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Nonlocal references

crates/ty_python_semantic/resources/mdtest/scopes/nonlocal.md

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Nonlocal references

One level up

py
def f():
    x = 1
    def g():
        reveal_type(x)  # revealed: Literal[1]

Two levels up

py
def f():
    x = 1
    def g():
        def h():
            reveal_type(x)  # revealed: Literal[1]

Skips class scope

py
def f():
    x = 1

    class C:
        x = 2
        def g():
            reveal_type(x)  # revealed: Literal[1]

Reads respect annotation-only declarations

py
def f():
    x: int = 1
    def g():
        # TODO: This example should actually be an unbound variable error. However to avoid false
        # positives, we'd need to analyze `nonlocal x` statements in other inner functions.
        x: str
        def h():
            reveal_type(x)  # revealed: str

Reads terminate at the global keyword in an enclosing scope, even if there's no binding in that scope

Unlike variables that are explicitly declared nonlocal (below), implicitly nonlocal ("free") reads can come from a variable that's declared global in an enclosing scope. It doesn't matter whether the variable is bound in that scope:

py
x: int = 1

def f():
    x: str = "hello"
    def g():
        global x
        def h():
            # allowed: this loads the global `x` variable due to the `global` declaration in the immediate enclosing scope
            y: int = x

The nonlocal keyword

Without the nonlocal keyword, bindings in an inner scope shadow variables of the same name in enclosing scopes. This example isn't a type error, because the inner x shadows the outer one:

py
def f():
    x: int = 1
    def g():
        x = "hello"  # allowed

With nonlocal it is a type error, because x refers to the same place in both scopes:

py
def f():
    x: int = 1
    def g():
        nonlocal x
        x = "hello"  # error: [invalid-assignment] "Object of type `Literal["hello"]` is not assignable to `int`"

Nested function after conditional nonlocal rebinding

An inner function should still resolve a name through an enclosing nonlocal declaration, even if that enclosing scope also conditionally rebinds the name:

py
def outer(flag: bool) -> None:
    x: int = 1

    def middle() -> None:
        nonlocal x

        if flag:
            x = 2
            return

        def inner() -> None:
            y: int = x

Generator expression after nonlocal rebinding

A nested eager scope such as a generator expression should see the rebound type of a nonlocal symbol:

py
from typing import Optional

class C:
    value: int

def check(x: Optional[C]) -> C:
    return C()

def outer(x: Optional[C]) -> None:
    def inner() -> None:
        nonlocal x
        x = check(x)
        all(reveal_type(x.value) == 1 for _ in [0])  # revealed: int

The types of nonlocal binding get unioned

Without a type declaration, we union the bindings in enclosing scopes to infer a type. But name resolution stops at the closest binding that isn't declared nonlocal, and we ignore bindings outside of that one:

py
def a():
    # This binding is shadowed in `b`, so we ignore it in inner scopes.
    x = 1

    def b():
        x = 2

        def c():
            nonlocal x
            x = 3

            def d():
                nonlocal x
                reveal_type(x)  # revealed: Literal[3, 2]
                x = 4
                reveal_type(x)  # revealed: Literal[4]

                def e():
                    reveal_type(x)  # revealed: Literal[4, 3, 2]

However, currently the union of types that we build is incomplete. We walk parent scopes, but not sibling scopes, child scopes, second-cousin-once-removed scopes, etc:

py
def a():
    x = 1
    def b():
        nonlocal x
        x = 2

    def c():
        def d():
            nonlocal x
            x = 3
        # TODO: This should include 2 and 3.
        reveal_type(x)  # revealed: Literal[1]

Local variable bindings "look ahead" to any assignment in the current scope

The binding x = 2 in g causes the earlier read of x to refer to g's not-yet-initialized binding, rather than to x = 1 in f's scope:

py
def f():
    x = 1
    def g():
        if x == 1:  # error: [unresolved-reference] "Name `x` used when not defined"
            x = 2

The nonlocal keyword makes this example legal (and makes the assignment x = 2 affect the outer scope):

py
def f():
    x = 1
    def g():
        nonlocal x
        if x == 1:
            x = 2

For the same reason, using the += operator in an inner scope is an error without nonlocal (unless you shadow the outer variable first):

py
def f():
    x = 1
    def g():
        x += 1  # error: [unresolved-reference] "Name `x` used when not defined"

def f():
    x = 1
    def g():
        x = 1
        x += 1  # allowed, but doesn't affect the outer scope

def f():
    x = 1
    def g():
        nonlocal x
        x += 1  # allowed, and affects the outer scope

nonlocal declarations must match an outer binding

nonlocal x isn't allowed when there's no binding for x in an enclosing scope:

py
def f():
    def g():
        nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"

def f():
    x = 1
    def g():
        nonlocal x, y  # error: [invalid-syntax] "no binding for nonlocal `y` found"

A global x doesn't work. The target must be in a function-like scope:

py
x = 1

def f():
    def g():
        nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"

def f():
    global x
    def g():
        nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"

def f():
    # A *use* of `x` in an enclosing scope isn't good enough. There needs to be a binding.
    print(x)
    def g():
        nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"

A class-scoped x also doesn't work:

py
class Foo:
    x = 1
    @staticmethod
    def f():
        nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"

