Set N818 to check error suffix on exception names.

README.rst

@@ -70,6 +70,8 @@ These error codes are emitted:
 +------+-------------------------------------------------------+
 | N817 | camelcase imported as acronym                         |
 +------+-------------------------------------------------------+
+| N818 | error suffix in exception names                       |
++------+-------------------------------------------------------+
 
 Options
 -------

run_tests.py

@@ -75,6 +75,10 @@ class OptionsManager(optparse.OptionParser):
     def __init__(self, *args, **kwargs):
         optparse.OptionParser.__init__(self, *args, **kwargs)
         self.config_options = []
+        self.ignore = []
+
+    def extend_default_ignore(self, new_ignores):
+        self.ignore += new_ignores
 
 
 def parse_options(checker, options):
@@ -85,6 +89,7 @@ def parse_options(checker, options):
     options_manager.add_option('--ignore', default=[])
     options_manager.add_option('--extend-ignore', default=[])
     options_manager.add_option('--enable-extensions', default=[])
+    options_manager.add_option('--extended-default-ignore', default=[])
     checker.add_options(options_manager)
     processed_options, _ = options_manager.parse_args(options)
     checker.parse_options(processed_options)

setup.py

@@ -44,7 +44,7 @@ def run_tests(self):
     license='Expat license',
     package_dir={'': 'src'},
     py_modules=['pep8ext_naming'],
-    install_requires=['flake8_polyfill>=1.0.2,<2'],
+    install_requires=['flake8>=3.9.1', 'flake8_polyfill>=1.0.2,<2'],
     zip_safe=False,
     entry_points={
         'flake8.extension': [

src/pep8ext_naming.py

@@ -167,6 +167,7 @@ def add_options(cls, parser):
                          help='List of method decorators pep8-naming plugin '
                               'should consider staticmethods (Defaults to '
                               '%default)')
+        parser.extend_default_ignore(['N818'])
 
     @classmethod
     def parse_options(cls, options):
@@ -291,6 +292,25 @@ class ClassNameCheck(BaseASTCheck):
     Classes for internal use have a leading underscore in addition.
     """
     N801 = "class name '{name}' should use CapWords convention"
+    N818 = "exception name '{name}' should be named with an Error suffix"
+
+    def get_class_def(self, name, parents):
+        for parent in parents:
+            for definition in parent.body:
+                is_class_definition = isinstance(definition, ast.ClassDef)
+                if is_class_definition and definition.name == name:
+                    return definition
+
+    def superclass_names(self, name, parents):
+        class_ids = set()
+        class_def = self.get_class_def(name, parents)
+        if not class_def:
+            return class_ids
+        for base in class_def.bases:
+            if hasattr(base, "id"):
+                class_ids.add(base.id)
+                class_ids.update(self.superclass_names(base.id, parents))
+        return class_ids
 
     def visit_classdef(self, node, parents, ignore=None):
         name = node.name
@@ -299,6 +319,9 @@ def visit_classdef(self, node, parents, ignore=None):
         name = name.strip('_')
         if not name[:1].isupper() or '_' in name:
             yield self.err(node, 'N801', name=name)
+        superclasses = self.superclass_names(name, parents)
+        if "Exception" in superclasses and not name.endswith("Error"):
+            yield self.err(node, 'N818', name=name)
 
 
 class FunctionNameCheck(BaseASTCheck):

testsuite/N818.py

@@ -0,0 +1,30 @@
+#: Okay
+class ActionError(Exception):
+    pass
+#: N818
+class ActionClass(Exception):
+    pass
+#: Okay
+class ActionError(Exception):
+    pass
+class DeepActionError(ActionError):
+    pass
+#: N818
+class ActionError(Exception):
+    pass
+class DeepActionClass(ActionError):
+    pass
+#: Okay
+class MixinError(Exception):
+    pass
+class Mixin:
+    pass
+class MixinActionError(Mixin, MixinError):
+    pass
+#: N818
+class MixinError(Exception):
+    pass
+class Mixin:
+    pass
+class MixinActionClass(Mixin, MixinError):
+    pass

tox.ini

@@ -6,15 +6,15 @@ commands = python run_tests.py
 
 [testenv:py27-flake8]
 deps =
-    flake8
+    flake8 >= 3.9.1
 commands =
     flake8 {posargs} src/pep8ext_naming.py
     python setup.py check --restructuredtext
 
 [testenv:py38-flake8]
 basepython = python3.8
 deps =
-    flake8
+    flake8 >= 3.9.1
 commands =
     flake8 {posargs} src/pep8ext_naming.py
     python setup.py check --restructuredtext