python-peps/pep-0526.txt

PEP: 526
Title: Syntax for Variable Annotations
Version: $Revision$
Last-Modified: $Date$
Author: Ryan Gonzalez <rymg19@gmail.com>, Philip House <phouse512@gmail.com>, Ivan Levkivskyi <levkivskyi@gmail.com>, Lisa Roach <lisaroach14@gmail.com>, Guido van Rossum <guido@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 09-Aug-2016
Python-Version: 3.6
Post-History: 30-Aug-2016


Notice for Reviewers
====================

This PEP was drafted in a separate repo:
https://github.com/phouse512/peps/tree/pep-0526.

There was preliminary discussion on python-ideas and at
https://github.com/python/typing/issues/258.

Before you bring up an objection in a public forum please at least
read the summary of rejected ideas listed at the end of this PEP.


Abstract
========

PEP 484 introduced type hints, a.k.a. type annotations.  While its
main focus was function annotations, it also introduced the notion of
type comments to annotate variables::

  # 'primes' is a list of integers
  primes = []  # type: List[int]

  # 'captain' is a string (Note: initial value is a problem)
  captain = ...  # type: str

  class Starship:
      # 'stats' is a class variable
      stats = {}  # type: Dict[str, int]

This PEP aims at adding syntax to Python for annotating the types of variables
(including non-method attributes), instead of expressing them through
comments::

  primes: List[int] = []

  captain: str  # Note: no initial value!

  class Starship:
      stats: ClassVar[Dict[str, int]] = {}


Rationale
=========

Although type comments work well enough, the fact that they're
expressed through comments has some downsides:

- Text editors often highlight comments differently from type annotations.

- There's no way to annotate the type of an undefined variable; one needs to
  initialize it to ``None`` (e.g. ``a = None # type: int``).

- Variables annotated in a conditional branch are difficult to read::

    if some_value:
        my_var = function() # type: Logger
    else:
        my_var = another_function() # Why isn't there a type here?

- Since type comments aren't actually part of the language, if a Python script
  wants to parse them, it would require a custom parser instead of just using
  ``ast``.

- Type comments are used a lot in typeshed. Migrating typeshed to use
  the variable annotation syntax instead of type comments would improve
  readability of stubs.

- In situations where normal comments and type comments used together, it is
  difficult to distinguish them::

    path = None  # type: Optional[str]  # Path to module source

- It's impossible to retrieve the annotations at runtime outside of
  attempting to find the module's source code and parse it at runtime,
  which is inelegant, to say the least.

The majority of these issues can be alleviated by making the syntax
a core part of the language. Moreover, having a dedicated annotation syntax
for class and instance variables (in addition to method annotations) will
pave the way to static duck-typing as a complement to nominal typing defined
by PEP 484.

Non-goals
*********

While the proposal is accompanied by an extension of ``typing.get_type_hints``
standard library function for runtime retrieval of annotations, the variable
annotations are not designed for runtime type checking. Third party packages
would have to be developed to implement such functionality.

It should also be emphasized that **Python will remain a dynamically typed
language, and the authors have no desire to ever make type hints mandatory,
even by convention.** The goal of annotation syntax is to provide an
easy way to specify the structured type metadata for third party tools.

This PEP does not require type checkers to change their type checking
rules. It merely provides a more readable syntax to replace type
comments.


Specification
=============

Type annotation can be added to an assignment statement or to a single
expression indicating the desired type of the annotation target to a third
party type checker::

  my_var: int
  my_var = 5  # Passes type check.
  other_var: int  = 'a'  # Flagged as error by type checker,
                         # but OK at runtime.

Below we specify the syntax of type annotations
in different contexts and their runtime effects.

We also suggest how type checkers might interpret annotations, but
compliance to these suggestions is not mandatory.  (This is in line
with the attitude towards compliance in PEP 484.)

