PEP: 526 Title: Syntax for Variable Annotations Version: $Revision$ Last-Modified: $Date$ Author: Ryan Gonzalez , Philip House , Ivan Levkivskyi , Lisa Roach , Guido van Rossum Status: Accepted Type: Standards Track Content-Type: text/x-rst Created: 09-Aug-2016 Python-Version: 3.6 Post-History: 30-Aug-2016, 02-Sep-2016 Status ====== This PEP has been provisionally accepted by the BDFL. See the acceptance message for more color: https://mail.python.org/pipermail/python-dev/2016-September/146282.html 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 class variables and instance variables), instead of expressing them through comments:: primes: List[int] = [] captain: str # Note: no initial value! class Starship: stats: ClassVar[Dict[str, int]] = {} PEP 484 explicitly states that type comments are intended to help with type inference in complex cases, and this PEP does not change this intention. However, since in practice type comments have also been adopted for class variables and instance variables, this PEP also discusses the use of type annotations for those variables. 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 requires 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 are 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 the ``typing.get_type_hints`` standard library function for runtime retrieval of annotations, variable annotations are not designed for runtime type checking. Third party packages will 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.** Type annotations should not be confused with variable declarations in statically typed languages. The goal of annotation syntax is to provide an easy way to specify 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. This syntax does not introduce any new semantics beyond PEP 484, so that the following three statements are equivalent:: var = value # type: annotation var: annotation; var = value var: annotation = value 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 initial value some_list: List[int] = [] # variable with initial value 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 the initial 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 defined by the typing module that indicates to the 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 might not be seen by a 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 this way a type checker may flag accidental assignments to attributes with the 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 (and convention), instance variables can be annotated in ``__init__`` or other methods, rather than in the 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, at least syntactically (it is up to the type checker what to do with this):: 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 is 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 not 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 as well as 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.) - **Use function based syntax**: It was proposed to annotate types of variables using ``var = cast(annotation[, value])``. Although this syntax alleviates some problems with type comments like absence of tne annotation in AST, it does not solve other problems such as readability and it introduces possible runtime overhead. - **Allow type annotations for tuple unpacking:** This causes ambiguity: it's not clear what this statement means:: 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 the types of ``y`` and ``z`` be? 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 annotations. And sometimes ``__init__`` is factored into more helper methods so it's even harder to chase them down. Putting the instance variable annotations 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`` store class variables to ``__annotations__``, so a simpler syntax was chosen. - **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. - **Annotate 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. - **Treating bare annotations the same as global or nonlocal:** The rejected proposal would prefer that the presence of an annotation without assignment in a function body should not involve *any* evaluation. In contrast, the PEP implies that if the target is more complex than a single name, its "left-hand part" should be evaluated at the point where it occurs in the function body, just to enforce that it is defined. For example, in this example:: def foo(self): slef.name: str the name ``slef`` should be evaluated, just so that if it is not defined (as is likely in this example :-), the error will be caught at runtime. This is more in line with what happens when there *is* an initial value, and thus is expected to lead to fewer surprises. (Also note that if the target was ``self.name`` (this time correctly spelled :-), an optimizing compiler has no obligation to evaluate ``self`` as long as it can prove that it will definitely be defined.) 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.