Friday 13 January 2017

Mypy 0.470 Released

We’ve just uploaded mypy 0.470 to PyPI. This release adds new features, bug fixes and library stub updates. You can install it as follows:

    python3 -m pip install mypy

Note: The package name is now “mypy” (no longer mypy-lang). If you have an older mypy version installed, remove it first, before installing the new package:

    python3 -m pip uninstall mypy-lang

Good news: New Package Name and Version Style

We have finally obtained ownership of the “mypy” package name on PyPI. In order to avoid conflicts with (hypothetical) users of the previous occupant of the “mypy” package name we’re changing the style of version numbers we’re using (at least until 1.0 comes along) — the new version is now 0.470. If you are using requirements.txt files, the proper incantation is now:


The old “mypy-lang” package will no longer be upgraded (the last version there is 0.4.6).

New Import Handling Options

The “silent imports” functionality, useful to avoid noisy errors about missing modules when annotating a large codebase, has been refactored and improved. There are now two separate flags (see the online docs for the full scoop):

  • --follow-imports=arg takes an argument which must be one of the following:
    • normal — the default behavior
    • silent — tries to find, parse and check imported modules (same as normal) but suppresses all errors for those modules that were not given on the command line
    • skip — roughly equivalent to the old --silent-imports flag
    • error — roughly equivalent to the old --almost-silent flag
  • --ignore-missing-imports suppresses error messages about imports that cannot be resolved at all. This was previously implied by --silent-imports unless --almost-silent was given.

Fast Parser Supported on Windows

The --fast-parser option now works also on Windows. It’s faster than the default parser and supports more Python syntax, including new syntax introduced in Python 3.6. The required package typed_ast is now installed by default as a mypy dependency (see below for more). We are planning to deprecate the current default parser in a future mypy release. (Also, the mypy project now runs tests for Windows on Appveyor.)

Improved Type Inference for lambda Expressions

A lambda expression without type context is now inferred as a Callable returning whatever type can be inferred for the expression in the body of the lambda. Previously such lambda expressions were given type Any . Note that lambda ...: None is now considered to have type Callable[..., None] which cannot be called to provide a value, only as a procedure call. The arguments are given type Any, so that e.g. the following two are now inferring the same type for a, i.e. Callable[[Any], List[Any]]:

    a = lambda x: [x]

    def a(x) -> List[Any]: return [x]

(Contributed by Elazar Gershuni)

Support for callable()

A condition using e.g. if callable(x) now causes mypy to infer a Callable type for x in the block controlled by the condition, and a non-Callable type in the block controlled by its negation. For example, this now type-checks:

    def maybecall(x: Union[int, Callable[[], int]]) -> int:
        if callable(x):
            return x()
            return x

(Contributed by Alex Frieder)

New Package Dependencies

Mypy now has automatic dependencies on typing and typed_ast. Previously typed_ast had to be manually installed using pip and typing was bundled with the mypy package.

Note that typed_ast requires Python 3.5+ on Windows. We’re dropping support for running mypy on Windows using Python 3.4 or older. (But checking still supports 2.7 and 3.2+ targets, using the --python-version flag.)

Notable Bugs Fixed

  • Callable type compatibility checking rewritten with several improvements (Naomi Seyfer)
  • Fix Callable arguments in overloaded functions
  • Fix lambda inference with simple Union contexts
  • Fix spurious warnings caused by type redeclarations (Łukasz Langa)
  • Fix handling of empty tuple type aliases
  • Fix crash on retrieving TypeVar from class (Tom Manderson)
  • Ensure required keyword-only arguments are provided (Naomi Seyfer)
  • Consider types like Tuple[t1, t2, ...] more consistently as subtypes of plain tuple (Ivan Levkivskyi)
  • Compare cached options to current per-file options when using --incremental
  • Fix NamedTuple defined in a method

New Experimental Features

There is work-in-progress support for dictionaries with literal string keys (TypedDict). Different keys can have items with different types, and mypy will figure out the right type for each key. For example, it will be possible to give a precise type for this dictionary:

    d = {'path': 'file.txt', 'size': 1254}

TypedDict is still very tentative and there’s no official documentation yet. You need to import TypedDict from mypy_extensions; use pip install -U mypy_extensions to install it. (Contributed by David Foster)

Descriptors are now supported. We're still not quite sure how well they work in practice. (Contributed by Calen Pennington)

Other Changes in This Release

  • Turn --hide-error-context on by default to generate more concise output from mypy. Add --show-error-context to re-enable the old behavior.
  • Arguments starting with __ (two underscores) are now positional-only. (Naomi Seyfer)
  • Add an API for calling mypy directly from another python application. (Jacques de Hooge)
  • Add ignore_errors config option to selectively ignore all non-fatal errors in some files.
  • Add Python 3 cheat sheet. (Ethan Smith)
  • Speed up the mypy test suite. (Łukasz Langa)
  • Miscellaneous other bug fixes.
  • Many updates to the library stubs in typeshed.


Thanks to all mypy contributors who contributed to this release:

  • Aleksander Vognild Burkow
  • Alex Frieder
  • Bertrand Bonnefoy-Claudet
  • Calen Pennington
  • Chris Oelmueller
  • David Euresti
  • David Foster
  • Elazar Gershuni
  • Ethan (ethanhs)
  • Ivan Levkivskyi
  • Jacques de Hooge
  • Jakub Stasiak
  • Jelle Zijlstra
  • Josiah Boning
  • Juanvulcano
  • Łukasz Langa
  • Naomi Seyfer
  • Roy Williams
  • Tetsuya Morimoto
  • Tom Manderson
  • TrueBrain

Additional thanks to these contributors to typeshed:

  • Alex Frieder
  • Alex Jurkiewicz
  • Anders Kaseorg
  • Bertrand Bonnefoy-Claudet
  • Cadel Watson
  • Calen Pennington
  • Daisuke Miyakawa
  • Danny Weinberg
  • David Euresti
  • Eric Moyer
  • George King
  • gotyaoi
  • Henri Dwyer
  • Hugo (hugovk)
  • Jason Fried
  • Jelle Zijlstra
  • Jon Dufresne
  • Joseph H Garvin
  • Josiah Boning
  • Kosaka Masayuki
  • lionel-github
  • Luiz Menezesf
  • Łukasz Langa
  • Madeleine Thompson
  • Mateusz Kurek
  • Matthias Kramm
  • Michael Lee
  • Mohab Usama
  • Naomi Seyfer
  • Nicolas Duchastel de Montrouge
  • Onno Kortmann
  • Peter Amstutz
  • Philip House
  • Reverb Chu
  • Richard Eames
  • Roy Williams
  • Ryan C. Thompson
  • rymdhund
  • Simon Ekstrand
  • Thomas Aynaud
  • Thomas Cellerier
  • Tom Manderson
  • TrueBrain
  • Wesley Bowman
  • z33ky

— Jukka (on behalf of the rest of the mypy team: Guido, David and Greg)