diff --git a/openapi_core/schema/content/exceptions.py b/openapi_core/schema/content/exceptions.py
index 4473ef0..7711ea0 100644
--- a/openapi_core/schema/content/exceptions.py
+++ b/openapi_core/schema/content/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIContentError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class MimeTypeNotFound(OpenAPIContentError):
     mimetype = attr.ib()
     availableMimetypes = attr.ib()
diff --git a/openapi_core/schema/content/models.py b/openapi_core/schema/content/models.py
index 1cabec1..4af6ed8 100644
--- a/openapi_core/schema/content/models.py
+++ b/openapi_core/schema/content/models.py
@@ -18,4 +18,4 @@ class Content(dict):
             if fnmatch.fnmatch(mimetype, key):
                 return value
 
-        raise MimeTypeNotFound(mimetype, self.keys())
+        raise MimeTypeNotFound(mimetype, list(self.keys()))
diff --git a/openapi_core/schema/media_types/exceptions.py b/openapi_core/schema/media_types/exceptions.py
index 97a0d85..60a0c20 100644
--- a/openapi_core/schema/media_types/exceptions.py
+++ b/openapi_core/schema/media_types/exceptions.py
@@ -1,19 +1,21 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIMediaTypeError(OpenAPIMappingError):
     pass
 
-@attr.s
+
+@attr.s(hash=True)
 class InvalidMediaTypeValue(OpenAPIMediaTypeError):
     original_exception = attr.ib()
 
     def __str__(self):
         return "Mimetype invalid: {0}".format(self.original_exception)
 
-@attr.s
+
+@attr.s(hash=True)
 class InvalidContentType(OpenAPIMediaTypeError):
     mimetype = attr.ib()
 
diff --git a/openapi_core/schema/operations/exceptions.py b/openapi_core/schema/operations/exceptions.py
index 30118b6..e78708a 100644
--- a/openapi_core/schema/operations/exceptions.py
+++ b/openapi_core/schema/operations/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIOperationError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class InvalidOperation(OpenAPIOperationError):
     path_pattern = attr.ib()
     http_method = attr.ib()
diff --git a/openapi_core/schema/parameters/exceptions.py b/openapi_core/schema/parameters/exceptions.py
index 853d067..bcaf13f 100644
--- a/openapi_core/schema/parameters/exceptions.py
+++ b/openapi_core/schema/parameters/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIParameterError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class MissingParameter(OpenAPIParameterError):
     name = attr.ib()
 
@@ -15,7 +15,7 @@ class MissingParameter(OpenAPIParameterError):
         return "Missing parameter (without default value): {0}".format(self.name)
 
 
-@attr.s
+@attr.s(hash=True)
 class MissingRequiredParameter(OpenAPIParameterError):
     name = attr.ib()
 
@@ -23,7 +23,7 @@ class MissingRequiredParameter(OpenAPIParameterError):
         return "Missing required parameter: {0}".format(self.name)
 
 
-@attr.s
+@attr.s(hash=True)
 class EmptyParameterValue(OpenAPIParameterError):
     name = attr.ib()
 
@@ -31,7 +31,7 @@ class EmptyParameterValue(OpenAPIParameterError):
         return "Value of parameter cannot be empty: {0}".format(self.name)
 
 
-@attr.s
+@attr.s(hash=True)
 class InvalidParameterValue(OpenAPIParameterError):
     name = attr.ib()
     original_exception = attr.ib()
diff --git a/openapi_core/schema/request_bodies/exceptions.py b/openapi_core/schema/request_bodies/exceptions.py
index 3b0e618..a84937b 100644
--- a/openapi_core/schema/request_bodies/exceptions.py
+++ b/openapi_core/schema/request_bodies/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIRequestBodyError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class MissingRequestBody(OpenAPIRequestBodyError):
     request = attr.ib()
 
diff --git a/openapi_core/schema/responses/exceptions.py b/openapi_core/schema/responses/exceptions.py
index 5e196e6..9ec50ee 100644
--- a/openapi_core/schema/responses/exceptions.py
+++ b/openapi_core/schema/responses/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIResponseError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class InvalidResponse(OpenAPIResponseError):
     http_status = attr.ib()
     responses = attr.ib()
@@ -16,7 +16,7 @@ class InvalidResponse(OpenAPIResponseError):
         return "Unknown response http status: {0}".format(str(self.http_status))
 
