extract _warn_about_missing_assertion into freestanding function

_pytest/config.py

@@ -54,7 +54,7 @@ def main(args=None, plugins=None):
             tw = py.io.TerminalWriter(sys.stderr)
             for line in traceback.format_exception(*e.excinfo):
                 tw.line(line.rstrip(), red=True)
-            tw.line("ERROR: could not load %s\n" % (e.path), red=True)
+            tw.line("ERROR: could not load %s\n" % (e.path,), red=True)
             return 4
         else:
             try:
@@ -1016,7 +1016,7 @@ class Config(object):
                 mode = 'plain'
             else:
                 self._mark_plugins_for_rewrite(hook)
-        self._warn_about_missing_assertion(mode)
+        _warn_about_missing_assertion(mode)
 
     def _mark_plugins_for_rewrite(self, hook):
         """
@@ -1043,23 +1043,6 @@ class Config(object):
         for name in _iter_rewritable_modules(package_files):
             hook.mark_rewrite(name)
 
-    def _warn_about_missing_assertion(self, mode):
-        try:
-            assert False
-        except AssertionError:
-            pass
-        else:
-            if mode == 'plain':
-                sys.stderr.write("WARNING: ASSERTIONS ARE NOT EXECUTED"
-                                 " and FAILING TESTS WILL PASS.  Are you"
-                                 " using python -O?")
-            else:
-                sys.stderr.write("WARNING: assertions not in test modules or"
-                                 " plugins will be ignored"
-                                 " because assert statements are not executed "
-                                 "by the underlying Python interpreter "
-                                 "(are you using python -O?)\n")
-
     def _preparse(self, args, addopts=True):
         if addopts:
             args[:] = shlex.split(os.environ.get('PYTEST_ADDOPTS', '')) + args
@@ -1233,6 +1216,29 @@ class Config(object):
         return self.getoption(name, skip=True)
 
 
+def _assertion_supported():
+    try:
+        assert False
+    except AssertionError:
+        return True
+    else:
+        return False
+
+
+def _warn_about_missing_assertion(mode):
+    if not _assertion_supported():
+        if mode == 'plain':
+            sys.stderr.write("WARNING: ASSERTIONS ARE NOT EXECUTED"
+                             " and FAILING TESTS WILL PASS.  Are you"
+                             " using python -O?")
+        else:
+            sys.stderr.write("WARNING: assertions not in test modules or"
+                             " plugins will be ignored"
+                             " because assert statements are not executed "
+                             "by the underlying Python interpreter "
+                             "(are you using python -O?)\n")
+
+
 def exists(path, ignore=EnvironmentError):
     try:
         return path.check()