diff --git a/jsonschema/_format.py b/jsonschema/_format.py
index bd2f95d..caae127 100644
--- a/jsonschema/_format.py
+++ b/jsonschema/_format.py
@@ -156,6 +156,7 @@ def is_email(instance):
 
 _ipv4_re = re.compile(r"^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$")
 
+
 @_checks_drafts(draft3="ip-address", draft4="ipv4")
 def is_ipv4(instance):
     if not isinstance(instance, str_types):
@@ -175,6 +176,7 @@ if hasattr(socket, "inet_pton"):
 
 _host_name_re = re.compile(r"^[A-Za-z0-9][A-Za-z0-9\.\-]{1,255}$")
 
+
 @_checks_drafts(draft3="host-name", draft4="hostname")
 def is_host_name(instance):
     if not isinstance(instance, str_types):
@@ -209,13 +211,13 @@ except ImportError:
         pass
     else:
         @_checks_drafts("date-time", raises=(ValueError, isodate.ISO8601Error))
-        def is_date(instance):
+        def is_datetime(instance):
             if not isinstance(instance, str_types):
                 return True
             return isodate.parse_datetime(instance)
 else:
     @_checks_drafts("date-time")
-    def is_date(instance):
+    def is_datetime(instance):
         if not isinstance(instance, str_types):
             return True
         return strict_rfc3339.validate_rfc3339(instance)
@@ -250,7 +252,6 @@ else:
     def is_css_color_code(instance):
         return webcolors.normalize_hex(instance)
 
-
     @_checks_drafts(draft3="color", raises=(ValueError, TypeError))
     def is_css21_color(instance):
         if (
@@ -260,7 +261,6 @@ else:
             return True
         return is_css_color_code(instance)
 
-
     def is_css3_color(instance):
         if instance.lower() in webcolors.css3_names_to_hex:
             return True
diff --git a/jsonschema/_validators.py b/jsonschema/_validators.py
index 40fe340..cfdf66a 100644
--- a/jsonschema/_validators.py
+++ b/jsonschema/_validators.py
@@ -255,7 +255,7 @@ def properties_draft3(validator, properties, instance, schema):
 
 def disallow_draft3(validator, disallow, instance, schema):
     for disallowed in _utils.ensure_list(disallow):
-        if validator.is_valid(instance, {"type" : [disallowed]}):
+        if validator.is_valid(instance, {"type": [disallowed]}):
             yield ValidationError(
                 "%r is disallowed for %r" % (disallowed, instance)
             )
diff --git a/jsonschema/cli.py b/jsonschema/cli.py
index 6975264..fb1b0f5 100644
--- a/jsonschema/cli.py
+++ b/jsonschema/cli.py
@@ -26,22 +26,28 @@ parser.add_argument(
     action="append",
     dest="instances",
     type=_json_file,
-    help="a path to a JSON instance (i.e. filename.json)"
-         "to validate (may be specified multiple times)",
+    help=(
+        "a path to a JSON instance (i.e. filename.json)"
+        "to validate (may be specified multiple times)"
+    ),
 )
 parser.add_argument(
     "-F", "--error-format",
     default="{error.instance}: {error.message}\n",
-    help="the format to use for each error output message, specified in "
-         "a form suitable for passing to str.format, which will be called "
-         "with 'error' for each error",
+    help=(
+        "the format to use for each error output message, specified in "
+        "a form suitable for passing to str.format, which will be called "
+        "with 'error' for each error"
+    ),
 )
 parser.add_argument(
     "-V", "--validator",
     type=_namedAnyWithDefault,
-    help="the fully qualified object name of a validator to use, or, for "
-         "validators that are registered with jsonschema, simply the name "
-         "of the class.",
+    help=(
+        "the fully qualified object name of a validator to use, or, for "
+        "validators that are registered with jsonschema, simply the name "
+        "of the class."
+    ),
 )
 parser.add_argument(
     "schema",
diff --git a/jsonschema/exceptions.py b/jsonschema/exceptions.py
index 829034c..da3e852 100644
--- a/jsonschema/exceptions.py
+++ b/jsonschema/exceptions.py
@@ -56,9 +56,6 @@ class _Error(Exception):
     def __repr__(self):
         return "<%s: %r>" % (self.__class__.__name__, self.message)
 
