Add check for unnecessary-default-type-args

doc/data/messages/u/unnecessary-default-type-args/bad.py

@@ -0,0 +1,4 @@
+from collections.abc import AsyncGenerator, Generator
+
+a1: AsyncGenerator[int, None]  # [unnecessary-default-type-args]
+b1: Generator[int, None, None]  # [unnecessary-default-type-args]

doc/data/messages/u/unnecessary-default-type-args/details.rst

@@ -0,0 +1,6 @@
+At the moment, this check only works for ``Generator`` and ``AsyncGenerator``.
+
+Starting with Python 3.13, the ``SendType`` and ``ReturnType`` default to ``None``.
+As such it's no longer necessary to specify them. The ``collections.abc`` variants
+don't validated the number of type arguments. Therefore the defaults for these
+can be used in earlier versions as well.

doc/data/messages/u/unnecessary-default-type-args/good.py

@@ -0,0 +1,4 @@
+from collections.abc import AsyncGenerator, Generator
+
+a1: AsyncGenerator[int]
+b1: Generator[int]

doc/data/messages/u/unnecessary-default-type-args/related.rst

@@ -0,0 +1,2 @@
+- `Python documentation for AsyncGenerator <https://docs.python.org/3.13/library/typing.html#typing.AsyncGenerator>`_
+- `Python documentation for Generator <https://docs.python.org/3.13/library/typing.html#typing.Generator>`_

doc/user_guide/checkers/extensions.rst

@@ -688,6 +688,9 @@ Typing checker Messages
 :consider-alternative-union-syntax (R6003): *Consider using alternative Union syntax instead of '%s'%s*
   Emitted when 'typing.Union' or 'typing.Optional' is used instead of the
   alternative Union syntax 'int | None'.
+:unnecessary-default-type-args (R6007): *Type `%s` has unnecessary default type args. Change it to `%s`.*
+  Emitted when types have default type args which can be omitted. Mainly used
+  for `typing.Generator` and `typing.AsyncGenerator`.
 :redundant-typehint-argument (R6006): *Type `%s` is used more than once in union type annotation. Remove redundant typehints.*
   Duplicated type arguments will be skipped by `mypy` tool, therefore should be
   removed to avoid confusion.

doc/user_guide/messages/messages_overview.rst

@@ -545,6 +545,7 @@ All messages in the refactor category:
    refactor/too-many-statements
    refactor/trailing-comma-tuple
    refactor/unnecessary-comprehension
+   refactor/unnecessary-default-type-args
    refactor/unnecessary-dict-index-lookup
    refactor/unnecessary-list-index-lookup
    refactor/use-a-generator

doc/whatsnew/fragments/9938.new_check

@@ -0,0 +1,4 @@
+Add ``unnecessary-default-type-args`` to the ``typing`` extension to detect the use
+of unnecessary default type args for ``typing.Generator`` and ``typing.AsyncGenerator``.
+
+Refs #9938

pylint/extensions/typing.py

@@ -85,6 +85,7 @@ class DeprecatedTypingAliasMsg(NamedTuple):
     parent_subscript: bool = False
 
 
+# pylint: disable-next=too-many-instance-attributes
 class TypingChecker(BaseChecker):
     """Find issue specifically related to type annotations."""
 
@@ -130,6 +131,12 @@ class TypingChecker(BaseChecker):
             "Duplicated type arguments will be skipped by `mypy` tool, therefore should be "
             "removed to avoid confusion.",
         ),
+        "R6007": (
+            "Type `%s` has unnecessary default type args. Change it to `%s`.",
+            "unnecessary-default-type-args",
+            "Emitted when types have default type args which can be omitted. "
+            "Mainly used for `typing.Generator` and `typing.AsyncGenerator`.",
+        ),
     }
     options = (
         (
@@ -174,6 +181,7 @@ def open(self) -> None:
         self._py37_plus = py_version >= (3, 7)
         self._py39_plus = py_version >= (3, 9)
         self._py310_plus = py_version >= (3, 10)
+        self._py313_plus = py_version >= (3, 13)
 
         self._should_check_typing_alias = self._py39_plus or (
             self._py37_plus and self.linter.config.runtime_typing is False
@@ -248,6 +256,33 @@ def visit_annassign(self, node: nodes.AnnAssign) -> None:
 
         self._check_union_types(types, node)
 
