make WSGIRequestHandler can use an easy way to replace the ServerHandler

Doc/library/wsgiref.rst

@@ -347,6 +347,14 @@ request.  (E.g., using the :func:`shift_path_info` function from
    :func:`make_server` function.  Some possibly relevant methods for overriding in
    subclasses:
 
+   :class:`WSGIRequestHandler` has the following class variables:
+
+   .. attribute:: server_handler
+
+      .. versionadded:: 3.9
+
+      The handler class to handle the request. This value should be the subclass
+      :class:`wsgiref.handlers.BaseHandler`
 
    .. method:: WSGIRequestHandler.get_environ()
 

Lib/test/test_wsgiref.py

@@ -46,6 +46,9 @@ class MockHandler(WSGIRequestHandler):
     def finish(self):
         pass
 
+class MockExceptionHandler(MockHandler):
+    server_handler = dict
+
 
 def hello_app(environ,start_response):
     start_response("200 OK", [
@@ -80,6 +83,18 @@ def run_amock(app=hello_app, data=b"GET / HTTP/1.0\n\n"):
 
     return out.getvalue(), err.getvalue()
 
+def run_amock_handler_with_incorrect_server_handler(app=hello_app, data=b"GET / HTTP/1.0\n\n"):
+    server = make_server("", 80, app, MockServer, MockExceptionHandler)
+    inp = BufferedReader(BytesIO(data))
+    out = BytesIO()
+    olderr = sys.stderr
+    err = sys.stderr = StringIO()
+
+    try:
+        server.finish_request((inp, out), ("127.0.0.1",8888))
+    finally:
+        sys.stderr = olderr
+
 def compare_generic_iter(make_it,match):
     """Utility to compare a generic 2.1/2.2+ iterator with an iterable
 
@@ -131,6 +146,9 @@ class IntegrationTests(TestCase):
             "\r\n"
             "Hello, world!").encode("iso-8859-1")
         )
+    def test_run_with_incorrect_server_handler(self):
+        with self.assertRaises(ValueError):
+            run_amock_handler_with_incorrect_server_handler()
 
     def test_plain_hello(self):
         out, err = run_amock()

Lib/wsgiref/simple_server.py

@@ -13,7 +13,7 @@ module.  See also the BaseHTTPServer module docs for other API information.
 from http.server import BaseHTTPRequestHandler, HTTPServer
 import sys
 import urllib.parse
-from wsgiref.handlers import SimpleHandler
+from wsgiref.handlers import BaseHandler, SimpleHandler
 from platform import python_implementation
 
 __version__ = "0.2"
@@ -71,6 +71,12 @@ class WSGIServer(HTTPServer):
 class WSGIRequestHandler(BaseHTTPRequestHandler):
 
     server_version = "WSGIServer/" + __version__
+    server_handler = ServerHandler
+
+    def __init__(self, *args, **kwargs):
+        if not issubclass(self.server_handler, BaseHandler):
+            raise ValueError("the server_handler should be the subclass of BaseHandler")
+        super().__init__(*args, **kwargs)
 
     def get_environ(self):
         env = self.server.base_environ.copy()
@@ -126,7 +132,7 @@ class WSGIRequestHandler(BaseHTTPRequestHandler):
         if not self.parse_request(): # An error code has been sent, just exit
             return
 
-        handler = ServerHandler(
+        handler = self.server_handler(
             self.rfile, self.wfile, self.get_stderr(), self.get_environ(),
             multithread=False,
         )

Misc/NEWS.d/next/Library/2020-05-21-00-18-03.bpo-40700.o88Tsp.rst

@@ -0,0 +1 @@
+make WSGIRequestHandler can use an easy way to replace the ServerHandler