Friday 9 October 2020

Mypy 0.790 Released

We’ve just uploaded mypy 0.790 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.

Note that mypy 0.790 doesn’t yet officially support the recently released Python 3.9.

Enabling and Disabling Error Codes

You can now globally disable and enable the generation of error messages with specific error codes by using --enable-error-code and --disable-error-code command line options (and config file options). This lets you disallow certain checks that don’t provide value or are too noisy, and in the future these can be used to enable optional, stricter checks. For more information, read the documentation.

This was contributed by Callum Wilkinson (PR 9172).

Bug Fixes

  • Fix disable_error_code config file option (Aristotelis Mikropoulos, PR 9538)
  • Fix partial type crash during protocol checking (Shantanu, PR 9495)
  • Store the type for assignment expression (walrus) targets (Lawrence Chan, PR 9479)
  • Always type check arguments when using --disallow-untyped-calls (PR 9510)
  • Make None compatible with Hashable (PR 9371)
  • Don't infinite loop on self dependencies in --follow-imports=normal (Michael J. Sullivan, PR 9302)
  • Don't simplify away Any when joining union types (PR 9301)
  • Check for deleted vars in raise from (Alexandre Viau, PR 9272)
  • Fix corner case for comparing nested overloads (Nikita Sobolev, PR 9259)
  • Fix issues with async for and with statements (Guido van Rossum, PR 9268)
  • Fix internal error on list/dict comprehension with walrus operator in global scope (dhood, PR 9062)
  • Fix propagation of module-specific options to the parser (Guido van Rossum, PR 9247)
  • Fix crash when super is called outside a method (Weiss, PR 9173)
  • Fix untyped decorator overload error on class decorator with __call__ overloads (Ran Benita, PR 9232)
  • Clean up terminal width handling (Florian Bruhin, PR 9143)
  • Add keyword arguments for functional Enum API (LiuYuhui, PR 9123)
  • Validate follow_imports values in mypy.ini (Michael J. Sullivan, PR 9165)
  • Fix *expr in an assigned expression (LiuYuhui, PR 8827)
  • Don't consider comparing True and False as a dangerous comparison (PR 9021)
  • Make reveal_type work with call expressions returning None (Jakub Stasiak, PR 8924)

Error Reporting Improvements

  • Improve missing module error for subdirectories (Ethan Leba, PR 8927)
  • Clarify bytes formatting error messages (Shantanu, PR 9243)
  • Fix misleading follow_imports error message in dmypy (Michael J. Sullivan, PR 9167)
  • Use [arg-type] error code for additional argument type error messages (PR 9090)
  • Report some additional serious errors in junit.xml (PR 8950)
  • Report note about binary operation on the same location as error message (PR 8936)

Documentation Updates

  • Document disable_error_code config file option (Aristotelis Mikropoulos, PR 9539)
  • Add cross references to config file values (Oleg Höfling, PR 7859)
  • Add cross references to additional config values, and missing plugins config value, and document --help option for stubgen (Oleg Höfling, PR 9241)
  • Remove note that Final is experimental and suggest importing it from typing (Ran Benita, PR 9138)
  • Discuss unreachable code as a common issue (Chetan Khanna, PR 8899)

Stubgen Improvements

  • Improve property type detection in extension modules (Antoine Prouvost, PR 8999)
  • Fix type stubgen crash caused by invalid type annotation (Henry Schreiner, PR 8888)
  • Import Iterable and Iterator from typing in generated stubs (Ashley Whetter, PR 9088)

Other Improvements

  • Speed up type checking of unions containing many literal string types (Akuli, PR 9192)
  • Add scripts to misc/ (in the mypy GitHub repository) for computing and applying diffs of mypy caches to enable incremental cache downloads (Michael J. Sullivan, PR 8906)

Internal Changes

This release includes several improvements to mypyc, the compiler we use to speed up mypy. Most notably, Xuanda Yang has been migrating mypyc to use a new, lower-level intermediate representation in his Google Summer of Code project. This provides a solid foundation for a variety of future mypyc improvements.

