Add Validator.evolve, deprecating passing _schema to methods.

A Validator should be thought of as encapsulating validation with a
single fixed schema.

Previously, iter_errors and is_valid allowed passing a second argument,
which was a different schema to use for one method call.

This was mostly for convenience, since the second argument is often
used during sub-validation whilst say, recursing.

The correct way to do so now is to say:

    validator.evolve(schema=new_schema).iter_errors(...)
    validator.evolve(schema=new_schema).is_valid(...)

instead, which is essentially equally convenient.

Closes: #522

1 files changed, 35 insertions, 1 deletions

diff --git a/jsonschema/tests/test_deprecations.py b/jsonschema/tests/test_deprecations.py
index 0ddc375..c1f4c21 100644
--- a/jsonschema/tests/test_deprecations.py
+++ b/jsonschema/tests/test_deprecations.py
@@ -1,6 +1,6 @@
 from unittest import TestCase
 
-from jsonschema.validators import RefResolver
+from jsonschema.validators import Draft7Validator, RefResolver
 
 
 class TestDeprecations(TestCase):
@@ -48,3 +48,37 @@ class TestDeprecations(TestCase):
                 "jsonschema.RefResolver.in_scope is deprecated ",
             ),
         )
+
+    def test_Validator_is_valid_two_arguments(self):
+        """
+        As of v4.0.0, calling is_valid with two arguments (to provide a
+        different schema) is deprecated.
+        """
+
+        validator = Draft7Validator({})
+        with self.assertWarns(DeprecationWarning) as w:
+            result = validator.is_valid("foo", {"type": "number"})
+
+        self.assertFalse(result)
+        self.assertTrue(
+            str(w.warning).startswith(
+                "Passing a schema to Validator.is_valid is deprecated ",
+            ),
+        )
+
+    def test_Validator_iter_errors_two_arguments(self):
+        """
+        As of v4.0.0, calling iter_errors with two arguments (to provide a
+        different schema) is deprecated.
+        """
+
+        validator = Draft7Validator({})
+        with self.assertWarns(DeprecationWarning) as w:
+            error, = validator.iter_errors("foo", {"type": "number"})
+
+        self.assertEqual(error.validator, "type")
+        self.assertTrue(
+            str(w.warning).startswith(
+                "Passing a schema to Validator.iter_errors is deprecated ",
+            ),
+        )