Workaround false "unreachable" positives

See Shoobx/mypy-zope#91

```
tests/http/test_proxyagent.py:626: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:762: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:826: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:838: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:845: error: Statement is unreachable  [unreachable]
tests/http/federation/test_matrix_federation_agent.py:151: error: Statement is unreachable  [unreachable]
tests/http/federation/test_matrix_federation_agent.py:452: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:60: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:93: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:127: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:152: error: Statement is unreachable  [unreachable]
```

tests/http/federation/test_matrix_federation_agent.py

@@ -63,7 +63,7 @@
     get_test_ca_cert_file,
 )
 from tests.server import FakeTransport, ThreadedMemoryReactorClock
-from tests.utils import default_config
+from tests.utils import default_config, checked_cast
 
 logger = logging.getLogger(__name__)
 
@@ -146,8 +146,10 @@ def _make_connection(
         #
         # Normally this would be done by the TCP socket code in Twisted, but we are
         # stubbing that out here.
-        client_protocol = client_factory.buildProtocol(dummy_address)
-        assert isinstance(client_protocol, _WrappingProtocol)
+        # NB: we use a checked_cast here to workaround https://github.com/Shoobx/mypy-zope/issues/91)
+        client_protocol = checked_cast(
+            _WrappingProtocol, client_factory.buildProtocol(dummy_address)
+        )
         client_protocol.makeConnection(
             FakeTransport(server_protocol, self.reactor, client_protocol)
         )
@@ -446,7 +448,6 @@ def _do_get_via_proxy(
         server_ssl_protocol = _wrap_server_factory_for_tls(
             _get_test_protocol_factory()
         ).buildProtocol(dummy_address)
-        assert isinstance(server_ssl_protocol, TLSMemoryBIOProtocol)
 
         # Tell the HTTP server to send outgoing traffic back via the proxy's transport.
         proxy_server_transport = proxy_server.transport
@@ -1529,7 +1530,7 @@ def _check_logcontext(context: LoggingContextOrSentinel) -> None:
 
 def _wrap_server_factory_for_tls(
     factory: IProtocolFactory, sanlist: Optional[List[bytes]] = None
-) -> IProtocolFactory:
+) -> TLSMemoryBIOFactory:
     """Wrap an existing Protocol Factory with a test TLSMemoryBIOFactory
     The resultant factory will create a TLS server which presents a certificate
     signed by our test CA, valid for the domains in `sanlist`

tests/http/test_proxyagent.py

@@ -14,7 +14,7 @@
 import base64
 import logging
 import os
-from typing import List, Optional
+from typing import List, Optional, cast
 from unittest.mock import patch
 
 import treq
@@ -43,6 +43,7 @@
 )
 from tests.server import FakeTransport, ThreadedMemoryReactorClock
 from tests.unittest import TestCase
+from tests.utils import checked_cast
 
 logger = logging.getLogger(__name__)
 
@@ -620,7 +621,6 @@ def _do_https_request_via_proxy(
         server_ssl_protocol = _wrap_server_factory_for_tls(
             _get_test_protocol_factory()
         ).buildProtocol(dummy_address)
-        assert isinstance(server_ssl_protocol, TLSMemoryBIOProtocol)
 
         # Tell the HTTP server to send outgoing traffic back via the proxy's transport.
         proxy_server_transport = proxy_server.transport
@@ -757,12 +757,14 @@ def test_https_request_via_uppercase_proxy_with_blacklist(self) -> None:
         assert isinstance(proxy_server, HTTPChannel)
 
         # fish the transports back out so that we can do the old switcheroo
-        s2c_transport = proxy_server.transport
-        assert isinstance(s2c_transport, FakeTransport)
-        client_protocol = s2c_transport.other
-        assert isinstance(client_protocol, _WrappingProtocol)
-        c2s_transport = client_protocol.transport
-        assert isinstance(c2s_transport, FakeTransport)
+        # To help mypy out with the various Protocols and wrappers and mocks, we do
+        # some explicit casting. Without the casts, we hit the bug I reported at
+        # https://github.com/Shoobx/mypy-zope/issues/91 .
+        # We also double-checked these casts at runtime (test-time) because I found it
+        # quite confusing to deduce these types in the first place!
+        s2c_transport = checked_cast(FakeTransport, proxy_server.transport)
+        client_protocol = checked_cast(_WrappingProtocol, s2c_transport.other)
+        c2s_transport = checked_cast(FakeTransport, client_protocol.transport)
 
         # the FakeTransport is async, so we need to pump the reactor
         self.reactor.advance(0)
@@ -822,9 +824,9 @@ def test_https_request_via_uppercase_proxy_with_blacklist(self) -> None:
     @patch.dict(os.environ, {"http_proxy": "proxy.com:8888"})
     def test_proxy_with_no_scheme(self) -> None:
         http_proxy_agent = ProxyAgent(self.reactor, use_proxy=True)
-        assert isinstance(http_proxy_agent.http_proxy_endpoint, HostnameEndpoint)
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._hostStr, "proxy.com")
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._port, 8888)
+        proxy_ep = checked_cast(HostnameEndpoint, http_proxy_agent.http_proxy_endpoint)
+        self.assertEqual(proxy_ep._hostStr, "proxy.com")
+        self.assertEqual(proxy_ep._port, 8888)
 
