Added comments and docstrings in accordance to feedback from @nicoddemus

.

_pytest/main.py

@@ -654,6 +654,13 @@ def _locate_module(self, modulename, searchpath):
         In case of a presumptive namespace package return all of its possible
         locations.
 
+        Note: The only reliable way to determine whether a package is a
+        namespace package, i.e., whether its ``__path__`` has more than one
+        element, is to import it.  This method does not do that and hence we
+        are talking of a *presumptive* namespace package.  The ``_parsearg``
+        method is aware of this and, quite conservatively, tends to raise an
+        exception in case of doubt.
+
         """
         try:
             fd, pathname, type_ = imp.find_module(modulename, searchpath)
@@ -666,6 +673,10 @@ def _locate_module(self, modulename, searchpath):
             return [pathname]
         else:
             init_file = os.path.join(pathname, '__init__.py')
+            # The following check is a little heuristic do determine whether a
+            # package is a namespace package.  If its '__init__.py' is empty
+            # then it should be treated as a regular package (see #1568 for
+            # further discussion):
             if os.path.getsize(init_file) == 0:
                 return [pathname]
             return [pathname] + self._locate_module(

testing/acceptance_test.py

@@ -559,7 +559,10 @@ def join_pythonpath(what):
         ])
 
     def test_cmdline_python_namespace_package(self, testdir, monkeypatch):
-        monkeypatch.delenv('PYTHONDONTWRITEBYTECODE', False)
+        """
+        test --pyargs option with namespace packages (#1567)
+        """
+        monkeypatch.delenv('PYTHONDONTWRITEBYTECODE', raising=False)
 
         search_path = []
         for dirname in "hello", "world":
@@ -599,16 +602,21 @@ def join_pythonpath(*dirs):
             monkeypatch.syspath_prepend(p)
 
         # mixed module and filenames:
-        result = testdir.runpytest("--pyargs", "ns_pkg.hello", "world/ns_pkg")
+        result = testdir.runpytest("--pyargs", "-v", "ns_pkg.hello", "world/ns_pkg")
         assert result.ret == 0
         result.stdout.fnmatch_lines([
+            "*test_hello.py::test_hello*PASSED",
+            "*test_hello.py::test_other*PASSED",
+            "*test_world.py::test_world*PASSED",
+            "*test_world.py::test_other*PASSED",
             "*4 passed*"
         ])
 
         # specify tests within a module
-        result = testdir.runpytest("--pyargs", "ns_pkg.world.test_world::test_other")
+        result = testdir.runpytest("--pyargs", "-v", "ns_pkg.world.test_world::test_other")
         assert result.ret == 0
         result.stdout.fnmatch_lines([
+            "*test_world.py::test_other*PASSED",
             "*1 passed*"
         ])