typing-protocol-intersection 0.3.4

Protocol intersection for mypy

typing-protocol-intersection

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.

Supported versions

The plugin supports python 3.7, 3.8, 3.9, 3.10 and 3.11 and mypy >= 0.920 and <= 1.4.x.

Installation

The typing-protocol-intersection package is pip-installable:

pip install typing-protocol-intersection 

Configuration

Add typing_protocol_intersection.mypy_plugin to plugins in mypy configuration:

> cat mypy.ini
[mypy]
plugins = typing_protocol_intersection.mypy_plugin

Examples

Simple example

from 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)

Complex example - valid program

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 file

Complex example - invalid program

And 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)

Recommended usage

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