Implement __cause__ for Py3.

Closes: #93

Author: Julian

jsonschema.py

@@ -76,7 +76,7 @@ def __init__(
         self.path = collections.deque(path)
         self.schema_path = collections.deque(schema_path)
         self.context = list(context)
-        self.cause = cause
+        self.cause = self.__cause__ = cause
         self.validator = validator
         self.validator_value = validator_value
         self.instance = instance
@@ -118,7 +118,7 @@ class FormatError(Exception):
     def __init__(self, message, cause=None):
         super(FormatError, self).__init__(message, cause)
         self.message = message
-        self.cause = cause
+        self.cause = self.__cause__ = cause
 
     def __str__(self):
         return self.message.encode("utf-8")
@@ -423,8 +423,8 @@ def validate_format(self, format, instance, schema):
         ):
             try:
                 self.format_checker.check(instance, format)
-            except FormatError as e:
-                yield ValidationError(unicode(e), cause=e.cause)
+            except FormatError as error:
+                yield ValidationError(error.message, cause=error.cause)
 
     def validate_minLength(self, mL, instance, schema):
         if self.is_type(instance, "string") and len(instance) < mL:
@@ -944,7 +944,8 @@ def check(self, instance, format):
                 cause = e
             if not result:
                 raise FormatError(
-                    "%r is not a %r" % (instance, format), cause=cause)
+                    "%r is not a %r" % (instance, format), cause=cause,
+                )
 
     def conforms(self, instance, format):
         """

tests.py

@@ -860,19 +860,31 @@ def test_it_can_register_checkers(self):
 
     def test_it_catches_registered_errors(self):
         checker = FormatChecker()
+        cause = self.fn.side_effect = ValueError()
+
         checker.checks("foo", raises=ValueError)(self.fn)
-        # Registered errors should be caught and turned into FormatErrors
-        cause = ValueError()
-        self.fn.side_effect = cause
+
         with self.assertRaises(FormatError) as cm:
             checker.check("bar", "foo")
-        # Original exception should be attached to cause attribute
+
         self.assertIs(cm.exception.cause, cause)
+        self.assertIs(cm.exception.__cause__, cause)
+
         # Unregistered errors should not be caught
         self.fn.side_effect = AttributeError
         with self.assertRaises(AttributeError):
             checker.check("bar", "foo")
 
+    def test_format_error_causes_become_validation_error_causes(self):
+        checker = FormatChecker()
+        checker.checks("foo", raises=ValueError)(self.fn)
+        cause = self.fn.side_effect = ValueError()
+        validator = Draft4Validator({"format" : "foo"}, format_checker=checker)
+
+        with self.assertRaises(ValidationError) as cm:
+            validator.validate("bar")
+
+        self.assertIs(cm.exception.__cause__, cause)
 
 
 def sorted_errors(errors):