Art resizer typings v2 (#5734)

A rebased & amended version of
#4649.

I left a FIXME note regarding a cleanup that I feel should be done at
some point, but that is out-of-scope here when just adding initial
typings.

Author: wisp3rwind

beets/util/__init__.py

@@ -40,6 +40,7 @@
     Any,
     AnyStr,
     Callable,
+    Generic,
     Iterable,
     NamedTuple,
     TypeVar,
@@ -1041,6 +1042,53 @@ def __get__(self, instance, owner):
         return self.cache[owner]
 
 
+class LazySharedInstance(Generic[T]):
+    """A descriptor that provides access to a lazily-created shared instance of
+    the containing class, while calling the class constructor to construct a
+    new object works as usual.
+
+    ```
+    ID: int = 0
+
+    class Foo:
+        def __init__():
+            global ID
+
+            self.id = ID
+            ID += 1
+
+        def func(self):
+            print(self.id)
+
+        shared: LazySharedInstance[Foo] = LazySharedInstance()
+
+    a0 = Foo()
+    a1 = Foo.shared
+    a2 = Foo()
+    a3 = Foo.shared
+
+    a0.func()  # 0
+    a1.func()  # 1
+    a2.func()  # 2
+    a3.func()  # 1
+    ```
+    """
+
+    _instance: T | None = None
+
+    def __get__(self, instance: T | None, owner: type[T]) -> T:
+        if instance is not None:
+            raise RuntimeError(
+                "shared instances must be obtained from the class property, "
+                "not an instance"
+            )
+
+        if self._instance is None:
+            self._instance = owner()
+
+        return self._instance
+
+
 def get_module_tempdir(module: str) -> Path:
     """Return the temporary directory for the given module.