Setup mypy in tox -e typing and get it to pass

This is the smallest possible change to get mypy passing on the
jsonschema codebase. The goal of this configuration is to enforce type
annotations anywhere that they appear.
That is, if a method is added to the codebase,

    def foo(x: int) -> str:
        return str(x)

then usages of `foo` will by type checked. If no annotations are
added, `mypy` will not type check functions.

For the most part, this keeps the impact low. The one exceptional case
is the use of `pyrsistent.pmap` as an argument to
`attr.ib(converter=...)`. Unfortunately, it causes `mypy` to
incorrectly deduce the type of the init parameter created by attrs. We
need to "explain the type of init" to mypy by creating a callable with
a concrete type to act as the converter. The callable in question
simply wraps `pmap` with a cast and presents the desired type
information to mypy.

Author: sirosen

.github/workflows/ci.yml

@@ -102,6 +102,8 @@ jobs:
             toxenv: secrets
           - name: 3.9
             toxenv: style
+          - name: 3.9
+            toxenv: typing
         exclude:
           - os: windows-latest
             python-version:

jsonschema/_format.py

@@ -3,9 +3,13 @@
 import datetime
 import ipaddress
 import re
+import typing
 
 from jsonschema.exceptions import FormatError
 
+_FormatCheckerFunc = typing.Callable[[typing.Any], bool]
+_CheckerRaises = typing.Union[Exception, typing.Tuple[Exception, ...]]
+
 
 class FormatChecker(object):
     """
@@ -30,7 +34,10 @@ class FormatChecker(object):
             limit which formats will be used during validation.
     """
 
-    checkers = {}
+    checkers: typing.Dict[
+        str,
+        typing.Tuple[_FormatCheckerFunc, _CheckerRaises],
+    ] = {}
 
     def __init__(self, formats=None):
         if formats is None:

jsonschema/_types.py

@@ -1,10 +1,25 @@
 import numbers
+import typing
 
 from pyrsistent import pmap
 import attr
 
 from jsonschema.exceptions import UndefinedTypeCheck
 
+# internal type declarations for annotations
+_TypeCheckerFunc = typing.Callable[["TypeChecker", typing.Any], bool]
+_TypeCheckerMapping = typing.Mapping[str, _TypeCheckerFunc]
+
+
+# unfortunately, the type of pmap is generic, and if used as the attr.ib
+# converter, the generic type is presented to mypy, which then fails to match
+# the concrete type of a type checker mapping
+# this "do nothing" wrapper presents the correct information to mypy
+def _typed_pmap_converter(
+    init_val: _TypeCheckerMapping,
+) -> _TypeCheckerMapping:
+    return typing.cast(_TypeCheckerMapping, pmap(init_val))
+
 
 def is_array(checker, instance):
     return isinstance(instance, list)
@@ -60,7 +75,10 @@ class TypeChecker(object):
 
             The initial mapping of types to their checking functions.
     """
-    _type_checkers = attr.ib(default=pmap(), converter=pmap)
+
+    _type_checkers: _TypeCheckerMapping = attr.ib(
+        default=pmap(), converter=_typed_pmap_converter,
+    )
 
     def is_type(self, instance, type):
         """

jsonschema/_utils.py

@@ -9,7 +9,7 @@
 if sys.version_info >= (3, 9):  # pragma: no cover
     from importlib import resources
 else:  # pragma: no cover
-    import importlib_resources as resources
+    import importlib_resources as resources  # type: ignore
 
 
 class URIDict(MutableMapping):

jsonschema/cli.py

@@ -12,7 +12,7 @@
 try:
     from importlib import metadata
 except ImportError:
-    import importlib_metadata as metadata
+    import importlib_metadata as metadata  # type: ignore
 
 import attr
 

jsonschema/exceptions.py

@@ -5,13 +5,14 @@
 from pprint import pformat
 from textwrap import dedent, indent
 import itertools
+import typing
 
 import attr
 
 from jsonschema import _utils
 
-WEAK_MATCHES = frozenset(["anyOf", "oneOf"])
-STRONG_MATCHES = frozenset()
+WEAK_MATCHES: typing.FrozenSet[str] = frozenset(["anyOf", "oneOf"])
+STRONG_MATCHES: typing.FrozenSet[str] = frozenset()
 
 _unset = _utils.Unset()
 

jsonschema/tests/test_cli.py

@@ -13,7 +13,7 @@
 try:  # pragma: no cover
     from importlib import metadata
 except ImportError:  # pragma: no cover
-    import importlib_metadata as metadata
+    import importlib_metadata as metadata  # type: ignore
 
 from pyrsistent import m
 

jsonschema/tests/test_validators.py

@@ -8,6 +8,7 @@
 import os
 import sys
 import tempfile
+import typing
 import unittest
 import warnings
 
@@ -1662,7 +1663,7 @@ def test_False_is_not_a_schema_even_if_you_forget_to_check(self):
 
 class TestDraft3Validator(AntiDraft6LeakMixin, ValidatorTestMixin, TestCase):
     Validator = validators.Draft3Validator
-    valid = {}, {}
+    valid: typing.Tuple[dict, dict] = {}, {}
     invalid = {"type": "integer"}, "foo"
 
     def test_any_type_is_valid_for_type_any(self):
@@ -1694,31 +1695,31 @@ def test_is_type_does_not_evade_bool_if_it_is_being_tested(self):
 
 class TestDraft4Validator(AntiDraft6LeakMixin, ValidatorTestMixin, TestCase):
     Validator = validators.Draft4Validator
-    valid = {}, {}
+    valid: typing.Tuple[dict, dict] = {}, {}
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft6Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft6Validator
-    valid = {}, {}
+    valid: typing.Tuple[dict, dict] = {}, {}
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft7Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft7Validator
-    valid = {}, {}
+    valid: typing.Tuple[dict, dict] = {}, {}
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft201909Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft201909Validator
-    valid = {}, {}
+    valid: typing.Tuple[dict, dict] = {}, {}
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft202012Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft202012Validator
-    valid = {}, {}
+    valid: typing.Tuple[dict, dict] = {}, {}
     invalid = {"type": "integer"}, "foo"
 
 

jsonschema/validators.py

@@ -10,6 +10,7 @@
 import contextlib
 import json
 import reprlib
+import typing
 import warnings
 
 import attr
@@ -22,9 +23,9 @@
     exceptions,
 )
 
-_VALIDATORS = {}
+_VALIDATORS: typing.Dict[str, typing.Any] = {}
 _META_SCHEMAS = _utils.URIDict()
-_VOCABULARIES = []
+_VOCABULARIES: typing.List[typing.Tuple[str, typing.Any]] = []
 
 
 def __getattr__(name):

setup.cfg

@@ -72,6 +72,9 @@ ignore =
     B306,  # See https://github.com/PyCQA/flake8-bugbear/issues/131
     W503,  # (flake8 default) old PEP8 boolean operator line breaks
 
+[mypy]
+ignore_missing_imports = true
+
 [pydocstyle]
 match = (?!(test_|_|compat|cli)).*\.py  # see PyCQA/pydocstyle#323
 add-select =