Handle a previously unhandled code scenario

Previously, we didn't handle the case where an error code was implicitly
ignored (by not being in --select) and implicitly selected (by not being
in --ignore). This means we need to update StyleGuide#_decision_for and
StyleGuide#is_user_selected to handle these cases.

Closes #242
Related-to #239
Related-to !132

src/flake8/style_guide.py

@@ -64,8 +64,9 @@ class StyleGuide(object):
         self.stats = statistics.Statistics()
         self._selected = tuple(options.select)
         self._extended_selected = tuple(options.extended_default_select)
-        self._ignored = tuple(options.ignore)
         self._enabled_extensions = tuple(options.enable_extensions)
+        self._all_selected = self._selected + self._enabled_extensions
+        self._ignored = tuple(options.ignore)
         self._decision_cache = {}
         self._parsed_diff = {}
 
@@ -82,17 +83,15 @@ class StyleGuide(object):
             Ignored.Implicitly if the selected list is not empty but no match
             was found.
         """
-        if not (self._selected or self._enabled_extensions):
-            return Selected.Implicitly
-
-        if code.startswith(self._selected + self._enabled_extensions):
+        if self._all_selected and code.startswith(self._all_selected):
             return Selected.Explicitly
 
-        # If it was not explicitly selected, it may have been implicitly
-        # selected because the check comes from a plugin that is enabled by
-        # default
-        if (self._extended_selected and
-                code.startswith(self._extended_selected)):
+        if (not self._all_selected and
+            (self._extended_selected and
+                code.startswith(self._extended_selected))):
+            # If it was not explicitly selected, it may have been implicitly
+            # selected because the check comes from a plugin that is enabled by
+            # default
             return Selected.Implicitly
 
         return Ignored.Implicitly
@@ -118,10 +117,22 @@ class StyleGuide(object):
     def _decision_for(self, code):
         # type: (Error) -> Decision
         startswith = code.startswith
-        selected = sorted([s for s in self._selected if startswith(s)])[0]
-        ignored = sorted([i for i in self._ignored if startswith(i)])[0]
+        try:
+            selected = sorted([s for s in self._selected if startswith(s)])[0]
+        except IndexError:
+            selected = None
+        try:
+            ignored = sorted([i for i in self._ignored if startswith(i)])[0]
+        except IndexError:
+            ignored = None
 
-        if selected.startswith(ignored):
+        if selected is None:
+            return Decision.Ignored
+
+        if ignored is None:
+            return Decision.Selected
+
+        if selected.startswith(ignored) and selected != ignored:
             return Decision.Selected
         return Decision.Ignored
 
@@ -150,8 +161,10 @@ class StyleGuide(object):
                  selected is Selected.Implicitly) and
                     ignored is Selected.Implicitly):
                 decision = Decision.Selected
-            elif (selected is Selected.Explicitly and
-                  ignored is Ignored.Explicitly):
+            elif ((selected is Selected.Explicitly and
+                  ignored is Ignored.Explicitly) or
+                  (selected is Ignored.Implicitly and
+                   ignored is Selected.Implicitly)):
                 decision = self._decision_for(code)
             elif (selected is Ignored.Implicitly or
                   ignored is Ignored.Explicitly):

tests/unit/test_style_guide.py

@@ -78,9 +78,14 @@ def test_is_user_selected_implicitly_selects_errors():
     """Verify we detect users implicitly selecting an error."""
     select_list = []
     error_code = 'E121'
-    guide = style_guide.StyleGuide(create_options(select=select_list),
-                                   listener_trie=None,
-                                   formatter=None)
+    guide = style_guide.StyleGuide(
+        create_options(
+            select=select_list,
+            extended_default_select=['E'],
+        ),
+        listener_trie=None,
+        formatter=None,
+    )
 
     assert (guide.is_user_selected(error_code) is
             style_guide.Selected.Implicitly)
@@ -114,9 +119,11 @@ def test_is_user_selected_excludes_errors(select_list, error_code):
     (['E111', 'E121'], ['E2'], 'E122', style_guide.Decision.Ignored),
     (['E11', 'E12'], ['E13'], 'E132', style_guide.Decision.Ignored),
     (['E1', 'E3'], ['E32'], 'E321', style_guide.Decision.Ignored),
-    ([], ['E2', 'E12'], 'E410', style_guide.Decision.Selected),
+    ([], ['E2', 'E12'], 'E410', style_guide.Decision.Ignored),
     (['E4'], ['E2', 'E12', 'E41'], 'E410', style_guide.Decision.Ignored),
     (['E41'], ['E2', 'E12', 'E4'], 'E410', style_guide.Decision.Selected),
+    (['E'], ['F'], 'E410', style_guide.Decision.Selected),
+    (['F'], [], 'E410', style_guide.Decision.Ignored),
 ])
 def test_should_report_error(select_list, ignore_list, error_code, expected):
     """Verify we decide when to report an error."""
@@ -172,7 +179,6 @@ def test_disable_is_inline_ignored():
     (['E111', 'E121'], [], 'E111'),
     (['E111', 'E121'], [], 'E121'),
     (['E11', 'E121'], ['E1'], 'E112'),
-    ([], ['E2', 'E12'], 'E410'),
     (['E41'], ['E2', 'E12', 'E4'], 'E410'),
 ])
 def test_handle_error_notifies_listeners(select_list, ignore_list, error_code):