However, class-scoped bindings don't break the nonlocal chain the way global declarations do:

py
def f():
    x: int = 1

    class Foo:
        x: str = "hello"

        @staticmethod
        def g():
            # Skips the class scope and reaches the outer function scope.
            nonlocal x
            x = 2  # allowed
            x = "goodbye"  # error: [invalid-assignment]

nonlocal uses the closest binding

py
def f():
    x = 1
    def g():
        x = 2
        def h():
            nonlocal x
            reveal_type(x)  # revealed: Literal[2]

nonlocal "chaining"

Multiple nonlocal statements can "chain" through nested scopes:

py
def f():
    x = 1
    def g():
        nonlocal x
        def h():
            nonlocal x
            reveal_type(x)  # revealed: Literal[1]

And the nonlocal chain can skip over a scope that doesn't bind the variable:

py
def f1():
    x = 1
    def f2():
        nonlocal x
        def f3():
            # No binding; this scope gets skipped.
            def f4():
                nonlocal x
                reveal_type(x)  # revealed: Literal[1]

But a global statement breaks the chain:

py
x = 1

def f():
    x = 2
    def g():
        global x
        def h():
            nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"

Assigning to a nonlocal respects the declared type from its defining scope, even without a binding in that scope

py
def f():
    x: int
    def g():
        nonlocal x
        x = "string"  # error: [invalid-assignment] "Object of type `Literal["string"]` is not assignable to `int`"

A complicated mixture of nonlocal chaining, empty scopes, class scopes, and the global keyword

py
# Global definitions of `x`, `y`, and `z`.
x: bool = True
y: bool = True
z: bool = True

def f1():
    # Local definitions of `x`, `y`, and `z`.
    x: int = 1
    y: int = 2
    z: int = 3

    def f2():
        # This scope doesn't touch `x`, `y`, or `z` at all.

        class Foo:
            # This class scope is totally ignored.
            x: str = "a"
            y: str = "b"
            z: str = "c"

            @staticmethod
            def f3():
                # This scope declares `x` nonlocal, shadows `y` without a type declaration, and
                # declares `z` global.
                nonlocal x
                x = 4
                y = 5
                global z

                def f4():
                    # This scope sees `x` from `f1` and `y` from `f3`. It *can't* declare `z`
                    # nonlocal, because of the global statement above, but it *can* load `z` as a
                    # "free" variable, in which case it sees the global value.
                    nonlocal x, y, z  # error: [invalid-syntax] "no binding for nonlocal `z` found"
                    x = "string"  # error: [invalid-assignment]
                    y = "string"  # allowed, because `f3`'s `y` is untyped

TODO: nonlocal affects the inferred type in the outer scope

Without nonlocal, g can't write to x, and the inferred type of x in f's scope isn't affected by g:

py
def f():
    x = 1
    def g():
        reveal_type(x)  # revealed: Literal[1]
    reveal_type(x)  # revealed: Literal[1]

But with nonlocal, g could write to x, and that affects its inferred type in f. That's true regardless of whether g actually writes to x. With a write:

py
def f():
    x = 1
    def g():
        nonlocal x
        reveal_type(x)  # revealed: Literal[1]
        x += 1
        reveal_type(x)  # revealed: Literal[2]
    # TODO: should be `Unknown | Literal[1]`
    reveal_type(x)  # revealed: Literal[1]

Without a write:

py
def f():
    x = 1
    def g():
        nonlocal x
        reveal_type(x)  # revealed: Literal[1]
    # TODO: should be `Unknown | Literal[1]`
    reveal_type(x)  # revealed: Literal[1]

Annotating a nonlocal binding is a syntax error

py
def f():
    x: int = 1
    def g():
        nonlocal x
        x: str = "foo"  # error: [invalid-syntax] "annotated name `x` can't be nonlocal"

Use before nonlocal

Using a name prior to its nonlocal declaration in the same scope is a syntax error:

py
def f():
    x = 1
    def g():
        x = 2
        nonlocal x  # error: [invalid-syntax] "name `x` is used prior to nonlocal declaration"

This is true even if there are multiple nonlocal declarations of the same variable, as long as any of them come after the usage:

py
def f():
    x = 1
    def g():
        nonlocal x
        x = 2
        nonlocal x  # error: [invalid-syntax] "name `x` is used prior to nonlocal declaration"

def f():
    x = 1
    def g():
        nonlocal x
        nonlocal x
        x = 2  # allowed

nonlocal before outer initialization

nonlocal x works even if x isn't bound in the enclosing scope until afterwards:

py
def f():
    def g():
        # This is allowed, because of the subsequent definition of `x`.
        nonlocal x
    x = 1

Narrowing nonlocal types to Never doesn't make them unbound

py
def foo():
    x: int = 1
    def bar():
        if isinstance(x, str):
            reveal_type(x)  # revealed: Never

Narrowing nonlocal variables with conditional assignments

When a nonlocal variable is conditionally reassigned and then narrowed via an assertion, the narrowing constraint should be applied correctly, even when the enclosing scope's type was itself narrowed (e.g., via an isinstance check).

py
def _(maybe_float: float | None, certain_int: int, flag: bool) -> None:
    if isinstance(maybe_float, int):
        return
    x = maybe_float
    def _() -> None:
        nonlocal x
        if flag:
            x = certain_int
        assert x is not None
        reveal_type(x)  # revealed: int | float
        +x