Additional mypyc improvements:

  • Dynamically disallow instantiating traits/interpreted subclasses (Michael J. Sullivan, PR 9248)
  • Basic support for protocols (Michael J. Sullivan, PR 8914)
  • Various optimizations (PR 8903, PR 8870; Michael J. Sullivan and Xuanda Yang, PR 9245)
  • Make it easier to write mypyc test cases (PR 9094)

Typeshed Updates

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


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:

  • Akuli
  • Alexandre Viau
  • Antoine Lecubin
  • Antoine Prouvost
  • Aristotelis Mikropoulos
  • Ashley Whetter
  • Ben Wolsieffer
  • Brian Mboya
  • Callum Wilkinson
  • Chetan Khanna
  • dhood
  • Ethan Leba
  • Florian Bruhin
  • Henry Schreiner
  • ishaan
  • Jakub Stasiak
  • Jelle Zijlstra
  • Lawrence Chan
  • LiuYuhui
  • Michael R. Crusoe
  • Michael Sullivan
  • Nikita Sobolev
  • Oleg Höfling
  • Ram Rachum
  • Ran Benita
  • Rune Tynan
  • Shantanu
  • Weiss
  • Xuanda Yang
  • 谭九鼎

Additional thanks to all contributors to typeshed:

  • Adam Hitchcock
  • Akuli
  • Alex Grönholm
  • Anders Kaseorg
  • Andre Delfino
  • Andreas Poehlmann
  • Artem Simonov
  • Batuhan Taskaya
  • Brad Solomon
  • Brian Wellington
  • Bruce Merry
  • Cerek Hillen
  • Christine
  • Christopher Head
  • Christopher Schramm
  • Dakkaron
  • Dan Palmer
  • David Hotham
  • Denis Laxalde
  • Dmitry Marakasov
  • Eric Masseran
  • Eric N. Vander Weele
  • Eric Traut
  • Federico Bond
  • frehoy
  • Gregory P. Smith
  • helafumpek
  • Ian Good
  • Ian Woloschin
  • Igor Nehoroshev
  • Ilaï Deutel
  • Ilya Konstantinov
  • Jakub Stasiak
  • James Weaver
  • Jaromir Latal
  • Jelle Zijlstra
  • Jia Chen
  • John Snow
  • Jonathan Slenders
  • Joshua Oreman
  • Julian Mehnle
  • Julin S
  • Jürgen Gmach
  • Karl Ding
  • Kevin Wojniak
  • Kunal Arya
  • Luciano Ramalho
  • Magnus Watn
  • MapleCCC
  • Mario Ishac
  • Mark Mendoza
  • Markus Pielmeier
  • Martijn Pieters
  • Matan Gover
  • Michael H
  • Mikhail Golubev
  • Milap Sheth
  • Nils K
  • Nipunn Koorapati
  • Omar Sandoval
  • Peter Law
  • Philipp Hahn
  • Phillip Huang
  • Ran Benita
  • Rebecca Chen
  • Rich Li
  • Rune Tynan
  • Russell Davis
  • Sabby6
  • Sam Bull
  • Sebastian Kreft
  • Sebastian Rittau
  • Selim Belhaouane
  • Shannon Zhu
  • Shantanu
  • Steve B
  • Steve Dignam
  • Tarcisio
  • Teddy Sudol
  • Toru Ogawa
  • Vegard Stikbakke
  • Vishal Kuo
  • Vlad Emelianov
  • Willy Wu
  • Wouter Bolsterlee
  • Áron Ricardo Perez-Lopez

Wednesday 3 June 2020

Mypy 0.780 Released

Mypy 0.780 Released

We’ve just uploaded mypy 0.780 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.

Following Imports in Mypy Daemon

You can now leave out the --follow-imports option when running dmypy, and mypy daemon will follow imports, similar to a non-daemon mypy run:

    dmypy run -- pkg/*.py

Previously mypy daemon did not support this, and it was necessary to use --follow-imports and explicitly mention all files to check on the command line. (Following imports in mypy daemon is an experimental feature.)

See the docs about following import in mypy.

Miscellaneous New Features

  • Support environment variable interpolation for junit_xml configuration key (Mattwmaster58, PR 8479)
  • Add no_site_packages config file setting (davidzwa, PR 8524)
  • Allow .mypy.ini (with a dot prefix) as an alternative name to mypy.ini (dosisod, PR 8515)
  • Disallow unpacking of strings in multiple assignment, as this is usually a bug (Abtin, PR 8589)
  • Allow type aliases in a runtime (non-type) context (PR 8779)
  • Add narrowing unions with bool literals via identity check (Steve Dignam, PR 8821)
  • Always allow to cast to Any without warnings (oda, PR 8544)
  • Suggest solutions for a typo in a key of a TypedDict (Felicián Németh, PR 8483)
  • Provide more context about why incompatible with supertype is an error (Chetan Khanna, PR 8866)

