mirror of https://github.com/astral-sh/ruff.git synced 2025-10-07 00:50:37 +00:00

[ty] Better error message for attempting to assign to a read-only property (#20150 )

2025-08-29 13:22:23 +00:00

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`NamedTuple`

NamedTuple is a type-safe way to define named tuples — a tuple where each field can be accessed by name, and not just by its numeric position within the tuple:

`typing.NamedTuple`

Basics

from typing import NamedTuple
from ty_extensions import static_assert, is_subtype_of, is_assignable_to

class Person(NamedTuple):
    id: int
    name: str
    age: int | None = None

alice = Person(1, "Alice", 42)
alice = Person(id=1, name="Alice", age=42)
bob = Person(2, "Bob")
bob = Person(id=2, name="Bob")

reveal_type(alice.id)  # revealed: int
reveal_type(alice.name)  # revealed: str
reveal_type(alice.age)  # revealed: int | None

# revealed: tuple[<class 'Person'>, <class 'tuple[int, str, int | None]'>, <class 'Sequence[int | str | None]'>, <class 'Reversible[int | str | None]'>, <class 'Collection[int | str | None]'>, <class 'Iterable[int | str | None]'>, <class 'Container[int | str | None]'>, typing.Protocol, typing.Generic, <class 'object'>]
reveal_type(Person.__mro__)

static_assert(is_subtype_of(Person, tuple[int, str, int | None]))
static_assert(is_subtype_of(Person, tuple[object, ...]))
static_assert(not is_assignable_to(Person, tuple[int, str, int]))
static_assert(not is_assignable_to(Person, tuple[int, str]))

reveal_type(len(alice))  # revealed: Literal[3]
reveal_type(bool(alice))  # revealed: Literal[True]

reveal_type(alice[0])  # revealed: int
reveal_type(alice[1])  # revealed: str
reveal_type(alice[2])  # revealed: int | None

# error: [index-out-of-bounds] "Index 3 is out of bounds for tuple `Person` with length 3"
reveal_type(alice[3])  # revealed: Unknown

reveal_type(alice[-1])  # revealed: int | None
reveal_type(alice[-2])  # revealed: str
reveal_type(alice[-3])  # revealed: int

# error: [index-out-of-bounds] "Index -4 is out of bounds for tuple `Person` with length 3"
reveal_type(alice[-4])  # revealed: Unknown

reveal_type(alice[1:])  # revealed: tuple[str, int | None]
reveal_type(alice[::-1])  # revealed: tuple[int | None, str, int]

alice_id, alice_name, alice_age = alice
reveal_type(alice_id)  # revealed: int
reveal_type(alice_name)  # revealed: str
reveal_type(alice_age)  # revealed: int | None

# error: [invalid-assignment] "Not enough values to unpack: Expected 4"
a, b, c, d = alice
# error: [invalid-assignment] "Too many values to unpack: Expected 2"
a, b = alice
*_, age = alice
reveal_type(age)  # revealed: int | None

# error: [missing-argument]
Person(3)

# error: [too-many-positional-arguments]
Person(3, "Eve", 99, "extra")

# error: [invalid-argument-type]
Person(id="3", name="Eve")

reveal_type(Person.id)  # revealed: property
reveal_type(Person.name)  # revealed: property
reveal_type(Person.age)  # revealed: property

# error: [invalid-assignment] "Cannot assign to read-only property `id` on object of type `Person`"
alice.id = 42
# error: [invalid-assignment]
bob.age = None

Alternative functional syntax:

Person2 = NamedTuple("Person", [("id", int), ("name", str)])
alice2 = Person2(1, "Alice")

# TODO: should be an error
Person2(1)

reveal_type(alice2.id)  # revealed: @Todo(functional `NamedTuple` syntax)
reveal_type(alice2.name)  # revealed: @Todo(functional `NamedTuple` syntax)

Definition

Fields without default values must come before fields with.

from typing import NamedTuple

class Location(NamedTuple):
    altitude: float = 0.0
    # error: [invalid-named-tuple] "NamedTuple field without default value cannot follow field(s) with default value(s): Field `latitude` defined here without a default value"
    latitude: float
    # error: [invalid-named-tuple] "NamedTuple field without default value cannot follow field(s) with default value(s): Field `longitude` defined here without a default value"
    longitude: float

class StrangeLocation(NamedTuple):
    altitude: float
    altitude: float = 0.0
    altitude: float
    altitude: float = 0.0
    latitude: float  # error: [invalid-named-tuple]
    longitude: float  # error: [invalid-named-tuple]

class VeryStrangeLocation(NamedTuple):
    altitude: float = 0.0
    latitude: float  # error: [invalid-named-tuple]
    longitude: float  # error: [invalid-named-tuple]
    altitude: float = 0.0