Global and local variable annotations
*************************************

The types of locals and globals can be annotated as follows::

  some_number: int           # variable without default
  some_list: List[int] = []  # variable with default

Being able to omit the initial value allows for easier typing of variables
assigned in conditional branches::

  sane_world: bool
  if 2+2 == 4:
      sane_world = True
  else:
      sane_world = False

Note that, although the syntax does allow tuple packing, it does *not* allow
one to annotate the types of variables when tuple unpacking is used::

  # Tuple packing with variable annotation syntax
  t: Tuple[int, ...] = (1, 2, 3)

  # Tuple unpacking with variable annotation syntax
  header: str
  kind: int
  body: Optional[List[str]]
  header, kind, body = message

Omitting a default value leaves the variable uninitialized::

  a: int
  print(a)  # raises NameError

However, annotating a local variable will cause the interpreter to always make
it a local::

  def f():
      a: int
      print(a)  # raises UnboundLocalError
      # Commenting out the a: int makes it a NameError.

as if the code were::

  def f():
      if False: a = 0
      print(a)  # raises UnboundLocalError

Duplicate type annotations will be ignored. However, static type
checkers may issue a warning for annotations of the same variable
by a different type::

  a: int
  a: str  # Static type checker may or may not warn about this.

.. _classvar:

Class and instance variable annotations
***************************************

Type annotations can also be used to annotate class and instance variables
in class bodies and methods. In particular, the value-less notation ``a: int``
allows one to annotate instance variables that should be initialized
in ``__init__`` or ``__new__``. The proposed syntax is as follows::

  class BasicStarship:
      captain: str = 'Picard'               # instance variable with default
      damage: int                           # instance variable without default
      stats: ClassVar[Dict[str, int]] = {}  # class variable

Here ``ClassVar`` is a special class in typing module that indicates to
static type checker that this variable should not be set on instances.
This could be illustrated with a more detailed example. In this class::

  class Starship:
      captain = 'Picard'
      stats = {}
      def __init__(self, damage, captain=None):
          self.damage = damage
          if captain:
              self.captain = captain  # Else keep the default
      def hit(self):
          Starship.stats['hits'] = Starship.stats.get('hits', 0) + 1

``stats`` is intended to be a class variable (keeping track of many different
per-game statistics), while ``captain`` is an instance variable with a default
value set in the class. This difference could not be seen by type
checker -- both get initialized in the class, but ``captain`` serves only
as a convenient default value for the instance variable, while ``stats``
is truly a class variable -- it is intended to be shared by all instances.

Since both variables happen to be initialized at the class level, it is
useful to distinguish them by marking class variables as annotated with
types wrapped in ``ClassVar[...]``. In such way type checker will prevent
accidental assignments to attributes with a same name on instances.
For example, annotating the discussed class::

  class Starship:
      captain: str = 'Picard'
      damage: int
      stats: ClassVar[Dict[str, int]] = {}
      def __init__(self, damage: int, captain: str = None):
          self.damage = damage
          if captain:
              self.captain = captain  # Else keep the default
      def hit(self):
          Starship.stats['hits'] = Starship.stats.get('hits', 0) + 1

      enterprise_d = Starship(3000)
      enterprise_d.stats = {} # Flagged as error by a type checker
      Starship.stats = {} # This is OK

As a matter of convenience, instance variables can be annotated in
``__init__`` or other methods, rather than in class::

  from typing import Generic, TypeVar
  T = TypeVar(’T’)

  class Box(Generic[T]):
      def __init__(self, content):
          self.content: T = content

Annotating expressions
**********************

The target of the annotation can be any valid single assignment target::

  class Cls:
      pass

  c = Cls()
  c.x: int = 0  # Annotates c.x with int.
  c.y: int      # Annotates c.y with int.

  d = {}
  d['a']: int = 0  # Annotates d['a'] with int.
  d['b']: int      # Annotates d['b'] with int.

Note that even a parenthesized name considered an expression,
not a simple name::

  (x): int      # Annotates x with int, (x) treated as expression by compiler.
  (y): int = 0  # Same situation here.

Where annotations aren't allowed
********************************

It is illegal to attempt to annotate variables subject to ``global``
or ``nonlocal`` in the same function scope::

  def f():
      global x: int  # SyntaxError

  def g():
      x: int  # Also a SyntaxError
      global x

The reason is that ``global`` and ``nonlocal`` don't own variables;
therefore, the type annotations belong in the scope owning the variable.

Only single assignment targets and single right hand side values are allowed.
In addition, one cannot annotate variables used in a ``for`` or ``with``
statement; they can be annotated ahead of time, in a similar manner to tuple
unpacking::

  a: int
  for a in my_iter:
      ...

  f: MyFile
  with myfunc() as f:
      ...


Changes to Standard Library and Documentation
=============================================

- A new covariant type ``ClassVar[T_co]`` is added to the ``typing``
  module. It accepts only a single argument that should be a valid type,
  and is used to annotate class variables that should no be set on class
  instances. This restriction is ensured by static checkers,
  but not at runtime. See the
  `classvar`_ section for examples and explanations for the usage of
  ``ClassVar``, and see the `rejected`_ section for more information on
  the reasoning behind ``ClassVar``.

