feat: add get_app class method to Application (#48)

* feat: add get_app

* test: add test

Author: tlambert03

src/app_model/_app.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import contextlib
-from typing import TYPE_CHECKING, ClassVar, Dict, Iterable, List, Tuple, Type
+from typing import TYPE_CHECKING, ClassVar, Dict, Iterable, List, Optional, Tuple, Type
 
 import in_n_out as ino
 from psygnal import Signal
@@ -122,6 +122,11 @@ def get_or_create(cls, name: str) -> Application:
         """Get app named `name` or create and return a new one if it doesn't exist."""
         return cls._instances[name] if name in cls._instances else cls(name)
 
+    @classmethod
+    def get_app(cls, name: str) -> Optional[Application]:
+        """Return app named `name` or None if it doesn't exist."""
+        return cls._instances.get(name)
+
     @classmethod
     def destroy(cls, name: str) -> None:
         """Destroy the `Application` named `name`.

tests/test_app.py

@@ -12,7 +12,9 @@
 
 
 def test_app_create():
+    assert Application.get_app("my_app") is None
     app = Application("my_app")
+    assert Application.get_app("my_app") is app
 
     # NOTE: for some strange reason, this test fails if I move this line
     # below the error assertion below... I don't know why.