Multiple Inheritance

Multiple inheritance is not supported for NamedTuple classes except with Generic:

from typing import NamedTuple, Protocol

# error: [invalid-named-tuple] "NamedTuple class `C` cannot use multiple inheritance except with `Generic[]`"
class C(NamedTuple, object):
    id: int

# fmt: off

class D(
    int,  # error: [invalid-named-tuple]
    NamedTuple
): ...

# fmt: on

# error: [invalid-named-tuple]
class E(NamedTuple, Protocol): ...

Inheriting from a `NamedTuple`

Inheriting from a NamedTuple is supported, but new fields on the subclass will not be part of the synthesized __new__ signature:

from typing import NamedTuple

class User(NamedTuple):
    id: int
    name: str

class SuperUser(User):
    level: int

# This is fine:
alice = SuperUser(1, "Alice")
reveal_type(alice.level)  # revealed: int

# This is an error because `level` is not part of the signature:
# error: [too-many-positional-arguments]
alice = SuperUser(1, "Alice", 3)

TODO: If any fields added by the subclass conflict with those in the base class, that should be flagged.

from typing import NamedTuple

class User(NamedTuple):
    id: int
    name: str
    age: int | None
    nickname: str

class SuperUser(User):
    # TODO: this should be an error because it implies that the `id` attribute on
    # `SuperUser` is mutable, but the read-only `id` property from the superclass
    # has not been overridden in the class body
    id: int

    # this is fine; overriding a read-only attribute with a mutable one
    # does not conflict with the Liskov Substitution Principle
    name: str = "foo"

    # this is also fine
    @property
    def age(self) -> int:
        return super().age or 42

    def now_called_robert(self):
        self.name = "Robert"  # fine because overridden with a mutable attribute

        # TODO: this should cause us to emit an error as we're assigning to a read-only property
        # inherited from the `NamedTuple` superclass (requires https://github.com/astral-sh/ty/issues/159)
        self.nickname = "Bob"

james = SuperUser(0, "James", 42, "Jimmy")

# fine because the property on the superclass was overridden with a mutable attribute
# on the subclass
james.name = "Robert"

# error: [invalid-assignment] "Cannot assign to read-only property `nickname` on object of type `SuperUser`"
james.nickname = "Bob"

Generic named tuples

[environment]
python-version = "3.12"

from typing import NamedTuple, Generic, TypeVar

class Property[T](NamedTuple):
    name: str
    value: T

reveal_type(Property("height", 3.4))  # revealed: Property[float]
reveal_type(Property.value)  # revealed: property
reveal_type(Property.value.fget)  # revealed: (self, /) -> Unknown
reveal_type(Property[str].value.fget)  # revealed: (self, /) -> str
reveal_type(Property("height", 3.4).value)  # revealed: float

T = TypeVar("T")

class LegacyProperty(NamedTuple, Generic[T]):
    name: str
    value: T

reveal_type(LegacyProperty("height", 42))  # revealed: LegacyProperty[int]
reveal_type(LegacyProperty.value)  # revealed: property
reveal_type(LegacyProperty.value.fget)  # revealed: (self, /) -> Unknown
reveal_type(LegacyProperty[str].value.fget)  # revealed: (self, /) -> str
reveal_type(LegacyProperty("height", 3.4).value)  # revealed: float

Attributes on `NamedTuple`

The following attributes are available on NamedTuple classes / instances:

from typing import NamedTuple

class Person(NamedTuple):
    name: str
    age: int | None = None

reveal_type(Person._field_defaults)  # revealed: dict[str, Any]
reveal_type(Person._fields)  # revealed: tuple[str, ...]
reveal_type(Person._make)  # revealed: bound method <class 'Person'>._make(iterable: Iterable[Any]) -> Person
reveal_type(Person._asdict)  # revealed: def _asdict(self) -> dict[str, Any]
reveal_type(Person._replace)  # revealed: def _replace(self, **kwargs: Any) -> Self@_replace

# TODO: should be `Person` once we support `Self`
reveal_type(Person._make(("Alice", 42)))  # revealed: Unknown

person = Person("Alice", 42)

reveal_type(person._asdict())  # revealed: dict[str, Any]
# TODO: should be `Person` once we support `Self`
reveal_type(person._replace(name="Bob"))  # revealed: Unknown