-    def __str__(self):
-        return unicode(self).encode("utf-8")
-
     def __unicode__(self):
         essential_for_verbose = (
             self.validator, self.validator_value, self.instance, self.schema,
@@ -86,6 +83,9 @@ class _Error(Exception):
 
     if PY3:
         __str__ = __unicode__
+    else:
+        def __str__(self):
+            return unicode(self).encode("utf-8")
 
     @classmethod
     def create_from(cls, other):
@@ -142,9 +142,6 @@ class UnknownType(Exception):
         self.instance = instance
         self.schema = schema
 
-    def __str__(self):
-        return unicode(self).encode("utf-8")
-
     def __unicode__(self):
         pschema = pprint.pformat(self.schema, width=72)
         pinstance = pprint.pformat(self.instance, width=72)
@@ -159,7 +156,9 @@ class UnknownType(Exception):
 
     if PY3:
         __str__ = __unicode__
-
+    else:
+        def __str__(self):
+            return unicode(self).encode("utf-8")
 
 
 class FormatError(Exception):
@@ -168,14 +167,14 @@ class FormatError(Exception):
         self.message = message
         self.cause = self.__cause__ = cause
 
-    def __str__(self):
-        return self.message.encode("utf-8")
-
     def __unicode__(self):
         return self.message
 
     if PY3:
         __str__ = __unicode__
+    else:
+        def __str__(self):
+            return self.message.encode("utf-8")
 
 
 class ErrorTree(object):
diff --git a/jsonschema/tests/test_exceptions.py b/jsonschema/tests/test_exceptions.py
index f277af8..1d7a6c6 100644
--- a/jsonschema/tests/test_exceptions.py
+++ b/jsonschema/tests/test_exceptions.py
@@ -10,11 +10,9 @@ class TestBestMatch(unittest.TestCase):
         errors = list(errors)
         best = exceptions.best_match(errors)
         reversed_best = exceptions.best_match(reversed(errors))
+        msg = "Didn't return a consistent best match!\nGot: {0}\n\nThen: {1}"
         self.assertEqual(
-            best,
-            reversed_best,
-            msg="Didn't return a consistent best match!\n"
-                "Got: {0}\n\nThen: {1}".format(best, reversed_best),
+            best, reversed_best, msg=msg.format(best, reversed_best),
         )
         return best
 
diff --git a/jsonschema/validators.py b/jsonschema/validators.py
index db76845..ee1ec94 100644
--- a/jsonschema/validators.py
+++ b/jsonschema/validators.py
@@ -14,7 +14,7 @@ from jsonschema.compat import (
     Sequence, urljoin, urlsplit, urldefrag, unquote, urlopen,
     str_types, int_types, iteritems, lru_cache,
 )
-from jsonschema.exceptions import ErrorTree  # Backwards compatibility  # noqa
+from jsonschema.exceptions import ErrorTree  # Backwards compat  # noqa: F401
 from jsonschema.exceptions import RefResolutionError, SchemaError, UnknownType
 
 
@@ -51,7 +51,7 @@ def validates(version):
     return _validates
 
 
-def create(meta_schema, validators=(), version=None, default_types=None):  # noqa
+def create(meta_schema, validators=(), version=None, default_types=None):  # noqa: C901, E501
     if default_types is None:
         default_types = {
             u"array": list, u"boolean": bool, u"integer": int_types,
@@ -329,7 +329,7 @@ class RefResolver(object):
             raise RefResolutionError(
                 "Failed to pop the scope from an empty stack. "
                 "`pop_scope()` should only be called once for every "
-                "`push_scope()`",
+                "`push_scope()`"
             )
 
     @property
diff --git a/tox.ini b/tox.ini
index fcc0954..b131119 100644
--- a/tox.ini
+++ b/tox.ini
@@ -60,4 +60,4 @@ commands =
 
 
 [flake8]
-ignore = E203,E302,E303,E701,F811
+exclude = jsonschema/_reflect.py