Pull decision making out of decision_for

Also, this further highlights why naming methods is so hard. I can't
think of a better name for 'more_specific_decision_for' that isn't
wildly long and unnecessarily verbose.

src/flake8/style_guide.py

@@ -240,21 +240,8 @@ class DecisionEngine(object):
             return Decision.Ignored
         return Decision.Selected
 
-    def decision_for(self, code):
+    def make_decision(self, code):
-        # type: (str) -> Decision
+        """Decide if code should be ignored or selected."""
-        """Determine if the error code should be reported or ignored.
-
-        This method only cares about the select and ignore rules as specified
-        by the user in their configuration files and command-line flags.
-
-        This method does not look at whether the specific line is being
-        ignored in the file itself.
-
-        :param str code:
-            The code for the check that has been run.
-        """
-        decision = self.cache.get(code)
-        if decision is None:
         LOG.debug('Deciding if "%s" should be reported', code)
         selected = self.was_selected(code)
         ignored = self.was_ignored(code)
@@ -273,7 +260,26 @@ class DecisionEngine(object):
         elif (selected is Ignored.Implicitly or
               ignored is Ignored.Explicitly):
             decision = Decision.Ignored  # pylint: disable=R0204
+        return decision
 
+    def decision_for(self, code):
+        # type: (str) -> Decision
+        """Return the decision for a specific code.
+
+        This method caches the decisions for codes to avoid retracing the same
+        logic over and over again. We only care about the select and ignore
+        rules as specified by the user in their configuration files and
+        command-line flags.
+
+        This method does not look at whether the specific line is being
+        ignored in the file itself.
+
+        :param str code:
+            The code for the check that has been run.
+        """
+        decision = self.cache.get(code)
+        if decision is None:
+            decision = self.make_decision(code)
             self.cache[code] = decision
             LOG.debug('"%s" will be "%s"', code, decision)
         return decision