typing: Use TYPE_CHECKING when dealing with cyclic dependencies.

tools/coveragerc

@@ -13,6 +13,8 @@ exclude_lines =
     def __str__[(]self[)] -> .*:
     # Don't require coverage for errors about unsupported webhook event types
     raise UnexpectedWebhookEventType
+    # Don't require coverage for blocks only run when type-checking
+    if TYPE_CHECKING:
 
 [run]
 omit =

zerver/lib/cache.py

@@ -9,7 +9,8 @@ from django.db.models import Q
 from django.core.cache.backends.base import BaseCache
 from django.http import HttpRequest
 
-from typing import Any, Callable, Dict, Iterable, List, Optional, TypeVar, Tuple
+from typing import Any, Callable, Dict, Iterable, List, \
+    Optional, TypeVar, Tuple, TYPE_CHECKING
 
 from zerver.lib.utils import statsd, statsd_key, make_safe_digest
 import time
@@ -19,7 +20,7 @@ import sys
 import os
 import hashlib
 
-if False:
+if TYPE_CHECKING:
     # These modules have to be imported for type annotations but
     # they cannot be imported at runtime due to cyclic dependency.
     from zerver.models import UserProfile, Realm, Message

zerver/lib/test_helpers.py

@@ -1,7 +1,7 @@
 from contextlib import contextmanager
 from typing import (
     Any, Callable, Dict, Generator, Iterable, Iterator, List, Mapping,
-    Optional, Tuple, Union, IO, TypeVar
+    Optional, Tuple, Union, IO, TypeVar, TYPE_CHECKING
 )
 
 from django.core import signing
@@ -38,7 +38,7 @@ from zerver.models import (
     UserProfile,
 )
 
-if False:
+if TYPE_CHECKING:
     # Avoid an import cycle; we only need these for type annotations.
     from zerver.lib.test_classes import ZulipTestCase, MigrationsTestCase
 

zerver/tornado/descriptors.py

@@ -1,6 +1,6 @@
-from typing import Dict, Optional
+from typing import Dict, Optional, TYPE_CHECKING
 
-if False:
+if TYPE_CHECKING:
     from zerver.tornado.event_queue import ClientDescriptor
 
 descriptors_by_handler_id = {}  # type: Dict[int, ClientDescriptor]