python · JelleZijlstra · Mar 14, 2023 · Nov 7, 2022 · Jan 12, 2023 · Jan 12, 2023
@@ -581,6 +581,7 @@ pep-0697.rst  @encukou
 pep-0698.rst  @jellezijlstra
 pep-0699.rst  @Fidget-Spinner
 pep-0700.rst  @pfmoore
+pep-0705.rst  @pablogsal
 # ...
 # pep-0754.txt
 # ...

@@ -0,0 +1,259 @@
+PEP: 9999
+Title: TypedMapping: Type Hints for Mappings with a Fixed Set of Keys
+Author: Alice Purcell <alicederyn@gmail.com>
+Sponsor: Pablo Galindo <pablogsal@gmail.com>
+Discussions-To: https://mail.python.org/archives/list/python-dev@python.org/thread/2P26R4VH2ZCNNNOQCBZWEM4RNF35OXOW/
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 07-Nov-2022
+Python-Version: 3.12
+Post-History: `30-Sep-2022 <https:/
8000
/mail.python.org/archives/list/typing-sig@python.org/thread/6FR6RKNUZU4UY6B6RXC2H4IAHKBU3UKV/>`__,
+              `02-Nov-2022 <https://mail.python.org/archives/list/python-dev@python.org/thread/2P26R4VH2ZCNNNOQCBZWEM4RNF35OXOW/>`__
+
+
+.. highlight:: rst
+
+Abstract
+========
+
+:pep:`589` defines the structural type ``TypedDict`` for dictionaries with a fixed set of keys.
+As it defines a mutable type, it is difficult to correctly annotate methods which accept read-only parameters in a way that doesn't prevent valid inputs.
+This PEP proposes a type constructor ``typing.TypedMapping`` to support this use case.
+
+For example, given the following definition::
+
+    from typing import NotRequired, TypedMapping
+
+    class Movie(TypedMapping):
+        name: str
+        year: NotRequired[int | None]
+
+    def movie_string(movie: Movie) -> str:
+        if movie.get("year") is None:
+            return movie["name"]
+        else:
+            return f'{movie["name"]} ({movie["year"
8000
]})'
+
+A type checker should then consider this code type safe::
+
+    from typing import TypedDict
+
+    class MovieRecord(TypedDict):
+        name: str
+        year: int
+
+    movie: MovieRecord = {'name': 'Blade Runner',
+                          'year': 1982}
+    print(movie_string(movie))
+
+Note that currently the code above will fail to type check if we define ``Movie`` based on ``TypedDict``, and there is no replacement.
+
+Motivation
+==========
+
+Representing structured data using (potentially nested) dictionaries with string keys is a common pattern in Python programs. :pep:`589` allows these values to be type checked when the exact type is known up-front, but it is hard to write read-only code code that accepts more specific variants: for instance, where fields may be subtypes or restrict a union of possible types. This is an especially common issue when writing APIs for services, which may support a wide range of input structures, and typically do not need to modify their input.
+
+Take for example the function ``movie_string`` defined above. Suppose its type signature is defined using a ``TypedDict``::
+
+    from typing import NotRequired, TypedDict
+
+    class Movie(TypedDict):
+        name: str
+        year: NotRequired[int | None]
+
+    class MovieRecord(TypedDict):
+        name: str
+        year: int
+
+    def movie_string(movie: Movie) -> str:
+        if movie.get("year") is None:
+            return movie["name"]
+        else:
+            return f'{movie["name"]} ({movie["year"]})'
+
+Attempting to pass a ``MovieRecord`` into ``movie_string`` results (using mypy) in the error::
+
+    Argument 1 to "movie_string" has incompatible type "MovieRecord"; expected "Movie"
+
+This particular use case should be type-safe, but the type checker correctly stops the
+user from passing a ``MovieRecord`` into a ``Movie`` argument in the general case because
+the ``Movie`` class has mutator methods that could potentially allow the function to break
+the type constraints in ``MovieRecord`` by setting ``movie["year"] = None`` or ``del movie["year"]``.
