Fix `reload()` with modules handled via `python_files`

If a module exists in `sys.modules` already, `load_module` has to return it.

Fixes https://bitbucket.org/pytest-dev/pytest/issue/435

--HG--
branch : fix-reload

_pytest/assertion/rewrite.py

@@ -146,6 +146,12 @@ class AssertionRewritingHook(object):
         return self
 
     def load_module(self, name):
+        # If there is an existing module object named 'fullname' in
+        # sys.modules, the loader must use that existing module. (Otherwise,
+        # the reload() builtin will not work correctly.)
+        if name in sys.modules:
+            return sys.modules[name]
+
         co, pyc = self.modules.pop(name)
         # I wish I could just call imp.load_compiled here, but __file__ has to
         # be set properly. In Python 3.2+, this all would be handled correctly

testing/test_assertrewrite.py

@@ -641,3 +641,27 @@ class TestAssertionRewriteHookDetails(object):
         pyc.write(contents[:strip_bytes], mode='wb')
 
         assert _read_pyc(source, str(pyc)) is None  # no error
+
+    def test_reload_is_same(self, testdir):
+        # A file that will be picked up during collecting.
+        testdir.tmpdir.join("file.py").ensure()
+        testdir.tmpdir.join("pytest.ini").write(py.std.textwrap.dedent("""
+            [pytest]
+            python_files = *.py
+        """))
+
+        testdir.makepyfile(test_fun="""
+            import sys
+            try:
+                from imp import reload
+            except ImportError:
+                pass
+
+            def test_loader():
+                import file
+                assert sys.modules["file"] is reload(file)
+            """)
+        result = testdir.runpytest('-s')
+        result.stdout.fnmatch_lines([
+            "* 1 passed*",
+        ])