Add better logic for loading entry points

As noted in the code, setuptools 11.3 deprecated EntryPoint.load and
its require parameter. Starting with 11.3 the preferred way is use
EntryPoint.require and EntryPoint.resolve as necessary. Unfortunately,
those methods do not exist in all versions of setuptools. So we have
to check for them and use them when they're available. Otherwise, we
fallback to using EntryPoint.load with the require parameter.

Closes #59
Closes #90

flake8/engine.py

@@ -17,6 +17,26 @@ _flake8_noqa = re.compile(r'\s*# flake8[:=]\s*noqa', re.I).search
 EXTRA_EXCLUDE = ['.tox', '.eggs', '*.egg']
 
 
+def _load_entry_point(entry_point, verify_requirements):
+    """Based on the version of setuptools load an entry-point correctly.
+
+    setuptools 11.3 deprecated `require=False` in the call to EntryPoint.load.
+    To load entry points correctly after that without requiring all
+    dependencies be present, the proper way is to call EntryPoint.resolve.
+
+    This function will provide backwards compatibility for older versions of
+    setuptools while also ensuring we do the right thing for the future.
+    """
+    if hasattr(entry_point, 'resolve') and hasattr(entry_point, 'require'):
+        if verify_requirements:
+            entry_point.require()
+        plugin = entry_point.resolve()
+    else:
+        plugin = entry_point.load(require=verify_requirements)
+
+    return plugin
+
+
 def _register_extensions():
     """Register all the extensions."""
     extensions = util.OrderedSet()
@@ -31,7 +51,7 @@ def _register_extensions():
     else:
         for entry in iter_entry_points('flake8.extension'):
             # Do not verify that the requirements versions are valid
-            checker = entry.load(require=False)
+            checker = _load_entry_point(entry, verify_requirements=False)
             pep8.register_check(checker, codes=[entry.name])
             extensions.add((checker.name, checker.version))
             if hasattr(checker, 'add_options'):

flake8/tests/test_engine.py

@@ -113,6 +113,30 @@ class TestEngine(unittest.TestCase):
             sg = engine.StyleGuide(parser=parser)
             assert 'X' not in sg.options.ignore
 
+    def test_load_entry_point_verifies_requirements(self):
+        entry_point = mock.Mock(spec=['require', 'resolve', 'load'])
+
+        engine._load_entry_point(entry_point, verify_requirements=True)
+        entry_point.require.assert_called_once_with()
+        entry_point.resolve.assert_called_once_with()
+
+    def test_load_entry_point_does_not_verify_requirements(self):
+        entry_point = mock.Mock(spec=['require', 'resolve', 'load'])
+
+        engine._load_entry_point(entry_point, verify_requirements=False)
+        self.assertFalse(entry_point.require.called)
+        entry_point.resolve.assert_called_once_with()
+
+    def test_load_entry_point_passes_require_argument_to_load(self):
+        entry_point = mock.Mock(spec=['load'])
+
+        engine._load_entry_point(entry_point, verify_requirements=True)
+        entry_point.load.assert_called_once_with(require=True)
+        entry_point.reset_mock()
+
+        engine._load_entry_point(entry_point, verify_requirements=False)
+        entry_point.load.assert_called_once_with(require=False)
+
 
 def oserror_generator(error_number, message='Ominous OSError message'):
     def oserror_side_effect(*args, **kwargs):