ruff/crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md

Narrowing for isinstance checks

Narrowing for isinstance(object, classinfo) expressions.

classinfo is a single type

def _(flag: bool):
    x = 1 if flag else "a"

    if isinstance(x, int):
        reveal_type(x)  # revealed: Literal[1]

    if isinstance(x, str):
        reveal_type(x)  # revealed: Literal["a"]
        if isinstance(x, int):
            reveal_type(x)  # revealed: Never

    if isinstance(x, (int, object)):
        reveal_type(x)  # revealed: Literal[1, "a"]

classinfo is a tuple of types

Note: isinstance(x, (int, str)) should not be confused with isinstance(x, tuple[(int, str)]). The former is equivalent to isinstance(x, int | str):

def _(flag: bool, flag1: bool, flag2: bool):
    x = 1 if flag else "a"

    if isinstance(x, (int, str)):
        reveal_type(x)  # revealed: Literal[1, "a"]
    else:
        reveal_type(x)  # revealed: Never

    if isinstance(x, (int, bytes)):
        reveal_type(x)  # revealed: Literal[1]

    if isinstance(x, (bytes, str)):
        reveal_type(x)  # revealed: Literal["a"]

    # No narrowing should occur if a larger type is also
    # one of the possibilities:
    if isinstance(x, (int, object)):
        reveal_type(x)  # revealed: Literal[1, "a"]
    else:
        reveal_type(x)  # revealed: Never

    y = 1 if flag1 else "a" if flag2 else b"b"
    if isinstance(y, (int, str)):
        reveal_type(y)  # revealed: Literal[1, "a"]

    if isinstance(y, (int, bytes)):
        reveal_type(y)  # revealed: Literal[1, b"b"]

    if isinstance(y, (str, bytes)):
        reveal_type(y)  # revealed: Literal["a", b"b"]

classinfo is a nested tuple of types

def _(flag: bool):
    x = 1 if flag else "a"

    if isinstance(x, (bool, (bytes, int))):
        reveal_type(x)  # revealed: Literal[1]
    else:
        reveal_type(x)  # revealed: Literal["a"]

classinfo is a PEP-604 union of types

[environment]
python-version = "3.10"

def _(x: int | str | bytes | memoryview | range):
    if isinstance(x, int | str):
        reveal_type(x)  # revealed: int | str
    elif isinstance(x, bytes | memoryview):
        reveal_type(x)  # revealed: bytes | memoryview[Unknown]
    else:
        reveal_type(x)  # revealed: range

Although isinstance() usually only works if all elements in the UnionType are class objects, at runtime a special exception is made for None so that isinstance(x, int | None) can work:

def _(x: int | str | bytes | range | None):
    if isinstance(x, int | str | None):
        reveal_type(x)  # revealed: int | str | None
    else:
        reveal_type(x)  # revealed: bytes | range

classinfo is an invalid PEP-604 union of types

Except for the None special case mentioned above, narrowing can only take place if all elements in the PEP-604 union are class literals. If any elements are generic aliases or other types, the isinstance() call may fail at runtime, so no narrowing can take place:

[environment]
python-version = "3.10"

from typing import Any, Literal, NamedTuple

def _(x: int | list[int] | bytes):
    # error: [invalid-argument-type]
    if isinstance(x, list[int] | int):
        reveal_type(x)  # revealed: int | list[int] | bytes
    # error: [invalid-argument-type]
    elif isinstance(x, Literal[42] | list[int] | bytes):
        reveal_type(x)  # revealed: int | list[int] | bytes
    # error: [invalid-argument-type]
    elif isinstance(x, Any | NamedTuple | list[int]):
        reveal_type(x)  # revealed: int | list[int] | bytes
    else:
        reveal_type(x)  # revealed: int | list[int] | bytes

PEP-604 unions on Python <3.10

PEP-604 unions were added in Python 3.10, so attempting to use them on Python 3.9 does not lead to any type narrowing.

