Define MetadataSourcePlugin methods on the subclass only

beets/metadata_plugins.py

@@ -8,7 +8,6 @@ implemented as plugins.
 from __future__ import annotations
 
 import abc
-import inspect
 import re
 import warnings
 from typing import TYPE_CHECKING, Generic, Literal, Sequence, TypedDict, TypeVar
@@ -421,13 +420,3 @@ class SearchApiMetadataSourcePlugin(
             query = unidecode.unidecode(query)
 
         return query
-
-
-# Dynamically copy methods to BeetsPlugin for legacy support
-# TODO: Remove this in the future major release, v3.0.0
-
-for name, method in inspect.getmembers(
-    MetadataSourcePlugin, predicate=inspect.isfunction
-):
-    if not hasattr(BeetsPlugin, name):
-        setattr(BeetsPlugin, name, method)

beets/plugins.py

@@ -158,6 +158,21 @@ class BeetsPlugin(metaclass=abc.ABCMeta):
     early_import_stages: list[ImportStageFunc]
     import_stages: list[ImportStageFunc]
 
+    def __init_subclass__(cls) -> None:
+        # Dynamically copy methods to BeetsPlugin for legacy support
+        # TODO: Remove this in the future major release, v3.0.0
+        if inspect.isabstract(cls):
+            return
+
+        from beets.metadata_plugins import MetadataSourcePlugin
+
+        abstractmethods = MetadataSourcePlugin.__abstractmethods__
+        for name, method in inspect.getmembers(
+            MetadataSourcePlugin, predicate=inspect.isfunction
+        ):
+            if name not in abstractmethods and not hasattr(cls, name):
+                setattr(cls, name, method)
+
     def __init__(self, name: str | None = None):
         """Perform one-time plugin setup."""