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With statements

crates/ty_python_semantic/resources/mdtest/with/sync.md

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With statements

Basic with statement

The type of the target variable in a with statement is the return type from the context manager's __enter__ method.

py
class Target: ...

class Manager:
    def __enter__(self) -> Target:
        return Target()

    def __exit__(self, exc_type, exc_value, traceback): ...

with Manager() as f:
    reveal_type(f)  # revealed: Target

Union context manager

py
def _(flag: bool):
    class Manager1:
        def __enter__(self) -> str:
            return "foo"

        def __exit__(self, exc_type, exc_value, traceback): ...

    class Manager2:
        def __enter__(self) -> int:
            return 42

        def __exit__(self, exc_type, exc_value, traceback): ...

    context_expr = Manager1() if flag else Manager2()

    with context_expr as f:
        reveal_type(f)  # revealed: str | int

Type aliases preserve context manager behavior

toml
[environment]
python-version = "3.12"
py
from typing import Self, TypeAlias
from typing_extensions import TypeAliasType

class A:
    def __enter__(self) -> Self:
        return self

    def __exit__(self, exc_type, exc_value, traceback) -> None: ...

class B:
    def __enter__(self) -> Self:
        return self

    def __exit__(self, exc_type, exc_value, traceback) -> None: ...

UnionAB1: TypeAlias = A | B
type UnionAB2 = A | B
UnionAB3 = TypeAliasType("UnionAB3", A | B)

def f1(x: UnionAB1) -> None:
    with x as y:
        reveal_type(y)  # revealed: A | B

def f2(x: UnionAB2) -> None:
    with x as y:
        reveal_type(y)  # revealed: A | B

def f3(x: UnionAB3) -> None:
    with x as y:
        reveal_type(y)  # revealed: A | B

Context manager without an __enter__ or __exit__ method

py
class Manager: ...

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not implement `__enter__` and `__exit__`"
with Manager():
    pass

Context manager without an __enter__ method

py
class Manager:
    def __exit__(self, exc_tpe, exc_value, traceback): ...

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not implement `__enter__`"
with Manager():
    pass

Context manager without an __exit__ method

py
class Manager:
    def __enter__(self): ...

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not implement `__exit__`"
with Manager():
    pass

Context manager with non-callable __enter__ attribute

py
class Manager:
    __enter__: int = 42

    def __exit__(self, exc_tpe, exc_value, traceback): ...

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not correctly implement `__enter__`"
with Manager():
    pass

Context manager with non-callable __exit__ attribute

py
from typing_extensions import Self

class Manager:
    def __enter__(self) -> Self:
        return self
    __exit__: int = 32

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not correctly implement `__exit__`"
with Manager():
    pass

Context expression with possibly-unbound union variants

<!-- snapshot-diagnostics -->
py
def _(flag: bool):
    class Manager1:
        def __enter__(self) -> str:
            return "foo"

        def __exit__(self, exc_type, exc_value, traceback): ...

    class NotAContextManager: ...
    context_expr = Manager1() if flag else NotAContextManager()

    # error: [invalid-context-manager] "Object of type `Manager1 | NotAContextManager` cannot be used with `with` because the methods `__enter__` and `__exit__` are possibly missing"
    with context_expr as f:
        reveal_type(f)  # revealed: str

Context expression with overlapping possibly-unbound union variants

<!-- snapshot-diagnostics -->
py
def _(flag1: bool, flag2: bool):
    class GoodManager:
        def __enter__(self) -> str:
            return "foo"

        def __exit__(self, exc_type, exc_value, traceback): ...

    class MissingExitManager:
        def __enter__(self) -> str:
            return "bar"

    class NotAContextManager: ...
    context_expr = GoodManager() if flag1 else MissingExitManager() if flag2 else NotAContextManager()

    # error: [invalid-context-manager] "Object of type `GoodManager | MissingExitManager | NotAContextManager` cannot be used with `with` because the methods `__enter__` and `__exit__` are possibly missing"
    with context_expr as f:
        reveal_type(f)  # revealed: str

Context expression where one union variant has a non-callable dunder

<!-- snapshot-diagnostics -->

If every union element implements the context manager protocol but at least one implements it incorrectly (e.g. with a non-callable __exit__ attribute), the diagnostic should reflect that — not report the dunder as "possibly missing".

py
def _(flag: bool):
    class GoodManager:
        def __enter__(self) -> str:
            return "foo"

        def __exit__(self, exc_type, exc_value, traceback): ...

    class BadManager:
        def __enter__(self) -> str:
            return "bar"

        # `__exit__` is present but not callable
        __exit__: int = 32

    context_expr = GoodManager() if flag else BadManager()

    # error: [invalid-context-manager] "Object of type `GoodManager | BadManager` cannot be used with `with` because it does not correctly implement `__exit__`"
    with context_expr as f:
        reveal_type(f)  # revealed: str

Context expression with "sometimes" callable __enter__ method

py
def _(flag: bool):
    class Manager:
        if flag:
            def __enter__(self) -> str:
                return "abcd"

        def __exit__(self, *args): ...

    # error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because the method `__enter__` may be missing"
    with Manager() as f:
        reveal_type(f)  # revealed: str

Invalid __enter__ signature

py
class Manager:
    def __enter__() -> str:
        return "foo"

    def __exit__(self, exc_type, exc_value, traceback): ...

context_expr = Manager()

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not correctly implement `__enter__`"
with context_expr as f:
    reveal_type(f)  # revealed: str

Accidental use of non-async with

If a synchronous with statement is used on a type with __aenter__ and __aexit__, we show a diagnostic hint that the user might have intended to use async with instead.

py
class Manager:
    async def __aenter__(self): ...
    async def __aexit__(self, *args): ...

# snapshot: invalid-context-manager
with Manager():
    pass
snapshot
error[invalid-context-manager]: Object of type `Manager` cannot be used with `with` because it does not implement `__enter__` and `__exit__`
 --> src/mdtest_snippet.py:6:6
  |
6 | with Manager():
  |      ^^^^^^^^^
  |
info: Objects of type `Manager` can be used as async context managers
info: Consider using `async with` here

Incorrect signatures

The sub-diagnostic is also provided if the signatures of __aenter__ and __aexit__ do not match the expected signatures for a context manager:

py
class Manager:
    async def __aenter__(self): ...
    async def __aexit__(self, typ: str, exc, traceback): ...

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not implement `__enter__` and `__exit__`"
with Manager():
    pass

Incorrect number of arguments

Similarly, we also show the hint if the functions have the wrong number of arguments:

py
class Manager:
    async def __aenter__(self, wrong_extra_arg): ...
    async def __aexit__(self, typ, exc, traceback, wrong_extra_arg): ...

# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it does not implement `__enter__` and `__exit__`"
with Manager():
    pass