 
-@attr.s
+@attr.s(hash=True)
 class MissingResponseContent(OpenAPIResponseError):
     response = attr.ib()
 
diff --git a/openapi_core/schema/schemas/exceptions.py b/openapi_core/schema/schemas/exceptions.py
index a370aac..c984578 100644
--- a/openapi_core/schema/schemas/exceptions.py
+++ b/openapi_core/schema/schemas/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPISchemaError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class NoValidSchema(OpenAPISchemaError):
     value = attr.ib()
 
@@ -15,7 +15,7 @@ class NoValidSchema(OpenAPISchemaError):
         return "No valid schema found for value: {0}".format(self.value)
 
 
-@attr.s
+@attr.s(hash=True)
 class UndefinedItemsSchema(OpenAPISchemaError):
     type = attr.ib()
 
@@ -23,7 +23,7 @@ class UndefinedItemsSchema(OpenAPISchemaError):
         return "Null value for schema type {0}".format(self.type)
 
 
-@attr.s
+@attr.s(hash=True)
 class InvalidSchemaValue(OpenAPISchemaError):
     msg = attr.ib()
     value = attr.ib()
@@ -32,7 +32,8 @@ class InvalidSchemaValue(OpenAPISchemaError):
     def __str__(self):
         return self.msg.format(value=self.value, type=self.type)
 
-@attr.s
+
+@attr.s(hash=True)
 class InvalidCustomFormatSchemaValue(InvalidSchemaValue):
     original_exception = attr.ib()
 
@@ -40,14 +41,15 @@ class InvalidCustomFormatSchemaValue(InvalidSchemaValue):
         return self.msg.format(value=self.value, type=self.type, exception=self.original_exception)
 
 
-@attr.s
+@attr.s(hash=True)
 class UndefinedSchemaProperty(OpenAPISchemaError):
     extra_props = attr.ib()
 
     def __str__(self):
         return "Extra unexpected properties found in schema: {0}".format(self.extra_props)
 
-@attr.s
+
+@attr.s(hash=True)
 class InvalidSchemaProperty(OpenAPISchemaError):
     property_name = attr.ib()
     original_exception = attr.ib()
@@ -55,7 +57,8 @@ class InvalidSchemaProperty(OpenAPISchemaError):
     def __str__(self):
         return "Invalid schema property {0}: {1}".format(self.property_name, self.original_exception)
 
-@attr.s
+
+@attr.s(hash=True)
 class MissingSchemaProperty(OpenAPISchemaError):
     property_name = attr.ib()
 
@@ -63,7 +66,7 @@ class MissingSchemaProperty(OpenAPISchemaError):
         return "Missing schema property: {0}".format(self.property_name)
 
 
-@attr.s
+@attr.s(hash=True)
 class NoOneOfSchema(OpenAPISchemaError):
     type = attr.ib()
 
@@ -71,7 +74,7 @@ class NoOneOfSchema(OpenAPISchemaError):
         return "Exactly one valid schema type {0} should be valid, None found.".format(self.type)
 
 
-@attr.s
+@attr.s(hash=True)
 class MultipleOneOfSchema(OpenAPISchemaError):
     type = attr.ib()
 
@@ -79,11 +82,10 @@ class MultipleOneOfSchema(OpenAPISchemaError):
         return "Exactly one schema type {0} should be valid, more than one found".format(self.type)
 
 
-class UnmarshallerError(Exception):
+class UnmarshallerError(OpenAPIMappingError):
     pass
 
