Add public generic type vars

This is to make documentation more readable, and to be able to document
those type variables.

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

Author: llucax

src/frequenz/channels/__init__.py

@@ -77,6 +77,14 @@
 from ._anycast import Anycast
 from ._broadcast import Broadcast
 from ._exceptions import ChannelClosedError, ChannelError, Error
+from ._generic import (
+    ChannelMessageT,
+    ErroredChannelT_co,
+    MappedMessageT_co,
+    ReceiverMessageT_co,
+    SenderMessageT_co,
+    SenderMessageT_contra,
+)
 from ._merge import Merger, merge
 from ._receiver import Receiver, ReceiverError, ReceiverStoppedError
 from ._select import (
@@ -93,15 +101,21 @@
     "Broadcast",
     "ChannelClosedError",
     "ChannelError",
+    "ChannelMessageT",
     "Error",
+    "ErroredChannelT_co",
+    "MappedMessageT_co",
     "Merger",
     "Receiver",
     "ReceiverError",
+    "ReceiverMessageT_co",
     "ReceiverStoppedError",
     "SelectError",
     "Selected",
     "Sender",
     "SenderError",
+    "SenderMessageT_co",
+    "SenderMessageT_contra",
     "UnhandledSelectedError",
     "merge",
     "select",

src/frequenz/channels/_generic.py

@@ -0,0 +1,24 @@
+# License: MIT
+# Copyright © 2024 Frequenz Energy-as-a-Service GmbH
+
+"""Generic type variables."""
+
+from typing import TypeVar
+
+ChannelMessageT = TypeVar("ChannelMessageT")
+"""The type of the message that can be sent across a channel."""
+
+ErroredChannelT_co = TypeVar("ErroredChannelT_co", covariant=True)
+"""The type of channel having an error."""
+
+MappedMessageT_co = TypeVar("MappedMessageT_co", covariant=True)
+"""The type of the message received by the receiver after being mapped."""
+
+ReceiverMessageT_co = TypeVar("ReceiverMessageT_co", covariant=True)
+"""The type of the message received by a receiver."""
+
+SenderMessageT_co = TypeVar("SenderMessageT_co", covariant=True)
+"""The type of the message sent by a sender."""
+
+SenderMessageT_contra = TypeVar("SenderMessageT_contra", contravariant=True)
+"""The type of the message sent by a sender."""