Using typed attributes

On AnyIO, streams and listeners can be layered on top of each other to provide extra functionality. But when you want to look up information from one of the layers down below, you might have to traverse the entire chain to find what you’re looking for, which is highly inconvenient. To address this, AnyIO has a system of typed attributes where you can look for a specific attribute by its unique key. If a stream or listener wrapper does not have the attribute you’re looking for, it will look it up in the wrapped instance, and that wrapper can look in its wrapped instance and so on, until the attribute is either found or the end of the chain is reached. This also lets wrappers override attributes from the wrapped objects when necessary.

A common use case is finding the IP address of the remote side of a TCP connection when the stream may be either SocketStream or TLSStream:

from anyio import connect_tcp
from anyio.abc import SocketAttribute


async def connect(host, port, tls: bool):
    stream = await connect_tcp(host, port, tls=tls)
    print('Connected to', stream.extra(SocketAttribute.remote_address))

Each typed attribute provider class should document the set of attributes it provides on its own.

Defining your own typed attributes

By convention, typed attributes are stored together in a container class with other attributes of the same category:

from anyio import TypedAttribute, TypedAttributeSet


class MyTypedAttribute:
    string_valued_attribute = TypedAttribute[str]()
    some_float_attribute = TypedAttribute[float]()

To provide values for these attributes, implement the extra_attributes() property in your class:

from anyio import TypedAttributeProvider


class MyAttributeProvider(TypedAttributeProvider):
    def extra_attributes():
        return {
            MyTypedAttribute.string_valued_attribute: lambda: 'my attribute value',
            MyTypedAttribute.some_float_attribute: lambda: 6.492
        }

If your class inherits from another typed attribute provider, make sure you include its attributes in the return value:

class AnotherAttributeProvider(MyAttributeProvider):
    def extra_attributes():
        return {
            **super().extra_attributes,
            MyTypedAttribute.string_valued_attribute: lambda: 'overridden attribute value'
        }