Make all select-related classes and functions covariant

When selecting, we should allow containers of more specific types to be
mixed with less specific types. We also use the new public type
variables.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/_select.py

@@ -133,13 +133,12 @@
 
 import asyncio
 from collections.abc import AsyncIterator
-from typing import Any, Generic, TypeGuard, TypeVar
+from typing import Any, Generic, TypeGuard
 
 from ._exceptions import Error
+from ._generic import ReceiverMessageT_co
 from ._receiver import Receiver, ReceiverStoppedError
 
-_T = TypeVar("_T")
-
 
 class _EmptyResult:
     """A sentinel value to distinguish between None and empty result.
@@ -152,7 +151,7 @@ def __repr__(self) -> str:
         return "<empty>"
 
 
-class Selected(Generic[_T]):
+class Selected(Generic[ReceiverMessageT_co]):
     """A result of a [`select()`][frequenz.channels.select] iteration.
 
     The selected receiver is consumed immediately and the received message is stored in
@@ -166,7 +165,7 @@ class Selected(Generic[_T]):
     Please see [`select()`][frequenz.channels.select] for an example.
     """
 
-    def __init__(self, receiver: Receiver[_T], /) -> None:
+    def __init__(self, receiver: Receiver[ReceiverMessageT_co], /) -> None:
         """Initialize this selected result.
 
         The receiver is consumed immediately when creating the instance and the received
@@ -178,10 +177,10 @@ def __init__(self, receiver: Receiver[_T], /) -> None:
         Args:
             receiver: The receiver that was selected.
         """
-        self._recv: Receiver[_T] = receiver
+        self._recv: Receiver[ReceiverMessageT_co] = receiver
         """The receiver that was selected."""
 
-        self._message: _T | _EmptyResult = _EmptyResult()
+        self._message: ReceiverMessageT_co | _EmptyResult = _EmptyResult()
         """The message that was received.
 
         If there was an exception while receiving the message, then this will be `None`.
@@ -198,7 +197,7 @@ def __init__(self, receiver: Receiver[_T], /) -> None:
         """Flag to indicate if this selected has been handled in the if-chain."""
 
     @property
-    def message(self) -> _T:
+    def message(self) -> ReceiverMessageT_co:
         """The message that was received, if any.
 
         Returns:
@@ -248,8 +247,8 @@ def __repr__(self) -> str:
 # `TypeGuard`s can't be used as methods. For more information see:
 # https://github.com/microsoft/pyright/discussions/3125
 def selected_from(
-    selected: Selected[Any], receiver: Receiver[_T]
-) -> TypeGuard[Selected[_T]]:
+    selected: Selected[Any], receiver: Receiver[ReceiverMessageT_co]
+) -> TypeGuard[Selected[ReceiverMessageT_co]]:
     """Check whether the given receiver was selected by [`select()`][frequenz.channels.select].
 
     This function is used in conjunction with the
@@ -283,23 +282,23 @@ class SelectError(Error):
     """
 
 
-class UnhandledSelectedError(SelectError, Generic[_T]):
+class UnhandledSelectedError(SelectError, Generic[ReceiverMessageT_co]):
     """A receiver was not handled in a [`select()`][frequenz.channels.select] iteration.
 
     This exception is raised when a [`select()`][frequenz.channels.select] iteration
     finishes without a call to [`selected_from()`][frequenz.channels.selected_from] for
     the selected receiver.
     """
 
-    def __init__(self, selected: Selected[_T]) -> None:
+    def __init__(self, selected: Selected[ReceiverMessageT_co]) -> None:
         """Initialize this error.
 
         Args:
             selected: The selected receiver that was not handled.
         """
         recv = selected._recv  # pylint: disable=protected-access
         super().__init__(f"Selected receiver {recv} was not handled in the if-chain")
-        self.selected: Selected[_T] = selected
+        self.selected: Selected[ReceiverMessageT_co] = selected
         """The selected receiver that was not handled."""