Validate asyncio marker loop_scope via _validate_scope

changelog.d/1584.fixed.rst

@@ -0,0 +1 @@
+Fixed an invalid ``loop_scope`` in the ``asyncio`` marker being reported as a bare ``AssertionError`` exposing a Python set repr, instead of a usage error that names the argument and lists the valid scopes.

pytest_asyncio/plugin.py

@@ -979,8 +979,7 @@ def _parse_asyncio_marker(
     scope = asyncio_marker.kwargs.get("loop_scope") or asyncio_marker.kwargs.get(
         "scope"
     )
-    if scope is not None:
-        assert scope in {"function", "class", "module", "package", "session"}
+    _validate_scope(scope, "loop_scope")
     marker_value = asyncio_marker.kwargs.get("loop_factories")
     if marker_value is None:
         return scope, None

tests/test_fixture_loop_scopes.py

@@ -126,3 +126,23 @@ def test_invalid_default_fixture_loop_scope_raises_error(pytester: Pytester):
             "function, class, module, package, session."
         ]
     )
+
+
+def test_invalid_marker_loop_scope_raises_error(pytester: Pytester):
+    pytester.makeini("[pytest]\nasyncio_default_fixture_loop_scope = function")
+    pytester.makepyfile("""\
+        import pytest
+
+        @pytest.mark.asyncio(loop_scope="invalid_scope")
+        async def test_invalid_loop_scope():
+            pass
+        """)
+    result = pytester.runpytest("--assert=plain")
+    # The exception class is rendered as pytest.UsageError on newer pytest and as
+    # _pytest.config.exceptions.UsageError on the oldest supported version.
+    result.stdout.fnmatch_lines(
+        [
+            "*UsageError: 'invalid_scope' is not a valid loop_scope. "
+            "Valid scopes are: function, class, module, package, session."
+        ]
+    )