Other Notable Improvements and Bug Fixes

  • Fix handling dependencies to __call__ in mypy daemon (PR 8494)
  • Various improvements to stubtest (PR 8502, PR 8886) (Shantanu)
  • Fix invalid type causing named tuple errors reported in wrong files (PR 8549)
  • Clarify documentation of Liskov substitution principle (PR 8563)
  • Fix type inference with lambda that returns None (PR 8562)
  • Fix incremental crash bug caused by NewType in functions (PR 8607)
  • Fix indexed assignment check when TypedDict is used as upper bound (Xuanda Yang, PR 8621)
  • Improve error message for bad indexed assignment (pando, PR 8641)
  • Fix crash when the same file is processed under multiple names (PR 8644)
  • Fix parser when using Python 3.9 (PR 8716)
  • Accept dict() as an empty dictionary in a TypedDict context (PR 8749)
  • Fix incorrect coroutine return type for nested function (Jens Widell, PR 8741)
  • Reject bytes literal as a TypedDict key (PR 8782)
  • Allow attrs keyword-only arguments at any position (Markus Schmaus, PR 8803)
  • Detect duplicate keys in call-based TypedDict definitions (PR 8849)
  • Look at arguments when generating constraints for an overload (PR 8845)
  • Fix potential wrong-file error message for unsupported types (PR 8854)

Typeshed Updates

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


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:

  • Abtin
  • Brad Solomon
  • Chetan Khanna
  • Claudio Jolowicz
  • davidzwa
  • Diego Elio Pettenò
  • dosisod
  • Ethan Smith
  • Felicián Németh
  • Jelle Zijlstra
  • Jens Widell
  • Joy Bhalla
  • LiuYuhui
  • Markus Schmaus
  • Marten Kenbeek
  • Mattwmaster58
  • Max R
  • oda
  • pando
  • PattenR
  • prostomarkeloff
  • Shantanu
  • Steve Dignam
  • Timofey Kukushkin
  • Ville Skyttä
  • Xuanda Yang

Additional thanks to all contributors to typeshed:

  • Alex McGrath Kraak
  • Alexandre Yang
  • Andrew
  • Andrew Svetlov
  • Anis
  • Anthony Sottile
  • Ben Leslie
  • Ben Motz
  • Benjamin Poirier
  • Brian Turek
  • Bruce Merry
  • Christopher Whelan
  • coiax
  • David Euresti
  • David T.H. Kao
  • Debjyoti Biswas
  • Denis Laxalde
  • Diego Elio Pettenò
  • dosisod
  • Ethan Smith
  • Florian Ludwig
  • Florimond Manca
  • Gal Ben David
  • Graham Bleaney
  • Jakub Stasiak
  • Jan Verbeek
  • Jaromir Latal
  • Javier Honduvilla Coto
  • Jeff Hunter
  • Jelle Zijlstra
  • Jia Chen
  • Jocelyn Boullier
  • Joost Cassee
  • Julian Andres Klode
  • Julin S
  • karl ding
  • Katelyn Gigante
  • Kazushi Kitaya
  • Kjell Braden
  • Lars
  • Lawrence
  • Luciano Ramalho
  • Mark Mendoza
  • Markus Wamser
  • Max R
  • Mickaël Schoentgen
  • Nguyễn Gia Phong
  • Nipunn Koorapati
  • Oleg Höfling
  • Paul Ganssle
  • Peter Pentchev
  • petsuter
  • PGijsbers
  • Philipp Hahn
  • Rahix
  • Ran Benita
  • Razzi Abuissa
  • Rebecca Chen
  • Rodrigo Castro
  • Rune Tynan
  • Sahith Nallapareddy
  • Sam Bull
  • Sebastian Rittau
  • Selim Belhaouane
  • Serhiy Storchaka
  • Shantanu
  • Steve Dignam
  • Tim Hatch
  • Vishal Kuo
  • Zhiming Wang

Tuesday 10 March 2020

Mypy 0.770 Released

We’ve just uploaded mypy 0.770 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.

Tagged Unions

Previously, the only way to distinguish between two types in a union was to use an isinstance check. Now, mypy supports distinguishing between two or more types based on the value of some common shared "tag" field.

This feature is most useful when you want to narrow down a union of TypedDicts: since TypedDicts are really just regular dicts at runtime, using isinstance checks won't work. Instead, you can now label or tag each TypedDict with a distinct Literal type and discriminate by checking that tag:

    from typing import Literal, TypedDict, Union
    class NewJob(TypedDict):
        event_type: Literal["new-job"]
        job_description: str
    class CancelJob(TypedDict):
        event_type: Literal["cancel-job"]
        job_id: int
    request: Union[NewJob, CancelJob]
    # We know request["event_type"] must be of type
    # Union[Literal["new-job"], Literal["cancel-job"]]
    if request["event_type"] == "new-job":
        # So if we narrow request["event_type"] down to type
        # Literal["new-job"], mypy will conclude 'request' must
        # therefore be of type 'NewJob' in this branch...
        print("Starting new job", request.job_description)
        # ...and inferred to be type 'CancelJob' in this one
        print("Cancelling job with id", request.job_id)

Note that you can import Literal from typing_extensions on Python 3.7 and earlier.

