Rely on iterable unpacking to extract one element from generator expression.

setuptools/build_meta.py

@@ -160,11 +160,11 @@ def build_wheel(wheel_directory, config_settings=None,
         shutil.rmtree(wheel_directory)
         shutil.copytree('dist', wheel_directory)
 
-    wheels = [f for f in os.listdir(wheel_directory)
-              if f.endswith('.whl')]
+    wheels = (f for f in os.listdir(wheel_directory)
+              if f.endswith('.whl'))
 
-    assert len(wheels) == 1
-    return wheels[0]
+    wheel, = wheels
+    return wheel
 
 
 def build_sdist(sdist_directory, config_settings=None):
@@ -175,8 +175,8 @@ def build_sdist(sdist_directory, config_settings=None):
         ["--dist-dir", sdist_directory]
     _run_setup()
 
-    sdists = [f for f in os.listdir(sdist_directory)
-              if f.endswith('.tar.gz')]
+    sdists = (f for f in os.listdir(sdist_directory)
+              if f.endswith('.tar.gz'))
 
-    assert len(sdists) == 1
-    return sdists[0]
+    sdist, = sdists
+    return sdist