Added validator_for to provide direct access to validators

4 files changed, 34 insertions, 5 deletions

diff --git a/docs/creating.rst b/docs/creating.rst
index 03b07c5..e18226f 100644
--- a/docs/creating.rst
+++ b/docs/creating.rst
@@ -45,6 +45,8 @@ Creating or Extending Validators
     :returns: an :class:`jsonschema.IValidator`
 
 
+.. autofunction:: validate_for
+
 .. autofunction:: validates
 
 
diff --git a/jsonschema/__init__.py b/jsonschema/__init__.py
index 4514d8f..9ace769 100644
--- a/jsonschema/__init__.py
+++ b/jsonschema/__init__.py
@@ -17,7 +17,7 @@ from jsonschema._format import (
 )
 from jsonschema.validators import (
     ErrorTree, Draft3Validator, Draft4Validator, RefResolver, ValidatorMixin,
-    validate, validates,
+    validator_for, validate, validates,
 )
 
 
diff --git a/jsonschema/tests/test_validators.py b/jsonschema/tests/test_validators.py
index 5162f6f..ba7da70 100644
--- a/jsonschema/tests/test_validators.py
+++ b/jsonschema/tests/test_validators.py
@@ -8,8 +8,8 @@ from jsonschema import FormatChecker, ValidationError
 from jsonschema.compat import PY3
 from jsonschema.tests.compat import mock, unittest
 from jsonschema.validators import (
-    RefResolutionError, UnknownType, ErrorTree, Draft3Validator,
-    Draft4Validator, RefResolver, ValidatorMixin, create, extend, validate,
+    _unset, RefResolutionError, UnknownType, ErrorTree, Draft3Validator,
+    Draft4Validator, RefResolver, ValidatorMixin, create, extend, validator_for, validate,
 )
 
 
@@ -624,6 +624,25 @@ class TestDraft4Validator(ValidatorTestMixin, unittest.TestCase):
     validator_class = Draft4Validator
 
 
+class TestValidatorFor(unittest.TestCase):
+    def do_it(self, schema_url, expected_draft):
+        schema = {"$schema" : schema_url}
+        with mock.patch.object(expected_draft, "check_schema") as chk_schema:
+            validator_for(schema)
+            chk_schema.assert_called_once_with(schema)
+
+    def test_validator_for(self):
+        self.do_it("http://json-schema.org/draft-03/schema#", Draft3Validator)
+        self.do_it("http://json-schema.org/draft-03/schema", Draft3Validator)
+        self.do_it("http://json-schema.org/draft-04/schema#", Draft4Validator)
+        self.do_it("http://json-schema.org/draft-04/schema", Draft4Validator)
+
+    def test_validator_unset(self):
+        schema = {}
+        validator = validator_for(schema)
+        self.assertEqual(validator, _unset)
+
+
 class TestValidate(unittest.TestCase):
     def test_draft3_validator_is_chosen(self):
         schema = {"$schema" : "http://json-schema.org/draft-03/schema#"}
diff --git a/jsonschema/validators.py b/jsonschema/validators.py
index c4424b4..9a68440 100644
--- a/jsonschema/validators.py
+++ b/jsonschema/validators.py
@@ -473,8 +473,16 @@ class ErrorTree(object):
         return len(self.errors) + child_errors
 
 
+def validator_for(schema, default=_unset):
+    cls = meta_schemas.get(schema.get("$schema", ""), default)
+    if not cls is _unset:
+        cls.check_schema(schema)
+    return cls
+
+
 def validate(instance, schema, cls=None, *args, **kwargs):
     if cls is None:
-        cls = meta_schemas.get(schema.get("$schema", ""), Draft4Validator)
-    cls.check_schema(schema)
+        cls = validator_for(schema)
+    else:
+        cls.check_schema(schema)
     cls(schema, *args, **kwargs).validate(instance)