 
-@attr.s
 class UnmarshallerStrictTypeError(UnmarshallerError):
     value = attr.ib()
     types = attr.ib()
diff --git a/openapi_core/schema/servers/exceptions.py b/openapi_core/schema/servers/exceptions.py
index 1ae0d3a..b8076e9 100644
--- a/openapi_core/schema/servers/exceptions.py
+++ b/openapi_core/schema/servers/exceptions.py
@@ -1,13 +1,13 @@
-from openapi_core.schema.exceptions import OpenAPIMappingError
-
 import attr
 
+from openapi_core.schema.exceptions import OpenAPIMappingError
+
 
 class OpenAPIServerError(OpenAPIMappingError):
     pass
 
 
-@attr.s
+@attr.s(hash=True)
 class InvalidServer(OpenAPIServerError):
     full_url_pattern = attr.ib()
 
diff --git a/tests/unit/schema/test_links.py b/tests/unit/schema/test_links.py
index df9f17d..346900f 100644
--- a/tests/unit/schema/test_links.py
+++ b/tests/unit/schema/test_links.py
@@ -40,5 +40,5 @@ class TestLinks(object):
     @pytest.mark.parametrize("request_body", request_body_list)
     def test_iteritems(self, link_factory, request_body, server):
         link = link_factory(request_body, server)
-        for par_name in link.parameters.keys():
+        for par_name in link.parameters:
             assert link[par_name] == link.parameters[par_name]
diff --git a/tests/unit/schema/test_operations.py b/tests/unit/schema/test_operations.py
index bca1da4..b84ce75 100644
--- a/tests/unit/schema/test_operations.py
+++ b/tests/unit/schema/test_operations.py
@@ -15,7 +15,7 @@ class TestSchemas(object):
         return Operation('get', '/path', {}, parameters=parameters)
 
     def test_iteritems(self, operation):
-        for name in operation.parameters.keys():
+        for name in operation.parameters:
             assert operation[name] == operation.parameters[name]
 
 
diff --git a/tests/unit/schema/test_paths.py b/tests/unit/schema/test_paths.py
index f7de1d5..bd6c83d 100644
--- a/tests/unit/schema/test_paths.py
+++ b/tests/unit/schema/test_paths.py
@@ -16,6 +16,6 @@ class TestPaths(object):
 
     @property
     def test_iteritems(self, path):
-        for http_method in path.operations.keys():
+        for http_method in path.operations:
             assert path[http_method] ==\
                 path.operations[http_method]
diff --git a/tests/unit/schema/test_request_bodies.py b/tests/unit/schema/test_request_bodies.py
index d47ccd2..cf82eda 100644
--- a/tests/unit/schema/test_request_bodies.py
+++ b/tests/unit/schema/test_request_bodies.py
@@ -16,6 +16,6 @@ class TestRequestBodies(object):
 
     @property
     def test_iteritems(self, request_body):
-        for mimetype in request_body.content.keys():
+        for mimetype in request_body.content:
             assert request_body[mimetype] ==\
                 request_body.content[mimetype]
diff --git a/tests/unit/schema/test_schemas.py b/tests/unit/schema/test_schemas.py
index 05dc599..0b747aa 100644
--- a/tests/unit/schema/test_schemas.py
+++ b/tests/unit/schema/test_schemas.py
@@ -27,7 +27,7 @@ class TestSchemaIteritems(object):
 
     @property
     def test_valid(self, schema):
-        for name in schema.properties.keys():
+        for name in schema.properties:
             assert schema[name] == schema.properties[name]
 
 
diff --git a/tests/unit/schema/test_specs.py b/tests/unit/schema/test_specs.py
index cede1ca..c8a0f0d 100644
--- a/tests/unit/schema/test_specs.py
+++ b/tests/unit/schema/test_specs.py
@@ -32,7 +32,7 @@ class TestSpecs(object):
         return Spec(servers, paths)
 
     def test_iteritems(self, spec):
-        for path_name in spec.paths.keys():
+        for path_name in spec.paths:
             assert spec[path_name] ==\
                 spec.paths[path_name]