Add boundary types for before hooks

** BREAKING CHANGE **

This adds typing information for the before hook as a proof of concept. I used
ParamSpec as a means to introspect the hooks arguments to type check the *args
and **kwargs arguments.

A problem with using ParamSpec is that additional arguments to the before
decorator become impossible (so far as I can see) to specify a type. is_async,
in needs to come after the positional args, but that is not allowed with
ParamSpec. The solution to this is to move is_async to be consumed through
**kwargs and no longer a special keyword argument. I don't think this is actually
a breaking change, but because it changes a signature it might be worth marking
it as such.

falcon/__init__.py

@@ -101,57 +101,57 @@
 
 
 __all__ = (
-    "API",
-    "App",
-    "after",
-    "before",
-    "HTTPError",
-    "HTTPStatus",
-    "CORSMiddleware",
-    "HTTPFound",
-    "HTTPMovedPermanently",
-    "HTTPPermanentRedirect",
-    "HTTPSeeOther",
-    "HTTPTemporaryRedirect",
-    "Forwarded",
-    "Request",
-    "RequestOptions",
-    "Response",
-    "ResponseOptions",
-    "BoundedStream",
-    "async_to_sync",
-    "BufferedReader",
-    "CaseInsensitiveDict",
-    "code_to_http_status",
-    "Context",
-    "create_task",
-    "deprecated",
-    "dt_to_http",
-    "ETag",
-    "get_argnames",
-    "get_bound_method",
-    "get_http_status",
-    "get_running_loop",
-    "http_cookies",
-    "http_date_to_dt",
-    "http_now",
-    "http_status_to_code",
-    "IS_64_BITS",
-    "is_python_func",
-    "misc",
-    "parse_header",
-    "reader",
-    "runs_sync",
-    "secure_filename",
-    "structures",
-    "sync",
-    "sync_to_async",
-    "sys",
-    "time",
-    "TimezoneGMT",
-    "to_query_str",
-    "uri",
-    "wrap_sync_to_async",
-    "wrap_sync_to_async_unsafe",
-    "__version__",
+    'API',
+    'App',
+    'after',
+    'before',
+    'HTTPError',
+    'HTTPStatus',
+    'CORSMiddleware',
+    'HTTPFound',
+    'HTTPMovedPermanently',
+    'HTTPPermanentRedirect',
+    'HTTPSeeOther',
+    'HTTPTemporaryRedirect',
+    'Forwarded',
+    'Request',
+    'RequestOptions',
+    'Response',
+    'ResponseOptions',
+    'BoundedStream',
+    'async_to_sync',
+    'BufferedReader',
+    'CaseInsensitiveDict',
+    'code_to_http_status',
+    'Context',
+    'create_task',
+    'deprecated',
+    'dt_to_http',
+    'ETag',
+    'get_argnames',
+    'get_bound_method',
+    'get_http_status',
+    'get_running_loop',
+    'http_cookies',
+    'http_date_to_dt',
+    'http_now',
+    'http_status_to_code',
+    'IS_64_BITS',
+    'is_python_func',
+    'misc',
+    'parse_header',
+    'reader',
+    'runs_sync',
+    'secure_filename',
+    'structures',
+    'sync',
+    'sync_to_async',
+    'sys',
+    'time',
+    'TimezoneGMT',
+    'to_query_str',
+    'uri',
+    'wrap_sync_to_async',
+    'wrap_sync_to_async_unsafe',
+    '__version__',
 )

falcon/hooks.py

@@ -15,21 +15,32 @@
 """Hook decorators."""
 
 from functools import wraps
-from inspect import getmembers
-from inspect import iscoroutinefunction
+from inspect import getmembers, iscoroutinefunction
 import re
 
+import typing_extensions as te
+
 from falcon.constants import COMBINED_METHODS
+from falcon.request import Request
+from falcon.response import Response
 from falcon.util.misc import get_argnames
 from falcon.util.sync import _wrap_non_coroutine_unsafe
 
-
 _DECORABLE_METHOD_NAME = re.compile(
     r'^on_({})(_\w+)?$'.format('|'.join(method.lower() for method in COMBINED_METHODS))
 )
 
 
-def before(action, *args, is_async=False, **kwargs):
+P = te.ParamSpec('P')
+R = te.TypeVar('R', bound=Request)
+S = te.TypeVar('S', bound=Response)
+
+
+def before(
+    action: te.Callable[te.Concatenate[R, S, te.Any, te.Dict[str, te.Any], P], None],
+    *args: P.args,
+    **kwargs: P.kwargs,
+) -> te.Callable[[te.Any], te.Any]:
     """Execute the given action function *before* the responder.
 
     The `params` argument that is passed to the hook
@@ -79,6 +90,8 @@ def do_something(req, resp, resource, params):
             *action*.
     """
 
+    is_async = kwargs.get('is_async', False)
+
     def _before(responder_or_resource):
         if isinstance(responder_or_resource, type):
             resource = responder_or_resource

pyproject.toml

@@ -4,6 +4,7 @@
         "setuptools>=47",
         "wheel>=0.34",
         "cython>=0.29.21; python_implementation == 'CPython'", # Skip cython when using pypy
+        "typing-extensions>=4"
     ]
 
 [tool.mypy]