Examples using typing.Protocol from PEP 544

Notes

Classic Example: a "file-like object"

From PEP 333 – Python Web Server Gateway Interface v1.0 (2003):

To be considered “file-like”, the object supplied by the application must have a read() method that takes an optional size argument.

The words "file-like" (or "file like") appear with similar implied meaning in the Python 3.12 distribution:

• 148 times in the documentation;
• 92 times in code comments (.py or .c source files).

Also, 30 times across 21 PEPs (100, 214, 258, 282, 305, 310, 333, 368, 400, 441, 444, 578, 680, 691, 3116, 3119, 3143, 3145, 3154, 3156, 3333).

Definition in Lib/wsgiref/types.py:

class _Readable(Protocol):
    def read(self, size: int = ..., /) -> bytes: ...

Examples as of 2024-05-23

typing.Protocol definitions found with ripgrep:

rg "Protocol\)" -g '*.pyi' | sort

• 120 definitions on typeshed/stdlib (Python standard library)
• 134 definitions on typeshed/stubs (external packages)

Cases to study

• importlib/resources/abc.py: class Traversable(Protocol) with several abstract and concrete methods, looks like an ABC but derives from typing.Protocol

Not typing.Protocol

• asyncio/protocols.py: defines classes BaseProtocol, Protocol etc. for network protocols