[environment]
python-version = "3.9"

def _(x: int | str | bytes):
    # error: [unsupported-operator]
    if isinstance(x, int | str):
        reveal_type(x)  # revealed: (int & Unknown) | (str & Unknown) | (bytes & Unknown)
    else:
        reveal_type(x)  # revealed: (int & Unknown) | (str & Unknown) | (bytes & Unknown)

classinfo is a typing.py special form

Certain special forms in typing.py are aliases to classes elsewhere in the standard library; these can be used in isinstance() and issubclass() checks. We support narrowing using them:

import typing as t

def f(x: dict[str, int] | list[str], y: object):
    if isinstance(x, t.Dict):
        reveal_type(x)  # revealed: dict[str, int]
    else:
        reveal_type(x)  # revealed: list[str]

    if isinstance(y, t.Callable):
        # TODO: a better top-materialization for `Callable`s (https://github.com/astral-sh/ty/issues/1426)
        reveal_type(y)  # revealed: () -> object

Class types

class A: ...
class B: ...
class C: ...

x = object()

if isinstance(x, A):
    reveal_type(x)  # revealed: A
    if isinstance(x, B):
        reveal_type(x)  # revealed: A & B
    else:
        reveal_type(x)  # revealed: A & ~B

if isinstance(x, (A, B)):
    reveal_type(x)  # revealed: A | B
elif isinstance(x, (A, C)):
    reveal_type(x)  # revealed: C & ~A & ~B
else:
    reveal_type(x)  # revealed: ~A & ~B & ~C

No narrowing for instances of builtins.type

def _(flag: bool, t: type):
    x = 1 if flag else "foo"

    if isinstance(x, t):
        reveal_type(x)  # revealed: Literal[1, "foo"]

Do not use custom isinstance for narrowing

def _(flag: bool):
    def isinstance(x, t):
        return True
    x = 1 if flag else "a"

    if isinstance(x, int):
        reveal_type(x)  # revealed: Literal[1, "a"]

Do support narrowing if isinstance is aliased

def _(flag: bool):
    isinstance_alias = isinstance

    x = 1 if flag else "a"

    if isinstance_alias(x, int):
        reveal_type(x)  # revealed: Literal[1]

Do support narrowing if isinstance is imported

from builtins import isinstance as imported_isinstance

def _(flag: bool):
    x = 1 if flag else "a"

    if imported_isinstance(x, int):
        reveal_type(x)  # revealed: Literal[1]

Do not narrow if second argument is not a type

def _(flag: bool):
    x = 1 if flag else "a"

    # error: [invalid-argument-type] "Argument to function `isinstance` is incorrect: Expected `type | UnionType | tuple[Unknown, ...]`, found `Literal["a"]"
    if isinstance(x, "a"):
        reveal_type(x)  # revealed: Literal[1, "a"]

    # error: [invalid-argument-type] "Argument to function `isinstance` is incorrect: Expected `type | UnionType | tuple[Unknown, ...]`, found `Literal["int"]"
    if isinstance(x, "int"):
        reveal_type(x)  # revealed: Literal[1, "a"]

Do not narrow if there are keyword arguments

def _(flag: bool):
    x = 1 if flag else "a"

    # error: [unknown-argument]
    if isinstance(x, int, foo="bar"):
        reveal_type(x)  # revealed: Literal[1, "a"]

type[] types are narrowed as well as class-literal types

def _(x: object, y: type[int]):
    if isinstance(x, y):
        reveal_type(x)  # revealed: int

Adding a disjoint element to an existing intersection

We used to incorrectly infer Literal booleans for some of these.

from ty_extensions import Not, Intersection, AlwaysTruthy, AlwaysFalsy

class P: ...

def f(
    a: Intersection[P, AlwaysTruthy],
    b: Intersection[P, AlwaysFalsy],
    c: Intersection[P, Not[AlwaysTruthy]],
    d: Intersection[P, Not[AlwaysFalsy]],
):
    if isinstance(a, bool):
        reveal_type(a)  # revealed: Never
    else:
        reveal_type(a)  # revealed: P & AlwaysTruthy

    if isinstance(b, bool):
        reveal_type(b)  # revealed: Never
    else:
        reveal_type(b)  # revealed: P & AlwaysFalsy

    if isinstance(c, bool):
        reveal_type(c)  # revealed: Never
    else:
        reveal_type(c)  # revealed: P & ~AlwaysTruthy

    if isinstance(d, bool):
        reveal_type(d)  # revealed: Never
    else:
        reveal_type(d)  # revealed: P & ~AlwaysFalsy

Narrowing if an object of type Any or Unknown is used as the second argument

In order to preserve the gradual guarantee, we intersect with the type of the second argument if the type of the second argument is a dynamic type:

from typing import Any
from something_unresolvable import SomethingUnknown  # error: [unresolved-import]

class Foo: ...

def f(a: Foo, b: Any):
    if isinstance(a, SomethingUnknown):
        reveal_type(a)  # revealed: Foo & Unknown

    if isinstance(a, b):
        reveal_type(a)  # revealed: Foo & Any

Narrowing if an object with an intersection/union/TypeVar type is used as the second argument

