Protocols (PEP 544)

1from typing import Protocol, runtime_checkable
2
3class Sized(Protocol):
4    def __len__(self) -> int: ...
5
6class Iterable(Protocol):
7    def __iter__(self): ...
8
9# Custom protocol
10@runtime_checkable
11class Closeable(Protocol):
12    def close(self) -> None: ...
13
14class File:
15    def close(self):
16        print("Closing file")
17
18class Connection:
19    def close(self):
20        print("Closing connection")
21
22def cleanup(resource: Closeable):
23    resource.close()
24
25# Works with any class that has close()
26cleanup(File())
27cleanup(Connection())
28
29# Runtime check
30isinstance(File(), Closeable)  # True