Move "__ in function name" check to new N807 code

This check appears controversial (#9) because PEP8 doesn't mention this
as an official naming rule. This change moves it to its own error code
so that it can be disabled separately.

README.rst

@@ -46,6 +46,8 @@ These error codes are emitted:
 +------+-------------------------------------------------------+
 | N806 | variable in function should be lowercase              |
 +------+-------------------------------------------------------+
+| N807 | function name should not start or end with '__'       |
++------+-------------------------------------------------------+
 +------+-------------------------------------------------------+
 | N811 | constant imported as non constant                     |
 +------+-------------------------------------------------------+

pep8ext_naming.py

@@ -232,22 +232,25 @@ class FunctionNameCheck(BaseASTCheck):
     """
     Function names should be lowercase, with words separated by underscores
     as necessary to improve readability.
-    Functions *not* beeing methods '__' in front and back are not allowed.
+
+    Functions *not* being methods '__' in front and back are not allowed.
     mixedCase is allowed only in contexts where that's already the
     prevailing style (e.g. threading.py), to retain backwards compatibility.
     """
     check = LOWERCASE_REGEX.match
     N802 = "function name '{name}' should be lowercase xxx"
+    N807 = "function name '{name}' should not start or end with '__'"
     def visit_functiondef(self, node, parents, ignore=None):
         function_type = getattr(node, 'function_type', _FunctionType.FUNCTION)
         name = node.name
         if ignore and name in ignore:
             return
-        if ((function_type == 'function' and '__' in (name[:2], name[-2:])) or
-                not self.check(name)):
+        if not self.check(name):
             yield self.err(node, 'N802', name)
+        if function_type == 'function' and '__' in (name[:2], name[-2:]):
+            yield self.err(node, 'N807', name)
 class FunctionArgNamesCheck(BaseASTCheck):

testsuite/N802.py

 #: Okay
 def ok():
     pass
-#: N802
-def __bad():
-    pass
-#: N802
-def bad__():
-    pass
-#: N802
-def __bad__():
-    pass
 #: Okay
 def _ok():
     pass
@@ -47,11 +38,6 @@ class ClassName(object):
 class ClassName(object):
     def notOk(self):
         pass
-#: N802
-class ClassName(object):
-    def method(self):
-        def __bad():
-            pass
 #: Okay
 def setUp():
     pass

testsuite/N807.py

+#: N807
+def __bad():
+    pass
+#: N807
+def bad__():
+    pass
+#: N807
+def __bad__():
+    pass
+#: N807
+class ClassName(object):
+    def method(self):
+        def __bad():
+            pass