add mypy plugin

setup.cfg

@@ -6,6 +6,7 @@ ignore_missing_imports = True
 warn_unused_configs = True
 warn_redundant_casts = True
 warn_unused_ignores = True
+plugins = idom.mypy
 
 [flake8]
 ignore = E203, E266, E501, W503, F811, N802, N806
@@ -41,6 +42,7 @@ exclude_lines =
     raise NotImplementedError
 omit =
     src/idom/__main__.py
+    src/idom/mypy.py
     src/idom/core/_fixed_jsonpatch.py
 
 [build_sphinx]

src/idom/mypy.py

@@ -0,0 +1,51 @@
+from typing import Callable, Final
+
+from mypy.errorcodes import ErrorCode
+from mypy.nodes import ArgKind
+from mypy.plugin import FunctionContext, Plugin
+from mypy.types import CallableType, Instance, Type
+
+
+KEY_IN_RENDER_FUNC: Final = ErrorCode(
+    "idom-key-in-render-func",
+    "Component render function has reserved 'key' parameter",
+    "IDOM",
+)
+
+
+class MypyPlugin(Plugin):
+    """MyPy plugin for IDOM"""
+
+    def get_function_hook(self, fullname: str) -> Callable[[FunctionContext], Type]:
+        if fullname == "idom.core.component.component":
+            return self.component_decorator_hook
+        return super().get_function_hook(fullname)
+
+    def component_decorator_hook(self, ctx: FunctionContext) -> CallableType:
+        if not ctx.arg_types or not ctx.arg_types[0]:
+            return ctx.default_return_type
+        render_func: CallableType = ctx.arg_types[0][0]
+
+        if render_func.argument_by_name("key") is not None:
+            ctx.api.msg.fail(
+                "Component render function has reserved 'key' parameter",
+                ctx.context,
+                code=KEY_IN_RENDER_FUNC,
+            )
+            return ctx.default_return_type
+
+        component_symbol = self.lookup_fully_qualified("idom.core.component.Component")
+        assert component_symbol is not None
+        assert component_symbol.node is not None
+
+        return render_func.copy_modified(
+            arg_kinds=render_func.arg_kinds + [ArgKind.ARG_NAMED_OPT],
+            arg_names=render_func.arg_names + ["key"],
+            arg_types=render_func.arg_types + [ctx.api.named_generic_type("str", [])],
+            ret_type=Instance(component_symbol.node, []),
+        )
+
+
+def plugin(version: str):
+    # ignore version argument if the plugin works with all mypy versions.
+    return MypyPlugin