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API reference for context.

Services: tags, references, and the context that holds them.

A tag names a service. Its identity is the class, and its type parameter is the service's shape, so two tags can share a shape and stay distinct::

class Database(Protocol):
    def find(self, key: str) -> str | None: ...

class Primary(Tag[Database]): ...
class Replica(Tag[Database]): ...

def program():
    db = yield from Primary()   # Database; adds Primary to R

Calling a tag class does not construct anything: it gives the effect that reads the service from the running fiber's context.

A Reference is a tag with a default value. Effects that read a reference do not need it provided, so it never appears in R::

class LogLevel(Reference[int]):
    @classmethod
    def default(cls) -> int:
        return 20
AttributeTypeDescription
S_inv= TypeVar('S_inv')
T_co= TypeVar('T_co', covariant=True)
R_contra= TypeVar('R_contra', contravariant=True, default=Any)

Classes

MissingServiceErrorServiceEffectTagReferenceTagLikeContext

Functions

tag_class

tag_class(tag) -> type[Tag[S]]

The class behind a TagLike. Every TagLike is a tag class; the protocol only exists so signatures can name its shape and tag.

ParameterTypeDescription
tagTagLike[S, T]

Returns type[Tag[S]]

def tag_class[S, T](tag: TagLike[S, T]) -> type[Tag[S]]:
    """The class behind a ``TagLike``. Every ``TagLike`` is a tag class;
    the protocol only exists so signatures can name its shape and tag."""
    if isinstance(tag, type) and issubclass(tag, Tag):
        return tag  # pyright: ignore[reportUnknownVariableType]
    raise TypeError(f"{tag!r} is not a Tag class")