testutil: extract find_one_subclass_in_module() helper

A small refactor to avoid shipping this duplicated code (this
one is easy to extract/reuse).

inputs/test/conftest.py

@@ -1,4 +1,3 @@
-import inspect
 import os
 import pathlib
 from types import ModuleType
@@ -28,14 +27,7 @@ def inputs_service(inputs_module) -> ModuleType:
     """inputs_service is a fixture that imports a inputs module by its name
     defined in INPUTS_NAME in the test module and returns a InputService
     """
-    service_cls = None
+    service_cls = testutil.find_one_subclass_in_module(inputs_module, inputs.InputService)
-    for memb in inspect.getmembers(
-            inputs_module,
-            predicate=lambda obj: inspect.isclass(obj) and issubclass(
-                obj, inputs.InputService)):
-        if service_cls:
-            raise ValueError(f"already have {service_cls}, also found {memb}")
-        service_cls = memb[1]
     fd = testutil.make_fake_service_fd()
     srv_obj = service_cls.from_args(["--service-fd", str(fd)])
     return srv_obj

osbuild/testutil/__init__.py

@@ -2,6 +2,7 @@
 Test related utilities
 """
 import contextlib
+import inspect
 import os
 import pathlib
 import re
@@ -10,6 +11,8 @@ import socket
 import subprocess
 import tempfile
 import textwrap
+from types import ModuleType
+from typing import Type
 
 
 def has_executable(executable: str) -> bool:
@@ -168,3 +171,18 @@ def make_fake_service_fd() -> int:
     sock = socket.socket(socket.AF_UNIX, socket.SOCK_SEQPACKET)
     fd = os.dup(sock.fileno())
     return fd
+
+
+def find_one_subclass_in_module(module: ModuleType, subclass: Type) -> object:
+    """Find the class in the given module that is a subclass of the given input
+
+    If multiple classes are found an error is raised.
+    """
+    cls = None
+    for name, memb in inspect.getmembers(
+            module,
+            predicate=lambda obj: inspect.isclass(obj) and issubclass(obj, subclass)):
+        if cls:
+            raise ValueError(f"already have {cls}, also found {name}:{memb}")
+        cls = memb
+    return cls

sources/test/conftest.py

@@ -1,4 +1,3 @@
-import inspect
 import os
 import pathlib
 from types import ModuleType
@@ -28,14 +27,7 @@ def sources_service(sources_module) -> ModuleType:
     """sources_service is a fixture that imports a sources module by its name
     defined in SOURCES_NAME in the test module and returns a SourcesService
     """
-    service_cls = None
+    service_cls = testutil.find_one_subclass_in_module(sources_module, sources.SourceService)
-    for memb in inspect.getmembers(
-            sources_module,
-            predicate=lambda obj: inspect.isclass(obj) and issubclass(
-                obj, sources.SourceService)):
-        if service_cls:
-            raise ValueError(f"already have {service_cls}, also found {memb}")
-        service_cls = memb[1]
     fd = testutil.make_fake_service_fd()
-    services_obj = service_cls.from_args(["--service-fd", str(fd)])
+    srv_obj = service_cls.from_args(["--service-fd", str(fd)])
-    return services_obj
+    return srv_obj