You can also use this technique to discriminate between unions of objects, tuples, or named tuples, so long as: 1. Every item in your union is tagged with a unique value 2. Mypy understands how to narrow the union of the tag values.

For more details and examples, see the documentation on tagged unions.

This feature was contributed by Michael Lee.

Type Inference Improvements

Type inference is now more powerful. Mypy can infer types in some cases where you previously needed an explicit type annotation.

Mypy can infer a built-in collection type from augmented assignment statements:

    x = []
    x += [1, 2]  # Infer type List[int] for x

Some dictionary construction idioms are better supported:

    d = {}  # No type annotation needed
    for k in things():
        if k not in d:
            d[k] = 0
            d[k] += 1

Mypy is better at inferring defaultdict types:

    from collections import defaultdict
    d = defaultdict(list)
    d['foo'].append(1)  # Infer type DefaultDict[str, List[int]] for d

Multiple Inheritance and isinstance() Checks

Previously, mypy assumed a variable cannot have two unrelated types when analyzing isinstance checks. For example, in the following example B is not a subclass of A, so mypy would incorrectly conclude the isinstance check is always false, and that the if-branch is unreachable — even though var could actually be a subtype of both types.

This meant that mypy skipped type checking anything inside the if statement, since it (by design) skips analyzing unreachable branches.

    class A: pass
    class B: pass
    class Ok(A, B): pass
    x: A = Ok()
    if isinstance(x, B):

Now, mypy will instead decide that the narrowed type of x is test.<subclass of "A" and "B">, instead of marking the branch as unreachable.

If it’s impossible for two types to be a subtype of one another, mypy will continue to not attempt to infer this "ad-hoc intersection":

    # Fails with a "TypeError: multiple bases have instance lay-out confict" error at runtime
    class Impossible(int, str): pass
    bad: int = Impossible()
    if instance(bad, str):
        # This branch is unreachable and so is not type checked.
        # If you want to be warned about unreachable branches, use
        # the --warn-unreachable flag.

This feature was contributed by Michael Lee.

Other Notable Improvements and Bug Fixes

  • Fix some crash bugs involving import * and import cycles (PR 8450)
  • Fix interaction of descriptor methods with plugins (Jan Verbeek, PR 8365)
  • Allow strict in config file (Ville Skyttä, PR 8192)
  • Don’t crash when a module shadows builtin libraries (such as typing or types, PR 8405)
  • Fix type join between subclasses of unicode and str (PR 8402)
  • Fix type join of fixed-length tuples with mismatching lengths (Marti Raudsepp, PR 8333)
  • Fix type join of Sequence (e.g. variadic tuple) and fixed-length tuple (Marti Raudsepp, PR 8335)
  • Make mypy.api.run_dmypy actually capture the output (PR 8375)
  • Support determining whether a literal is truthy (Jan Verbeek, PR 8368)
  • Fix covariant overriding of decorated methods (Xuanda Yang, PR 8350)
  • Support typing.Annotated in addition to typing_extensions.Annotated (Jakub Stasiak, PR 8371)
  • Add add_method_to_class function to plugins.common (useful when writing mypy plugins) (Maksim Kurnikov, PR 8245)
  • Fix module alias as instance attribute (Uwe L. Korn, PR 8259)
  • Automatically write a file .gitignore file to cache directory, ignoring everything (Ville Skyttä , PR 8193)
  • Don't make dunder attributes enum members (Xuanda Yang, PR 8302)
  • Allow redefining TypedDict keys (while still generating an error) (Cohen Karnell, PR 8109)
  • Fix some crashes in dataclasses (PR 8271)
  • Use TypedDict union as type context when unambiguous (PattenR, PR 8212)
  • Fix false positive for subclasses of bytes overriding __str__ (Shantanu, PR 8222)
  • Fix deeply nested InitVar definitions in dataclasses with init=False (Jacob Beck, PR 8208)
  • Narrow types for walrus assignment in if statements in some cases (Shantanu, PR 8258)
  • Narrow types for walrus assignment in if statements in most of the rest of cases (PR 8458)
  • Fix incorrect error code indexing (Xuanda Yang, PR 8248)
  • Fix regression in container check logic (Michael Lee, PR 8232)
  • Make reachability code understand chained comparisons (Michael Lee, PR 8148)
  • Fix incorrect name lookup for decorated methods (Xuanda Yang, PR 8175)
  • Fix simplifying unions with type aliases (Xuanda Yang, PR 8146)
  • Fix crash when overriding __init__ in a dataclass subclass (Jacob Beck, PR 8159)
  • Fix some daemon crashes involving classes becoming generic (PR 8157)

