directory>, look for source files recursively also inside directories without a __init__.py (PR 9614)
Support namespace packages with -p (PR 9683)
Log encountered source files when running mypy with -v (PR 9672)
Document the new module finding behavior (PR 9923)

Other Notable Improvements and Bug Fixes

Only treat import X as X as a re-export in stubs (Shantanu, PR 9515)
Fix package imports with aliases in stubgen (Chad Dombrova, PR 9534)
Require first argument of namedtuple() to match the variable name (Momoko Hattori, PR 9577)
Add error code for name mismatches in named tuples and TypedDicts to make it easy to disable these error messages (Jukka Lehtosalo, PR 9811)
Document local_partial_types config option (Momoko Hattori, PR 9551)
Improve ambiguous kwargs checking (Erik Soma, PR 9573)
Disable unreachable warnings in boolean operators for type variables with value restrictions (Vincent Barbaresi, PR 9572)
Allow assignment to an empty tuple (Tobin Yehle, PR 5617)
Use absolute path when checking source duplication error (Yuki Igarashi, PR 9059)
Add get_function_signature_hook() to the plugin API (Nikita Sobolev, PR 9102)
Speed up type checking of dictionary, set, and list expressions (Hugues, PR 9477)
Allow non-types as arguments in Annotated (Patrick Arminio, PR 9625)
Add support for next generation attrs API (David Euresti, PR 9396)
Fix case folding of missing keys error message for TypedDicts (Marti Raudsepp, PR 9757)
Fix generic inheritance of __init__() methods in dataclasses and attrs classes (Nate McMaster, PR 9383, PR 9380)
Add more information to error message on too few arguments (Abhinay Pandey, PR 9796)
Document PEP 585, 563, 604, and related functionality (Shantanu, PR 9763)

Mypyc Improvements

We use mypyc to compile mypy into fast C extension modules. This release includes many mypyc improvements.

Xuanda Yang finished the migration to use a new, lower-level compiler intermediate representation in his Google Summer of Code project.

New supported Python features:

Support the walrus operator (:=) (Michael J. Sullivan, PR 9624)

Performance improvements:

Add primitives for list sort and list reverse (Jukka Lehtosalo, PR 9897)
Recognize six.moves.xrange again as an alias of range (Jukka Lehtosalo, PR 9896)
Speed up some integer primitives (Jukka Lehtosalo, PR 9801)
Speed up if x for int values (Jukka Lehtosalo, PR 9854)
Implement dict clear primitive (Vasileios Sakkas, PR 9724)
Implement list insert primitive (Vasileios Sakkas, PR 9741)
Implement float abs primitive (Xuanda Yang, PR 9695)
Implement str-to-float primitive (Xuanda Yang, PR 9685)
Specialize some calls to frozenset (Michael J. Sullivan, PR 9623)
Speed up multiple assignment from tuple (Xuanda Yang, PR 9575)
Speed up multiple assignment from sequence (Jukka Lehtosalo, PR 9800)
Optimize startswith and endswith (Tomer Chachamu, PR 9557)
Add primitives for bitwise ops (Jukka Lehtosalo, PR 9529)
Speed up in operations for list/tuple (Johan Dahlin, PR 9004)
Add primitives for list, str and tuple slicing (Jukka Lehtosalo, PR 9283)
Speed up tuple equality checks (Xuanda Yang, PR 9343)

Bug fixes:

Always add implicit None return type to __init__ method (Thomas Johnson, PR 9866)
Fix deallocation of deeply nested data structures (Michael J. Sullivan, PR 9839)
Fix using package imported inside a function (Jukka Lehtosalo, PR 9782)
Fix type of for loop index register in for over range (Jukka Lehtosalo, PR 9634)

Typeshed Updates

Many improvements were made to typeshed — too many to list. Browse the typeshed commit log here.

Acknowledgments

First of all, we’d like to thank our employer, Dropbox, for funding the mypy core team.

Thanks to all mypy contributors who contributed to this release:

Abdullah Selek
Abhinay Pandey
Adam
aghast
Akuli
Alexander
Allan Daemon
Aristotelis Mikropoulos
Ashley Whetter
Brian Mboya
Bryan Forbes
cdce8p
Chad Dombrova
David Euresti
Denis Laxalde
Eisuke Kawashima
Erik Soma
Ethan Pronovost
Florian Bruhin
Frank Dana
Greg Compestine
Guido van Rossum
Hugues
Jake Bailey
Jakub Stasiak
Jelle Zijlstra
Jeremy Metz
Johan Dahlin
Jon Shea
Jonathan Wong
Jürgen Gmach
Kamil Turek
Krzysztof Przybyła
Lawrence Chan
Marti Raudsepp
Matan Gover
Matt Gilson
Michael J. Sullivan
Momoko Hattori
Nate McMaster
Nikita Sobolev
Nils K
Nipunn Koorapati
Oleg Höfling
Patrick Arminio
Rajiv Singh
rhkleijn
Roland van Laar
Shantanu
Tobin Yehle
Tom Scogland
Tomer Chachamu
Tripp Horbinski
Ville Skyttä
Vincent Barbaresi
vsakkas
Wes Turner
willtryagain
Xiaodong DENG
Xuanda Yang
Yash Chhabria
Yuki Igarashi

Additional thanks to all contributors to typeshed:

Abraham Francis
Adam Dangoor
Adam Kliś
Adam Lichtl
Akuli
alexander-held
an onion
Anders Kaseorg
Andrew Mitchell
Árni Már Jónsson
Ash Berlin-Taylor
Ashwin Vishnu
Avery
cdce8p
Cebtenzzre
Changsheng
Christine
coiax
cptpcrd
crusaderky
Daniel O'Neel
David Caro
Dominic Davis-Foster
Eric Traut
Ethan Pronovost
Frank Maximilian
Gal Ben David
github-actions[bot]
Guido van Rossum
henribru
Hong Xu
Hugues
Hynek Schlawack
jack1142
Jake Bailey
Jason Fried
Jelle Zijlstra
Jia Chen
Jon Dufresne
Jonathan Schoonhoven
Jonathan Slenders
Joseph Haaga
Julien Danjou
Jun Jia
Jérome Perrin
karl ding
Katelyn Gigante
Kaushal Rohit
Kevin Wojniak
ky-gog
Kyle Fuller
kylec1
Lam Son Ho
Lourens Veen
Mahmoud Abduljawad
Mariam Maarouf
Marti Raudsepp
melassa
Mickaël Schoentgen
Mikhail Sveshnikov
Mikołaj Kuranowski
Moriyoshi Koizumi
Nate McMaster
Neel Somani
nicolas-harraudeau-sonarsource
Nikolaus Waxweiler
Nils K
Nipunn Koorapati
Oleg Höfling
Omar Sandoval
Paul
Pete Scopes
Peter Law
Philipp Hahn
Phillip Huang
proost
PythonCoderAS
Rajiv Bakulesh Shah
Ran Benita
Raphael Geronimi
Rebecca Chen
Sam Bull
Sebastian Rittau
Sergei Lebedev
Shantanu
Stefano Chiodino
Steve Dignam
Sténio Jacinto
Timur Kushukov
Tom Most
turettn
Unrud
Utsav
Vasily Zakharov
Vincent Barbaresi
Vincent Meurisse
愚氓
Yuri Khan

The Mypy Blog

Friday, 22 January 2021

Mypy 0.800 Released