- Function ``get_type_hints`` in the ``typing`` module will be extended,
  so that one can retrieve type annotations at runtime from modules
  and classes in addition to functions.
  Annotations are returned as a dictionary mapping from variable or arguments
  to their type hints with forward references evaluated.
  For classes it returns a mapping (perhaps ``collections.ChainMap``)
  constructed from annotations in method resolution order.

- Recommended guidelines for using annotations will be added to the
  documentation, containing a pedagogical recapitulation of specifications
  described in this PEP and in PEP 484. In addition, a helper script for
  translating type comments into type annotations will be published
  separately from the standard library.


Runtime Effects of Type Annotations
===================================

Annotating a local variable will cause
the interpreter to treat it as a local, even if it was never assigned to.
Annotations for local variables will not be evaluated::

  def f():
      x: NonexistentName  # No error.

However, if it is at a module or class level, then the type *will* be
evaluated::

  x: NonexistentName  # Error!
  class X:
      var: NonexistentName  # Error!

In addition, at the module or class level, if the item being annotated is a
*simple name*, then it and the annotation will be stored in the
``__annotations__`` attribute of that module or class as a dictionary mapping
from names to evaluated annotations. Here is an example::

  from typing import Dict
  class Player:
      ...
  players: Dict[str, Player]

  print(__annotations__)
  # prints: {'players': typing.Dict[str, __main__.Player]}

``__annotations__`` is writable, so this is permitted::

  __annotations__['s'] = str

But attempting to update ``__annotations__`` to something other than a dict
may result in a TypeError::

  class C:
      __annotations__ = 42
      x: int = 5  # raises TypeError

(Note that the assignment to ``__annotations__``, which is the
culprit, is accepted by the Python interpreter without questioning it
-- but the subsequent type annotation expects it to be a
``MutableMapping`` and will fail.)

The recommended way of getting annotations at runtime is by using
``typing.get_type_hints`` function; as with all dunder attributes,
any undocummented use of ``__annotations__`` is subject to breakage
without warning::

  from typing import Dict, ClassVar, get_type_hints
  class Starship:
      hitpoints: int = 50
      stats: ClassVar[Dict[str, int]] = {}
      shield: int = 100
      captain: str
      def __init__(self, captain: str) -> None:
          ...

  assert get_type_hints(Starship) == {'hitpoints': int,
                                      'stats': ClassVar[Dict[str, int]],
                                      'shield': int,
                                      'captain': str}

  assert get_type_hints(Starship.__init__) == {'captain': str,
                                               'return': None}

Note that if annotations are not found statically, then the
``__annotations__`` dictionary is not created at all. Also the
value of having annotations available locally does not offset
the cost of having to create and populate the annotations dictionary
on every function call. Therefore annotations at function level are
not evaluated and not stored.

Other uses of annotations
*************************

While Python with this PEP will not object to::

  alice: 'well done' = 'A+'
  bob: 'what a shame' = 'F-'

since it will not care about the type annotation beyond "it evaluates
without raising", a type checker that encounters it will flag it,
unless disabled with ``# type: ignore`` or ``@no_type_check``.

However, since Python won't care what the "type" is,
if the above snippet is at the global level or in a class, ``__annotations__``
will include ``{'alice': 'well done', 'bob': 'what a shame'}``.

These stored annotations might be used for other purposes,
but with this PEP we explicitly recommend type hinting as the
preferred use of annotations.

.. _rejected:

Rejected/Postponed Proposals
============================

- **Should we introduce variable annotations at all?**
  Variable annotations have *already* been around for almost two years
  in the form of type comments, sanctioned by PEP 484.  They are
  extensively used by third party type checkers (mypy, pytype,
  PyCharm, etc.) and by projects using the type checkers. However, the
  comment syntax has many downsides listed in Rationale. This PEP is
  not about the need for type annotations, it is about what should be
  the syntax for such annotations.

- **Introduce a new keyword:**
  The choice of a good keyword is hard,
  e.g. it can't be ``var`` because that is way too common a variable name,
  and it can't be ``local`` if we want to use it for class variables or
  globals. Second, no matter what we choose, we'd still need
  a ``__future__`` import.