`collections.namedtuple`

from collections import namedtuple

Person = namedtuple("Person", ["id", "name", "age"], defaults=[None])

alice = Person(1, "Alice", 42)
bob = Person(2, "Bob")

The symbol `NamedTuple` itself

At runtime, NamedTuple is a function, and we understand this:

import types
import typing

def expects_functiontype(x: types.FunctionType): ...

expects_functiontype(typing.NamedTuple)

This means we also understand that all attributes on function objects are available on the symbol typing.NamedTuple:

reveal_type(typing.NamedTuple.__name__)  # revealed: str
reveal_type(typing.NamedTuple.__qualname__)  # revealed: str
reveal_type(typing.NamedTuple.__kwdefaults__)  # revealed: dict[str, Any] | None

# TODO: this should cause us to emit a diagnostic and reveal `Unknown` (function objects don't have an `__mro__` attribute),
# but the fact that we don't isn't actually a `NamedTuple` bug (https://github.com/astral-sh/ty/issues/986)
reveal_type(typing.NamedTuple.__mro__)  # revealed: tuple[<class 'FunctionType'>, <class 'object'>]

By the normal rules, NamedTuple and type[NamedTuple] should not be valid in type expressions -- there is no object at runtime that is an "instance of NamedTuple", nor is there any class at runtime that is a "subclass of NamedTuple" -- these are both impossible, since NamedTuple is a function and not a class. However, for compatibility with other type checkers, we allow NamedTuple in type expressions and understand it as describing an interface that all NamedTuple classes would satisfy:

def expects_named_tuple(x: typing.NamedTuple):
    reveal_type(x)  # revealed: tuple[object, ...] & NamedTupleLike
    reveal_type(x._make)  # revealed: bound method type[NamedTupleLike]._make(iterable: Iterable[Any]) -> NamedTupleLike
    reveal_type(x._replace)  # revealed: bound method NamedTupleLike._replace(**kwargs) -> NamedTupleLike
    # revealed: Overload[(value: tuple[object, ...], /) -> tuple[object, ...], (value: tuple[_T@__add__, ...], /) -> tuple[object, ...]]
    reveal_type(x.__add__)
    reveal_type(x.__iter__)  # revealed: bound method tuple[object, ...].__iter__() -> Iterator[object]

def _(y: type[typing.NamedTuple]):
    reveal_type(y)  # revealed: @Todo(unsupported type[X] special form)

# error: [invalid-type-form] "Special form `typing.NamedTuple` expected no type parameter"
def _(z: typing.NamedTuple[int]): ...

Any instance of a NamedTuple class can therefore be passed for a function parameter that is annotated with NamedTuple:

from typing import NamedTuple, Protocol, Iterable, Any
from ty_extensions import static_assert, is_assignable_to

class Point(NamedTuple):
    x: int
    y: int

reveal_type(Point._make)  # revealed: bound method <class 'Point'>._make(iterable: Iterable[Any]) -> Point
reveal_type(Point._asdict)  # revealed: def _asdict(self) -> dict[str, Any]
reveal_type(Point._replace)  # revealed: def _replace(self, **kwargs: Any) -> Self@_replace

static_assert(is_assignable_to(Point, NamedTuple))

expects_named_tuple(Point(x=42, y=56))  # fine

# error: [invalid-argument-type] "Argument to function `expects_named_tuple` is incorrect: Expected `tuple[object, ...] & NamedTupleLike`, found `tuple[Literal[1], Literal[2]]`"
expects_named_tuple((1, 2))

The type described by NamedTuple in type expressions is understood as being assignable to tuple[object, ...] and tuple[Any, ...]:

static_assert(is_assignable_to(NamedTuple, tuple))
static_assert(is_assignable_to(NamedTuple, tuple[object, ...]))
static_assert(is_assignable_to(NamedTuple, tuple[Any, ...]))

def expects_tuple(x: tuple[object, ...]): ...
def _(x: NamedTuple):
    expects_tuple(x)  # fine

NamedTuple with custom `getattr`

This is a regression test for https://github.com/astral-sh/ty/issues/322. Make sure that the __getattr__ method does not interfere with the NamedTuple behavior.

from typing import NamedTuple

class Vec2(NamedTuple):
    x: float = 0.0
    y: float = 0.0

    def __getattr__(self, attrs: str): ...

Vec2(0.0, 0.0)

12 KiB Raw Blame History

NamedTuple

typing.NamedTuple