Use contravariant generic types for Senders

This allows narrowing of the message type, such that senders for
sending broad types can be typed as senders for narrower types.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/channels/_sender.py

@@ -54,14 +54,14 @@
 
 from ._exceptions import Error
 
-_T = TypeVar("_T")
+_T_contra = TypeVar("_T_contra", contravariant=True)
 
 
-class Sender(ABC, Generic[_T]):
+class Sender(ABC, Generic[_T_contra]):
     """An endpoint to sends messages."""
 
     @abstractmethod
-    async def send(self, msg: _T) -> None:
+    async def send(self, msg: _T_contra) -> None:
         """Send a message to the channel.
 
         Args:
@@ -72,13 +72,13 @@ async def send(self, msg: _T) -> None:
         """
 
 
-class SenderError(Error, Generic[_T]):
+class SenderError(Error, Generic[_T_contra]):
     """An error produced in a [Sender][frequenz.channels.Sender].
 
     All exceptions generated by senders inherit from this exception.
     """
 
-    def __init__(self, message: str, sender: Sender[_T]):
+    def __init__(self, message: str, sender: Sender[_T_contra]):
         """Create an instance.
 
         Args:
@@ -87,5 +87,5 @@ def __init__(self, message: str, sender: Sender[_T]):
                 happened.
         """
         super().__init__(message)
-        self.sender: Sender[_T] = sender
+        self.sender: Sender[_T_contra] = sender
         """The sender where the error happened."""