allow opt-in option '--backends=...' in test runner instead of requiring opt-out '--no-...' options

runtests.py

@@ -204,6 +204,8 @@ COMPILER = None
 INCLUDE_DIRS = [ d for d in os.getenv('INCLUDE', '').split(os.pathsep) if d ]
 CFLAGS = os.getenv('CFLAGS', '').split()
 
+BACKENDS = ['c', 'cpp']
+
 def memoize(f):
     uncomputed = object()
     f._cache = {}
@@ -1248,12 +1250,15 @@ def main():
                       help="do not run the Cython compiler, only the C compiler")
     parser.add_option("--compiler", dest="compiler", default=None,
                       help="C compiler type")
+    backend_list = ','.join(BACKENDS)
+    parser.add_option("--backends", dest="backends", default=backend_list,
+                      help="select backends to test (default: %s)" % backend_list)
     parser.add_option("--no-c", dest="use_c",
                       action="store_false", default=True,
-                      help="do not test C compilation")
+                      help="do not test C compilation backend")
     parser.add_option("--no-cpp", dest="use_cpp",
                       action="store_false", default=True,
-                      help="do not test C++ compilation")
+                      help="do not test C++ compilation backend")
     parser.add_option("--no-unit", dest="unittests",
                       action="store_false", default=True,
                       help="do not run the unit tests")
@@ -1402,8 +1407,6 @@ def main():
     if WITH_CYTHON and options.language_level == 3:
         sys.stderr.write("Using Cython language level 3.\n")
 
-    sys.stderr.write("\n")
-
     test_bugs = False
     if options.tickets:
         for ticket_number in options.tickets:
@@ -1438,11 +1441,23 @@ def main():
     global COMPILER
     if options.compiler:
         COMPILER = options.compiler
-    languages = []
-    if options.use_c:
-        languages.append('c')
-    if options.use_cpp:
-        languages.append('cpp')
+
+    selected_backends = [ name.strip() for name in options.backends.split(',') if name.strip() ]
+    backends = []
+    for backend in selected_backends:
+        if backend == 'c' and not options.use_c:
+            continue
+        elif backend == 'cpp' and not options.use_cpp:
+            continue
+        elif backend not in BACKENDS:
+            sys.stderr.write("Unknown backend requested: '%s' not one of [%s]\n" % (
+                backend, ','.join(BACKENDS)))
+            sys.exit(1)
+        backends.append(backend)
+    sys.stderr.write("Backends: %s\n" % ','.join(backends))
+    languages = backends
+
+    sys.stderr.write("\n")
 
     test_suite = unittest.TestSuite()