Add a typing test for the Validator protocol

The typing tests are a new subdirectory of `jsonschema/tests/` which
are checked under a more stringent mypy configuration
(`--warn-unused-ignores`) in order to allow certain kinds of negative
tests against the declared types. [^1]

The first new test confirms that each validator matches the Validator
protocol, and furthermore that this is not vacuously true by way of
`Any`.

The test failed at first, as the return type of `create()` was not
annotated, and therefore under the declared types in `jsonschema`, all
of the validators were of type `Any`. To resolve, the return type of
`create` is now annotated as `type[Validator]`.

[^1]: Technically, the new nox configuration checks these files twice,
but only the second check, with `--warn-unused-ignores`, is doing all
of the necessary work.

Author: sirosen

jsonschema/tests/typing/__init__.py

@@ -0,0 +1,38 @@
+"""
+This module acts as a test that type checkers will allow each validator
+class to be assigned to a variable of type `type[Validator]`
+
+The assignation is only valid if type checkers recognize each Validator
+implementation as a valid implementer of the protocol.
+"""
+from jsonschema.protocols import Validator
+from jsonschema.validators import (
+    Draft3Validator,
+    Draft4Validator,
+    Draft6Validator,
+    Draft7Validator,
+    Draft201909Validator,
+    Draft202012Validator,
+)
+
+my_validator: type[Validator]
+
+my_validator = Draft3Validator
+my_validator = Draft4Validator
+my_validator = Draft6Validator
+my_validator = Draft7Validator
+my_validator = Draft201909Validator
+my_validator = Draft202012Validator
+
+
+# in order to confirm that none of the above were incorrectly typed as 'Any'
+# ensure that each of these assignments to a non-validator variable requires an
+# ignore
+none_var: None
+
+none_var = Draft3Validator  # type: ignore[assignment]
+none_var = Draft4Validator  # type: ignore[assignment]
+none_var = Draft6Validator  # type: ignore[assignment]
+none_var = Draft7Validator  # type: ignore[assignment]
+none_var = Draft201909Validator  # type: ignore[assignment]
+none_var = Draft202012Validator  # type: ignore[assignment]

jsonschema/tests/typing/test_all_concrete_validators_match_protocol.py

@@ -147,7 +147,7 @@ def create(
     applicable_validators: _typing.ApplicableValidators = methodcaller(
         "items",
     ),
-):
+) -> type[Validator]:
     """
     Create a new validator class.
 
@@ -511,7 +511,7 @@ def is_valid(self, instance, _schema=None):
         Validator.__name__ = Validator.__qualname__ = f"{safe}Validator"
         Validator = validates(version)(Validator)  # type: ignore[misc]
 
-    return Validator
+    return Validator  # type: ignore[return-value]
 
 
 def extend(

jsonschema/validators.py

@@ -6,6 +6,7 @@
 
 ROOT = Path(__file__).parent
 PACKAGE = ROOT / "jsonschema"
+TYPING_TESTS= ROOT / "jsonschema" / "tests" / "typing"
 BENCHMARKS = PACKAGE / "benchmarks"
 PYPROJECT = ROOT / "pyproject.toml"
 CHANGELOG = ROOT / "CHANGELOG.rst"
@@ -181,6 +182,9 @@ def typing(session):
     """
     session.install("mypy", "types-requests", ROOT)
     session.run("mypy", "--config", PYPROJECT, PACKAGE)
+    session.run(
+        "mypy", "--config", PYPROJECT, "--warn-unused-ignores", TYPING_TESTS
+    )
 
 
 @session(tags=["docs"])