Documentation and Error Reporting Improvements

  • Use fully qualified names in error messages for class names resembling builtins(Mukuntha N S, PR 8425)
  • Improve diagnostics involving missing stubs for a library that is installed in site-packages (Michael Lee, PR 8238)
  • Add a section to the documentation about incompatible overrides (RAHUL RAJA, PR 8377)
  • Add variable-sized tuples to the cheat sheet (Marcio Mazza, PR 8364)
  • Improve documentation of decorators (add decorator factories) (Marti Raudsepp, PR 8336)
  • Update documentation of variables and aliases (Xuanda Yang, PR 8200)
  • Report an error if a final class has abstract attributes (Denys Halenok, PR 8332)
  • Update common issues to include __init__ without arguments (Tan Yuanhong, PR 8303)
  • Add a new error code for errors about unreachability (Denys Halenok, PR 8312)
  • Fix error indicator position in code with tabs (dosisod, PR 8307)
  • Document --local-partial-types (Xuanda Yang, PR 8201)
  • Update documentation for Literal types (Michael Lee, PR 8152)

Stubtest rewrite

stubtest is a tool that compares stub definitions to what it finds at runtime with introspection and reports back inconsistencies. It got a complete rewrite. Some features of the new stubtest are:
  • Find missing, extraneous or mistyped classes, methods, functions and attributes in the stubs
  • Check presence, names and kinds of function arguments, accounting for overloads, decorators, *args and kwargs. Checks argument types against their default values, accounting for type vars.
  • Checks @property, @classmethod, @staticmethod declarations
  • Check types of module level and class level attributes and enums

Some results of this:

  • We now run stubtest in typeshed CI (for stdlib)
  • We’ve fixed about 1900 issues in typeshed definitions
  • We’ve greatly improved Python 3.8 support in typeshed and it’ll be easier to make the changes needed for future Python versions
  • We’ve uncovered a handful of issues in Python itself

The stubtest rewrite was contributed by Shanatu.

Typeshed Updates

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


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:

  • Anthony Sottile
  • Cohen Karnell
  • Denys Halenok
  • dosisod
  • Ethan Smith
  • FooTaGe
  • hauntsaninja
  • HeShan
  • Jacob Beck
  • Jakub Stasiak
  • Jan Verbeek
  • Jérome Perrin
  • lazytype
  • Maksim Kurnikov
  • Marcio Mazza
  • Marti Raudsepp
  • Michael Lee
  • Mukuntha N S
  • Nikita Sobolev
  • PattenR
  • Shantanu
  • Tan Yuanhong
  • Uwe L. Korn
  • Ville Skyttä
  • Xuanda Yang

Additional thanks to all contributors to typeshed:

  • abe
  • Alan Du
  • Alex Grönholm
  • Alex Willmer
  • Andrew Svetlov
  • Anthony Sottile
  • Artjoms Iskovs
  • Batuhan Taşkaya
  • Benjamin Peterson
  • Brian Maissy
  • Cal Paterson
  • Christopher Dignam
  • crusaderky
  • Cyril Roelandt
  • Daniel Däschle
  • Daniel Farley
  • Daniel Hahler
  • Daniël van Eeden
  • Dave Halter
  • dave-shawley
  • Erick
  • Faidon Liambotis
  • Fionn Fitzmaurice
  • François Freitag
  • Gisle Aas
  • hauntsaninja
  • Ian Good
  • ijl
  • Ilaï Deutel
  • Jacob Beck
  • Jacob Ilias Komissar
  • Jakub Stasiak
  • Jan Verbeek
  • Jaromir Latal
  • Jason
  • Jason Gilholme
  • Jelle Zijlstra
  • Jens Hedegaard Nielsen
  • Jeppe Fihl-Pearson
  • layday
  • lazytype
  • Maarten ter Huurne
  • Mark
  • Michael Heyns
  • Mickaël Schoentgen
  • Niklas Fiekas
  • Oleg Höfling
  • Ophir LOJKINE
  • Pavel Savchenko
  • Rebecca Chen
  • Reid Swan
  • rikhilraithatha
  • Romain
  • Rune Tynan
  • Sebastian Rittau
  • Shantanu
  • Squirrel
  • Thomas Schaper
  • tikki
  • Vury Leo
  • Wolf Honore