Support exception aliases properly in B014 (#129)

* Add tests

* Support exception aliases properly

A good example is OSError. Since Python 3.3, IOError, EnvironmentError,
select.error, etc merged into OSError and became aliases. Before this
change, both the aliases and the primary exceptions were removed from
the message.

* Functionalize

bugbear.py

@@ -174,13 +174,22 @@ def visit_ExceptHandler(self, node):
                 #    (MyError, MyError)  # duplicate names
                 #    (MyError, BaseException)  # everything derives from the Base
                 #    (Exception, TypeError)  # builtins where one subclasses another
+                #    (IOError, OSError)  # IOError is an alias of OSError since Python3.3
                 # but note that other cases are impractical to hande from the AST.
                 # We expect this is mostly useful for users who do not have the
                 # builtin exception hierarchy memorised, and include a 'shadowed'
                 # subtype without realising that it's redundant.
                 good = sorted(set(names), key=names.index)
                 if "BaseException" in good:
                     good = ["BaseException"]
+                # Find and remove aliases exceptions and only leave the primary alone
+                primaries = filter(
+                    lambda primary: primary in good, B014.exception_aliases.keys()
+                )
+                for primary in primaries:
+                    aliases = B014.exception_aliases[primary]
+                    good = list(filter(lambda e: e not in aliases, good))
+
                 for name, other in itertools.permutations(tuple(good), 2):
                     if issubclass(
                         getattr(builtins, name, type), getattr(builtins, other, ())
@@ -639,6 +648,16 @@ def visit(self, node):
         "Write `except {2}{1}:`, which catches exactly the same exceptions."
     )
 )
+B014.exception_aliases = {
+    "OSError": {
+        "IOError",
+        "EnvironmentError",
+        "WindowsError",
+        "mmap.error",
+        "socket.error",
+        "select.error",
+    }
+}
 
 # Those could be false positives but it's more dangerous to let them slip
 # through if they're not.

tests/b014.py

@@ -1,6 +1,6 @@
 """
 Should emit:
-B014 - on lines 10, 16, 27, 41, and 48
+B014 - on lines 10, 16, 27, 41, 48, and 55
 """
 
 import re
@@ -48,3 +48,14 @@ class MyError(Exception):
 except (re.error, re.error):
     # Duplicate exception types as attributes
     pass
+
+
+try:
+    pass
+except (IOError, EnvironmentError, OSError):
+    # Detect if a primary exception and any its aliases are present.
+    #
+    # Since Python 3.3, IOError, EnvironmentError, WindowsError, mmap.error,
+    # socket.error and select.error are aliases of OSError. See PEP 3151 for
+    # more info.
+    pass

tests/test_bugbear.py

@@ -178,6 +178,7 @@ def test_b014(self):
             B014(27, 0, vars=("MyError, MyError", "", "MyError")),
             B014(41, 0, vars=("MyError, BaseException", " as e", "BaseException")),
             B014(48, 0, vars=("re.error, re.error", "", "re.error")),
+            B014(55, 0, vars=("IOError, EnvironmentError, OSError", "", "OSError"),),
         )
         self.assertEqual(errors, expected)