PEP: 482
Title: Literature Overview for Type Hints
Author: Łukasz Langa <lukasz@python.org>
Discussions-To: python-ideas@python.org
Status: Final
Type: Informational
Topic: Typing
Created: 08-Jan-2015
Post-History:


Abstract
========

This PEP is one of three related to type hinting.  This PEP gives a
literature overview of related work.  The main spec is :pep:`484`.


Existing Approaches for Python
==============================


mypy
----

(This section is a stub, since `mypy`__ is essentially what we're
proposing.)

__ https://mypy-lang.org


Reticulated Python
------------------

`Reticulated Python`__ by Michael Vitousek is an example of
a slightly different approach to gradual typing for Python. It is
described in an actual `academic paper`__ written by
Vitousek with Jeremy Siek and Jim Baker (the latter of Jython fame).

__ https://github.com/mvitousek/reticulated
__ http://wphomes.soic.indiana.edu/jsiek/files/2014/03/retic-python.pdf

PyCharm
-------

PyCharm by JetBrains has been providing a way to specify and check
types for about four years.  The type system suggested by PyCharm__
grew from simple class types to tuple types, generic types,
function types, etc. based on feedback of many users who shared their
experience of using type hints in their code.

__ https://github.com/JetBrains/python-skeletons#types

Others
------

TBD: Add sections on pyflakes__, pylint__, numpy__,
`Argument Clinic`__, pytypedecl__, numba__, obiwan__.

__ https://github.com/pyflakes/pyflakes/
__ https://www.pylint.org
__ https://www.numpy.org
__ https://docs.python.org/3/howto/clinic.html
__ https://github.com/google/pytypedecl
__ https://numba.pydata.org
__ https://pypi.org/project/obiwan

Existing Approaches in Other Languages
======================================

ActionScript
------------

ActionScript__ is a class-based, single inheritance,
object-oriented superset of ECMAScript.  It supports interfaces and
strong runtime-checked static typing.  Compilation supports a “strict
dialect” where type mismatches are reported at compile-time.

__ https://livedocs.adobe.com/specs/actionscript/3/

Example code with types::

  package {
    import flash.events.Event;

    public class BounceEvent extends Event {
      public static const BOUNCE:String = "bounce";
      private var _side:String = "none";

      public function get side():String {
        return _side;
      }

      public function BounceEvent(type:String, side:String){
        super(type, true);
        _side = side;
      }

      public override function clone():Event {
        return new BounceEvent(type, _side);
      }
    }
  }

Dart
----

Dart__ is a class-based, single inheritance, object-oriented
language with C-style syntax.  It supports interfaces, abstract classes,
reified generics, and optional typing.

__ https://www.dartlang.org

Types are inferred when possible.  The runtime differentiates between two
modes of execution: *checked mode* aimed for development (catching type
errors at runtime) and *production mode* recommended for speed execution
(ignoring types and asserts).

Example code with types::

  class Point {
      final num x, y;

      Point(this.x, this.y);

      num distanceTo(Point other) {
          var dx = x - other.x;
          var dy = y - other.y;
          return math.sqrt(dx * dx + dy * dy);
      }
  }

Hack
----

Hack__ is a programming language that interoperates seamlessly
with PHP.  It provides opt-in static type checking, type aliasing,
generics, nullable types, and lambdas.

__ https://hacklang.org

Example code with types::

  <?hh
  class MyClass {
    private ?string $x = null;

    public function alpha(): int {
      return 1;
    }

    public function beta(): string {
      return 'hi test';
    }
  }

  function f(MyClass $my_inst): string {
    // Will generate a hh_client error
    return $my_inst->alpha();
  }

TypeScript
----------

TypeScript__ is a typed superset of JavaScript that adds
interfaces, classes, mixins and modules to the language.

__ http://www.typescriptlang.org

Type checks are duck typed.  Multiple valid function signatures are
specified by supplying overloaded function declarations.  Functions and
classes can use generics as type parameterization.  Interfaces can have
optional fields.  Interfaces can specify array and dictionary types.
Classes can have constructors that implicitly add arguments as fields.
Classes can have static fields.  Classes can have private fields.
Classes can have getters/setters for fields (like property).  Types are
inferred.

Example code with types::

  interface Drivable {
      start(): void;
      drive(distance: number): boolean;
      getPosition(): number;
  }

  class Car implements Drivable {
      private _isRunning: boolean;
      private _distanceFromStart: number;

      constructor() {
          this._isRunning = false;
          this._distanceFromStart = 0;
      }

      public start() {
          this._isRunning = true;
      }

      public drive(distance: number): boolean {
          if (this._isRunning) {
              this._distanceFromStart += distance;
              return true;
          }
          return false;
      }

      public getPosition(): number {
          return this._distanceFromStart;
      }
  }


Copyright
=========

This document has been placed in the public domain.