+The problem disappears if we don't have mutator methods in ``Movie``. This could be achieved by defining an immutable interface using a :pep:`544` Protocol::
+
+    from typing import Literal, Protocol, overload
+
+    class Movie(Protocol):
+        @overload
+        def get(self, key: Literal["name"]) -> str: ...
+
+        @overload
+        def get(self, key: Literal["year"]) -> int | None: ...
+
+        @overload
+        def __getitem__(self, key: Literal["name"]) -> str: ...
+
+        @overload
+        def __getitem__(self, key: Literal["year"]) -> int | None: ...
+
+This is very repetitive, easy to get wrong, and is still missing important method definitions like ``__contains__()`` and ``keys()``.
+
+The proposed ``TypedMapping`` type allows a straightforward way of defining these types that should be familiar to existing users of ``TypedDict`` and support the cases exemplified above.
+In addition to those benefits, by flagging arguments of a function as ``TypedMapping``, it makes explicit not just to typecheckers but also to users that the function is not going to modify its inputs, which is usually a desirable property of a function interface.
+Finally, this allows bringing the benefits of ``TypedDict`` to other mapping types that are unrelated to ``dict``.
+
+Specification
+=============
+
+The ``TypedMapping`` type is a protocol behaving almost identically to ``TypedDict``, except:
+
+1. The runtime type of a TypedMapping object is not constrained to be a ``dict``
+2. no mutator methods (``__setitem__``, ``__delitem__``, ``update``, etc.) will be generated
+3. a class definition defines a ``TypedMapping`` protocol if and only if ``TypedMapping`` appears directly in its class bases
+4. subclasses can narrow field types, in the same manner as other protocols
+
+All current and future features of TypedDict are applicable to TypedMapping, including class-based and alternative syntax, totality, and ``Required`` and ``NotRequired`` from :pep:`655`.
+
+As with :pep:`589`, this PEP provides a sketch of how a type checker is expected to support type checking operations involving TypedMapping and TypedDict objects, but details are left to implementors. In particular, type compatibility should be based on structural compatibility.
+
+
+Multiple inheritance and TypedDict
+----------------------------------
+
+A type that inherits from a TypedMapping protocol and from TypedDict (either directly or indirectly):
+
+5. is the structural intersection of its parents, or invalid if no such intersection exists
+6. instances must be a dict subclass
+7. adds mutator methods only for fields it explicitly (re)declares
+
+For example::
+
+    class Movie(TypedMapping):
+        name: str
+        year: int | None
+
+    class MovieRecord(A, TypedDict):
+        year: int
+
+    movie: MovieRecord = { "name": "Blade Runner",
+                           "year": 1982 }
+
+    movie["year"] = 1985  # Fine, mutator methods added in definition
+    movie["name"] = "Terminator"  # Type check error, "name" mutator not declared
+
+Inheriting, directly or indirectly, from both TypedDict and Protocol will continue to fail at runtime, and should continue to be rejected by type checkers.
+
+
+Multiple inheritance and Protocol
+---------------------------------
+
+* A type that inherits from a TypedMapping protocol and from a Protocol protocol must satisfy the protocols defined by both, but is not itself a protocol unless it inherits directly from TypedMapping or Protocol.
+* A type that inherits from a TypedMapping protocol and from Protocol itself is configured as a Protocol. Methods and properties may be defined; keys may not::
+
+    class A(Movie, Protocol):
+        # Declare a mutable property called 'year'
+        # This does not affect the dictionary key 'year'
+        year: str
+
+* A type that inherits from a Protocol protocol and from TypedMapping itself is configured as a TypedMapping. Keys may be defined; methods and properties may not::
+
+    class B(A, TypedMapping):
+        # Declare a key 'year'
+        # This does not affect the property 'year'
+        year: int
+
+
+Type Consistency Rules
+----------------------
+
+Informally speaking, *type consistency* is a generalization of the is-subtype-of relation to support the ``Any`` type. It is defined more formally in :pep:`483`. This section introduces the new, non-trivial rules needed to support type consistency for TypedMapping types.
+
+First, any TypedMapping type is consistent with ``Mapping[str, object]``.