+    @only_required_for_messages("unnecessary-default-type-args")
+    def visit_subscript(self, node: nodes.Subscript) -> None:
+        inferred = safe_infer(node.value)
+        if (  # pylint: disable=too-many-boolean-expressions
+            isinstance(inferred, nodes.ClassDef)
+            and (
+                inferred.qname() in {"typing.Generator", "typing.AsyncGenerator"}
+                and self._py313_plus
+                or inferred.qname()
+                in {"_collections_abc.Generator", "_collections_abc.AsyncGenerator"}
+            )
+            and isinstance(node.slice, nodes.Tuple)
+            and all(
+                (isinstance(el, nodes.Const) and el.value is None)
+                for el in node.slice.elts[1:]
+            )
+        ):
+            suggested_str = (
+                f"{node.value.as_string()}[{node.slice.elts[0].as_string()}]"
+            )
+            self.add_message(
+                "unnecessary-default-type-args",
+                args=(node.as_string(), suggested_str),
+                node=node,
+                confidence=HIGH,
+            )
+
     @staticmethod
     def _is_deprecated_union_annotation(
         annotation: nodes.NodeNG, union_name: str

tests/functional/ext/typing/unnecessary_default_type_args.py

@@ -0,0 +1,17 @@
+# pylint: disable=missing-docstring,deprecated-typing-alias
+import collections.abc as ca
+import typing as t
+
+a1: t.Generator[int, str, str]
+a2: t.Generator[int, None, None]
+a3: t.Generator[int]
+b1: t.AsyncGenerator[int, str]
+b2: t.AsyncGenerator[int, None]
+b3: t.AsyncGenerator[int]
+
+c1: ca.Generator[int, str, str]
+c2: ca.Generator[int, None, None]  # [unnecessary-default-type-args]
+c3: ca.Generator[int]
+d1: ca.AsyncGenerator[int, str]
+d2: ca.AsyncGenerator[int, None]  # [unnecessary-default-type-args]
+d3: ca.AsyncGenerator[int]

tests/functional/ext/typing/unnecessary_default_type_args.rc

@@ -0,0 +1,2 @@
+[main]
+load-plugins=pylint.extensions.typing

tests/functional/ext/typing/unnecessary_default_type_args.txt

@@ -0,0 +1,2 @@
+unnecessary-default-type-args:13:4:13:33::Type `ca.Generator[int, None, None]` has unnecessary default type args. Change it to `ca.Generator[int]`.:HIGH
+unnecessary-default-type-args:16:4:16:32::Type `ca.AsyncGenerator[int, None]` has unnecessary default type args. Change it to `ca.AsyncGenerator[int]`.:HIGH

tests/functional/ext/typing/unnecessary_default_type_args_py313.py

@@ -0,0 +1,17 @@
+# pylint: disable=missing-docstring,deprecated-typing-alias
+import collections.abc as ca
+import typing as t
+
+a1: t.Generator[int, str, str]
+a2: t.Generator[int, None, None]  # [unnecessary-default-type-args]
+a3: t.Generator[int]
+b1: t.AsyncGenerator[int, str]
+b2: t.AsyncGenerator[int, None]  # [unnecessary-default-type-args]
+b3: t.AsyncGenerator[int]
+
+c1: ca.Generator[int, str, str]
+c2: ca.Generator[int, None, None]  # [unnecessary-default-type-args]
+c3: ca.Generator[int]
+d1: ca.AsyncGenerator[int, str]
+d2: ca.AsyncGenerator[int, None]  # [unnecessary-default-type-args]
+d3: ca.AsyncGenerator[int]

tests/functional/ext/typing/unnecessary_default_type_args_py313.rc

@@ -0,0 +1,3 @@
+[main]
+py-version=3.13
+load-plugins=pylint.extensions.typing

tests/functional/ext/typing/unnecessary_default_type_args_py313.txt

@@ -0,0 +1,4 @@
+unnecessary-default-type-args:6:4:6:32::Type `t.Generator[int, None, None]` has unnecessary default type args. Change it to `t.Generator[int]`.:HIGH
+unnecessary-default-type-args:9:4:9:31::Type `t.AsyncGenerator[int, None]` has unnecessary default type args. Change it to `t.AsyncGenerator[int]`.:HIGH
+unnecessary-default-type-args:13:4:13:33::Type `ca.Generator[int, None, None]` has unnecessary default type args. Change it to `ca.Generator[int]`.:HIGH
+unnecessary-default-type-args:16:4:16:32::Type `ca.AsyncGenerator[int, None]` has unnecessary default type args. Change it to `ca.AsyncGenerator[int]`.:HIGH