A tiny Python 3 package that introduces Protocol intersections (for Protocols themselves
see PEP 544).
The ProtocolIntersection type tells mypy that an object implements multiple protocols.
It can be used either as a function parameter or as a return value.
A mypy plugin that ships with the package is required for this to work.
See the examples section below.
The plugin supports python 3.8 up to 3.12 and mypy >= 0.920 and <= 1.10.x.
The typing-protocol-intersection package is pip-installable:
pip install typing-protocol-intersection Add typing_protocol_intersection.mypy_plugin to plugins in mypy configuration:
> cat mypy.ini
[mypy]
plugins = typing_protocol_intersection.mypy_pluginfrom typing import Protocol
from typing_protocol_intersection import ProtocolIntersection as Has
class X(Protocol):
x: str
class Y(Protocol):
y: str
def foo(xy: Has[X, Y]) -> None:
# Note xy implements both X and Y, not just one of them
print(xy.x, xy.y)Here's a more complex example showing what you can write with the help of this mypy plugin:
from types import SimpleNamespace
from typing import Protocol, Generic, TypeVar, Dict
from typing_protocol_intersection import ProtocolIntersection as Has
class X(Protocol):
x: str
class Y(Protocol):
y: str
T = TypeVar("T")
class Builder(Generic[T]):
def __init__(self) -> None:
super().__init__()
self._d: Dict[str, str] = {}
def with_x(self) -> "Builder[Has[T, X]]":
self._d["x"] = "X"
return self # type: ignore
def with_y(self) -> "Builder[Has[T, Y]]":
self._d["y"] = "Y"
return self # type: ignore
def build(self) -> T:
return SimpleNamespace(**self._d) # type: ignore
class DesiredObject(X, Y, Protocol):
pass
def get_x_y_1(o: DesiredObject) -> None:
print(f"{o.x=}; {o.y=}")
def get_x_y_2(o: Has[X, Y]) -> None:
print(f"{o.x=}; {o.y=}")
def main() -> None:
valid_o = Builder().with_x().with_y().build()
get_x_y_1(valid_o)
get_x_y_2(valid_o)
if __name__ == "__main__":
main()> # with plugin
> mypy example.py
Success: no issues found in 1 source fileAnd here's how would the plugin help if you forgot to include one of the protocols while building an object:
from types import SimpleNamespace
from typing import Protocol, Generic, TypeVar, Dict
from typing_protocol_intersection import ProtocolIntersection as Has
class X(Protocol):
x: str
class Y(Protocol):
y: str
T = TypeVar("T")
class Builder(Generic[T]):
def __init__(self) -> None:
super().__init__()
self._d: Dict[str, str] = {}
def with_x(self) -> "Builder[Has[T, X]]":
self._d["x"] = "X"
return self # type: ignore
def with_y(self) -> "Builder[Has[T, Y]]":
self._d["y"] = "Y"
return self # type: ignore
def build(self) -> T:
return SimpleNamespace(**self._d) # type: ignore
class DesiredObject(X, Y, Protocol):
pass
def get_x_y_1(o: DesiredObject) -> None:
print(f"{o.x=}; {o.y=}")
def get_x_y_2(o: Has[X, Y]) -> None:
print(f"{o.x=}; {o.y=}")
def main() -> None:
valid_o = Builder().with_x().build() # <-- note no .with_y()
get_x_y_1(valid_o)
get_x_y_2(valid_o)
if __name__ == "__main__":
main()> # Note the real output would contain some invisible characters which were removed here.
> mypy example.py
example.py:40:15: error: Argument 1 to "get_x_y_1" has incompatible type "ProtocolIntersection[X]"; expected "DesiredObject" [arg-type]
example.py:40:15: note: "ProtocolIntersection" is missing following "DesiredObject" protocol member:
example.py:40:15: note: y
example.py:41:15: error: Argument 1 to "get_x_y_2" has incompatible type "typing_protocol_intersection.types.ProtocolIntersection[X]"; expected "typing_protocol_intersection.types.ProtocolIntersection[Y, X]" [arg-type]
example.py:41:15: note: "ProtocolIntersection" is missing following "ProtocolIntersection" protocol member:
example.py:41:15: note: y
Found 2 errors in 1 file (checked 1 source file)The ProtocolIntersection class name might seem a bit lengthy, but it's explicit, which is good.
For brevity and better readability, it's recommended to use an alias when importing, as seen in the examples above.
from typing_protocol_intersection import ProtocolIntersection as Has