+Second, a TypedMapping or TypedDict type ``A`` is consistent with TypedMapping ``B`` if ``A`` is structurally compatible with ``B``. This is true if and only if both of these conditions are satisfied:
+
+* For each key in ``A``, ``B`` has the corresponding key and the corresponding value type in ``B`` is consistent with the value type in ``A``.
+
+* For each required key in ``A``, the corresponding key is required in ``B``.
+
+Discussion:
+
+* Value types behave covariantly, since TypedMapping objects have no mutator methods. This is similar to container types such as ``Mapping``, and different from relationships between two TypedDicts. Example::
+
+    class A(TypedMapping):
+        x: int | None
+
+    class B(TypedDict):
+        x: int
+
+    def f(a: A) -> None:
+        print(a['x'] or 0)
+
+    b: B = {'x': 0}
+    f(b)  # Accepted by type checker
+
+* A TypedDict or TypedMapping type with a required key is consistent with a TypedMapping type where the same key is a non-required key, again unlike relationships between two TypedDicts. Example::
+
+    class A(TypedMapping, total=False):
+        x: int
+
+    class B(TypedDict):
+        x: int
+
+    def f(a: A) -> None:
+        print(a.get('x', 0))
+
+    b: B = {'x': 0}
+    f(b)  # Accepted by type checker
+
+* A TypedMapping type ``A`` with no key ``'x'`` is not consistent with a TypedMapping type with a non-required key ``'x'``, since at runtime the key ``'x'`` could be present and have an incompatible type (which may not be visible through ``A`` due to structural subtyping). This is the same as for TypedDicts. Example::
+
+    class A(TypedMapping, total=False):
+        x: int
+        y: int
+
+    class B(TypedMapping, total=False):
+        x: int
+
+    class C(TypedMapping, total=False):
+        x: int
+        y: str
+
+    def f(a: A) -> None:
+        print(a.get('y') + 1)
+
+    def g(b: B) -> None:
+        f(b)  # Type check error: 'B' incompatible with 'A'
+
+    c: C = {'x': 0, 'y': 'foo'}
+    g(c)  # Runtime error: str + int
+
+* A TypedMapping with all ``int`` values is not consistent with ``Mapping[str, int]``, since there may be additional non-``int`` values not visible through the type, due to structural subtyping. This mirrors TypedDict. Example::
+
+    class A(TypedMapping):
+        x: int
+
+    class B(TypedMapping):
+        x: int
+        y: str
+
+    def sum_values(m: Mapping[str, int]) -> int:
+        return sum(m.values())
+
+    def f(a: A) -> None:
+        sum_values(a)  # Type check error: 'A' incompatible with Mapping[str, int]
+
+    b: B = {'x': 0, 'y': 'foo'}
+    f(b)  # Runtime error: int + str
+
+
+Rejected Alternatives
+=====================
+
+Several variations were considered and discarded:
+
+* A ``readonly`` parameter to ``TypedDict``, behaving much like TypedMapping but with the additional constraint that instances must be dictionaries at runtime. This was discarded as less flexible due to the extra constraint; additionally, the new type nicely mirrors the existing ``Mapping``/``Dict`` types.
+* Inheriting from a ``TypedMapping`` subclass and ``TypedDict`` resulting in mutator methods being added for all fields, not just those actively (re)declared in the class body. Discarded as less flexible, and not matching how inheritance works in other cases for TypedDict (e.g. total=False and total=True do not affect fields not specified in the class body).
+* A generic type that removes mutator methods from its parameter, e.g. ``Readonly[MovieRecord]``. This would naturally want to be defined for a wider set of types than just ``TypedDict`` subclasses, and also raises questions about whether and how it applies to nested types. We decided to keep the scope of this PEP narrower.
+* Declaring methods directly on a ``TypedMapping`` class. Methods are a kind of property, but declarations on a ``TypedMapping`` class are defining keys, so mixing the two is potentially confusing. Banning methods also makes it very easy to decide whether a ``TypedDict`` subclass can mix in a protocol or not (yes if it's just TypedMappings, no if there's a Protocol)
+