- **Use 'def' as a keyword:**
  The proposal would be::

    def primes: List[int] = []
    def captain: str

  The problem with this is that ``def`` means "define a function" to
  generations of Python programmers (and tools!), and using it also to
  define variables does not increase clarity.  (Though this is of
  course subjective.)

- **Allow type annotations for tuple unpacking:**
  This causes an ambiguity: It's not clear what meaning should be
  assigned to this statement::

    x, y: T

  Are ``x`` and ``y`` both of type ``T``, or do we expect ``T`` to be
  a tuple type of two items that are distributed over ``x`` and ``y``,
  or perhaps ``x`` has type ``Any`` and ``y`` has type ``T``? (The
  latter is what this would mean if this occurred in a function
  signature.)  Rather than leave the (human) reader guessing, we
  forbid this, at least for now.

- **Parenthesized form ``(var: type)`` for annotations:**
  It was brought up on python-ideas as a remedy for the above-mentioned
  ambiguity, but it was rejected since such syntax would be hairy,
  the benefits are slight, and the readability would be poor.

- **Allow annotations in chained assignments:**
  This has problems of ambiguity and readability similar to tuple
  unpacking, for example in::

    x: int = y = 1
    z = w: int = 1

  it is ambiguous, what should be the type of ``y``, and what should
  be the type of ``z``. Also the second line is difficult to parse.

- **Allow annotations in ``with`` and ``for`` statement:**
  This was rejected because in ``for`` it would make it hard to spot the actual
  iterable, and in ``with`` it would confuse the CPython's LL(1) parser.

- **Evaluate local annotations at function definition time:**
  This has been rejected by Guido because the placement of the annotation
  strongly suggests that it's in the same scope as the surrounding code.

- **Store variable annotations also in function scope:**
  The value of having the annotations available locally is just not enough
  to significantly offset the cost of creating and populating the dictionary
  on *each* function call.

- **Initialize variables annotated without assignment:**
  It was proposed on python-ideas to initialize ``x`` in ``x: int`` to
  ``None`` or to an additional special constant like Javascript's
  ``undefined``. However, adding yet another singleton value to the language
  would needed to be checked for everywhere in the code. Therefore,
  Guido just said plain "No" to this.

- **Add also** ``InstanceVar`` **to the typing module:**
  This is redundant because instance variables are way more common than
  class variables. The more common usage deserves to be the default.

- **Allow instance variable annotations only in methods:**
  The problem is that many ``__init__`` methods do a lot of things besides
  initializing instance variables, and it would be harder (for a human)
  to find all the instance variable declarations.
  And sometimes ``__init__`` is factored into more helper methods
  so it's even harder to chase them down. Putting the instance variable
  declarations together in the class makes it easier to find them,
  and helps a first-time reader of the code.

- **Use syntax** ``x: class t = v`` **for class variables:**
  This would require a more complicated parser and the ``class``
  keyword would confuse simple-minded syntax highlighters. Anyway we
  need to have ``ClassVar`` to store class variables to
  ``__annotations__``, so that it was decided to go with a simpler
  syntax.

- **Forget about** ``ClassVar`` **altogether:**
  This was proposed since mypy seems to be getting along fine without a way
  to distinguish between class and instance variables. But a type checker
  can do useful things with the extra information, for example flag
  accidental assignments to a class variable via the instance
  (which would create an instance variable shadowing the class variable).
  It could also flag instance variables with mutable defaults,
  a well-known hazard.

- **Do not evaluate annotations, treat them as strings:**
  This would be inconsistent with the behavior of function annotations that
  are always evaluated. Although this might be reconsidered in future,
  it was decided in PEP 484 that this would have to be a separate PEP.

- **Declare variable types in class docstring:**
  Many projects already use various docstring conventions, often without
  much consistency and generally without conforming to the PEP 484 annotation
  syntax yet. Also this would require a special sophisticated parser.
  This, in turn, would defeat the purpose of the PEP --
  collaborating with the third party type checking tools.

- **Implement ``__annotations__`` as a descriptor:**
  This was proposed to prohibit setting ``__annotations__`` to something
  non-dictionary or non-None. Guido has rejected this idea as unnecessary;
  instead a TypeError will be raised if an attempt is made to update
  ``__annotations__`` when it is anything other than a dict.


Backwards Compatibility
=======================

This PEP is fully backwards compatible.


Implementation
==============

An implementation for Python 3.6 is found on GitHub repo at
https://github.com/ilevkivskyi/cpython/tree/pep-526


Copyright
=========

This document has been placed in the public domain.