     @patch.dict(os.environ, {"http_proxy": "socks://proxy.com:8888"})
     def test_proxy_with_unsupported_scheme(self) -> None:
@@ -834,25 +836,21 @@ def test_proxy_with_unsupported_scheme(self) -> None:
     @patch.dict(os.environ, {"http_proxy": "http://proxy.com:8888"})
     def test_proxy_with_http_scheme(self) -> None:
         http_proxy_agent = ProxyAgent(self.reactor, use_proxy=True)
-        assert isinstance(http_proxy_agent.http_proxy_endpoint, HostnameEndpoint)
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._hostStr, "proxy.com")
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._port, 8888)
+        proxy_ep = checked_cast(HostnameEndpoint, http_proxy_agent.http_proxy_endpoint)
+        self.assertEqual(proxy_ep._hostStr, "proxy.com")
+        self.assertEqual(proxy_ep._port, 8888)
 
     @patch.dict(os.environ, {"http_proxy": "https://proxy.com:8888"})
     def test_proxy_with_https_scheme(self) -> None:
         https_proxy_agent = ProxyAgent(self.reactor, use_proxy=True)
-        assert isinstance(https_proxy_agent.http_proxy_endpoint, _WrapperEndpoint)
-        self.assertEqual(
-            https_proxy_agent.http_proxy_endpoint._wrappedEndpoint._hostStr, "proxy.com"
-        )
-        self.assertEqual(
-            https_proxy_agent.http_proxy_endpoint._wrappedEndpoint._port, 8888
-        )
+        proxy_ep = checked_cast(_WrapperEndpoint, https_proxy_agent.http_proxy_endpoint)
+        self.assertEqual(proxy_ep._wrappedEndpoint._hostStr, "proxy.com")
+        self.assertEqual(proxy_ep._wrappedEndpoint._port, 8888)
 
 
 def _wrap_server_factory_for_tls(
     factory: IProtocolFactory, sanlist: Optional[List[bytes]] = None
-) -> IProtocolFactory:
+) -> TLSMemoryBIOFactory:
     """Wrap an existing Protocol Factory with a test TLSMemoryBIOFactory
 
     The resultant factory will create a TLS server which presents a certificate

tests/logging/test_remote_handler.py

@@ -21,6 +21,7 @@
 from tests.logging import LoggerCleanupMixin
 from tests.server import FakeTransport, get_clock
 from tests.unittest import TestCase
+from tests.utils import checked_cast
 
 
 def connect_logging_client(
@@ -56,8 +57,8 @@ def test_log_output(self) -> None:
         client, server = connect_logging_client(self.reactor, 0)
 
         # Trigger data being sent
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # One log message, with a single trailing newline
         logs = server.data.decode("utf8").splitlines()
@@ -89,8 +90,8 @@ def test_log_backpressure_debug(self) -> None:
 
         # Allow the reconnection
         client, server = connect_logging_client(self.reactor, 0)
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # Only the 7 infos made it through, the debugs were elided
         logs = server.data.splitlines()
@@ -123,8 +124,8 @@ def test_log_backpressure_info(self) -> None:
 
         # Allow the reconnection
         client, server = connect_logging_client(self.reactor, 0)
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # The 10 warnings made it through, the debugs and infos were elided
         logs = server.data.splitlines()
@@ -148,8 +149,8 @@ def test_log_backpressure_cut_middle(self) -> None:
 
         # Allow the reconnection
         client, server = connect_logging_client(self.reactor, 0)
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # The first five and last five warnings made it through, the debugs and
         # infos were elided

tests/utils.py

@@ -15,7 +15,7 @@
 
 import atexit
 import os
-from typing import Any, Callable, Dict, List, Tuple, Union, overload
+from typing import Any, Callable, Dict, List, Tuple, Union, overload, TypeVar, Type
 
 import attr
 from typing_extensions import Literal, ParamSpec
@@ -338,3 +338,27 @@ async def create_room(hs: HomeServer, room_id: str, creator_id: str) -> None:
     event, context = await event_creation_handler.create_new_client_event(builder)
 
     await persistence_store.persist_event(event, context)
+
+
+T = TypeVar("T")
+
+
+def checked_cast(type: Type[T], x: object) -> T:
+    """A version of typing.cast that is checked at runtime.
+
+    We have our own function for this for two reasons:
+
+    1. typing.cast itself is deliberately a no-op at runtime, see
+       https://docs.python.org/3/library/typing.html#typing.cast
+    2. To help workaround a mypy-zope bug https://github.com/Shoobx/mypy-zope/issues/91
+       where mypy would erroneously consider `isinstance(x, type)` to be false in all
+       circumstances.
+
+    For this to make sense, `T` needs to be something that `isinstance` can check; see
+        https://docs.python.org/3/library/functions.html?highlight=isinstance#isinstance
+        https://docs.python.org/3/glossary.html#term-abstract-base-class
+        https://docs.python.org/3/library/typing.html#typing.runtime_checkable
+    for more details.
+    """
+    assert isinstance(x, type)
+    return x