If an intersection with only positive members is used as the second argument, and all positive members of the intersection are valid arguments for the second argument to isinstance(), we intersect with each positive member of the intersection:

[environment]
python-version = "3.12"

from typing import Any
from ty_extensions import Intersection

class Foo: ...

class Bar:
    attribute: int

class Baz:
    attribute: str

def f(x: Foo, y: Intersection[type[Bar], type[Baz]], z: type[Any]):
    if isinstance(x, y):
        reveal_type(x)  # revealed: Foo & Bar & Baz

    if isinstance(x, z):
        reveal_type(x)  # revealed: Foo & Any

The same if a union type is used:

def g(x: Foo, y: type[Bar | Baz]):
    if isinstance(x, y):
        reveal_type(x)  # revealed: (Foo & Bar) | (Foo & Baz)

And even if a TypeVar is used, providing it has valid upper bounds/constraints:

from typing import TypeVar

T = TypeVar("T", bound=type[Bar])

def h_old_syntax(x: Foo, y: T) -> T:
    if isinstance(x, y):
        reveal_type(x)  # revealed: Foo & Bar
        reveal_type(x.attribute)  # revealed: int

    return y

def h[U: type[Bar | Baz]](x: Foo, y: U) -> U:
    if isinstance(x, y):
        reveal_type(x)  # revealed: (Foo & Bar) | (Foo & Baz)
        reveal_type(x.attribute)  # revealed: int | str

    return y

Or even a tuple of tuple of typevars that have intersection bounds...

from ty_extensions import Intersection

class Spam: ...
class Eggs: ...
class Ham: ...
class Mushrooms: ...

def i[T: Intersection[type[Bar], type[Baz | Spam]], U: (type[Eggs], type[Ham])](x: Foo, y: T, z: U) -> tuple[T, U]:
    if isinstance(x, (y, (z, Mushrooms))):
        reveal_type(x)  # revealed: (Foo & Bar & Baz) | (Foo & Bar & Spam) | (Foo & Eggs) | (Foo & Ham) | (Foo & Mushrooms)

    return (y, z)

Narrowing with generics

[environment]
python-version = "3.12"

Narrowing to a generic class using isinstance() uses the top materialization of the generic. With a covariant generic, this is equivalent to using the upper bound of the type parameter (by default, object):

from typing import Self

class Covariant[T]:
    def get(self) -> T:
        raise NotImplementedError

def _(x: object):
    if isinstance(x, Covariant):
        reveal_type(x)  # revealed: Covariant[object]
        reveal_type(x.get())  # revealed: object

Similarly, contravariant type parameters use their lower bound of Never:

class Contravariant[T]:
    def push(self, x: T) -> None: ...

def _(x: object):
    if isinstance(x, Contravariant):
        reveal_type(x)  # revealed: Contravariant[Never]
        # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Expected `Never`, found `Literal[42]`"
        x.push(42)

Invariant generics are trickiest. The top materialization, conceptually the type that includes all instances of the generic class regardless of the type parameter, cannot be represented directly in the type system, so we represent it with the internal Top[] special form.

class Invariant[T]:
    def push(self, x: T) -> None: ...
    def get(self) -> T:
        raise NotImplementedError

def _(x: object):
    if isinstance(x, Invariant):
        reveal_type(x)  # revealed: Top[Invariant[Unknown]]
        reveal_type(x.get())  # revealed: object
        # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Expected `Never`, found `Literal[42]`"
        x.push(42)

When more complex types are involved, the Top[] type may get simplified away.

def _(x: list[int] | set[str]):
    if isinstance(x, list):
        reveal_type(x)  # revealed: list[int]
    else:
        reveal_type(x)  # revealed: set[str]

Though if the types involved are not disjoint bases, we necessarily keep a more complex type.

def _(x: Invariant[int] | Covariant[str]):
    if isinstance(x, Invariant):
        reveal_type(x)  # revealed: Invariant[int] | (Covariant[str] & Top[Invariant[Unknown]])
    else:
        reveal_type(x)  # revealed: Covariant[str] & ~Top[Invariant[Unknown]]

The behavior of issubclass() is similar.

def _(x: type[object], y: type[object], z: type[object]):
    if issubclass(x, Covariant):
        reveal_type(x)  # revealed: type[Covariant[object]]
    if issubclass(y, Contravariant):
        reveal_type(y)  # revealed: type[Contravariant[Never]]
    if issubclass(z, Invariant):
        reveal_type(z)  # revealed: type[Top[Invariant[Unknown]]]