python: Convert assignment type annotations to Python 3.6 style.

This commit was split by tabbott; this piece covers the vast majority
of files in Zulip, but excludes scripts/, tools/, and puppet/ to help
ensure we at least show the right error messages for Xenial systems.

We can likely further refine the remaining pieces with some testing.

Generated by com2ann, with whitespace fixes and various manual fixes
for runtime issues:

-    invoiced_through: Optional[LicenseLedger] = models.ForeignKey(
+    invoiced_through: Optional["LicenseLedger"] = models.ForeignKey(

-_apns_client: Optional[APNsClient] = None
+_apns_client: Optional["APNsClient"] = None

-    notifications_stream: Optional[Stream] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)
-    signup_notifications_stream: Optional[Stream] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)
+    notifications_stream: Optional["Stream"] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)
+    signup_notifications_stream: Optional["Stream"] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)

-    author: Optional[UserProfile] = models.ForeignKey('UserProfile', blank=True, null=True, on_delete=CASCADE)
+    author: Optional["UserProfile"] = models.ForeignKey('UserProfile', blank=True, null=True, on_delete=CASCADE)

-    bot_owner: Optional[UserProfile] = models.ForeignKey('self', null=True, on_delete=models.SET_NULL)
+    bot_owner: Optional["UserProfile"] = models.ForeignKey('self', null=True, on_delete=models.SET_NULL)

-    default_sending_stream: Optional[Stream] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)
-    default_events_register_stream: Optional[Stream] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)
+    default_sending_stream: Optional["Stream"] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)
+    default_events_register_stream: Optional["Stream"] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)

-descriptors_by_handler_id: Dict[int, ClientDescriptor] = {}
+descriptors_by_handler_id: Dict[int, "ClientDescriptor"] = {}

-worker_classes: Dict[str, Type[QueueProcessingWorker]] = {}
-queues: Dict[str, Dict[str, Type[QueueProcessingWorker]]] = {}
+worker_classes: Dict[str, Type["QueueProcessingWorker"]] = {}
+queues: Dict[str, Dict[str, Type["QueueProcessingWorker"]]] = {}

-AUTH_LDAP_REVERSE_EMAIL_SEARCH: Optional[LDAPSearch] = None
+AUTH_LDAP_REVERSE_EMAIL_SEARCH: Optional["LDAPSearch"] = None

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/data_import/import_util.py

@@ -21,8 +21,8 @@ ZerverFieldsT = Dict[str, Any]
 
 class SubscriberHandler:
     def __init__(self) -> None:
-        self.stream_info = dict()  # type: Dict[int, Set[int]]
-        self.huddle_info = dict()  # type: Dict[int, Set[int]]
+        self.stream_info: Dict[int, Set[int]] = dict()
+        self.huddle_info: Dict[int, Set[int]] = dict()
 
     def set_info(self,
                  users: Set[int],
@@ -105,7 +105,7 @@ def make_subscriber_map(zerver_subscription: List[ZerverFieldsT]) -> Dict[int, S
     This can be convenient for building up UserMessage
     rows.
     '''
-    subscriber_map = dict()  # type: Dict[int, Set[int]]
+    subscriber_map: Dict[int, Set[int]] = dict()
     for sub in zerver_subscription:
         user_id = sub['user_profile']
         recipient_id = sub['recipient']
@@ -163,7 +163,7 @@ def build_public_stream_subscriptions(
     users to every public stream.  This returns a list of Subscription
     dicts.
     '''
-    subscriptions = []  # type: List[ZerverFieldsT]
+    subscriptions: List[ZerverFieldsT] = []
 
     public_stream_ids = {
         stream['id']
@@ -199,7 +199,7 @@ def build_stream_subscriptions(
         zerver_recipient: List[ZerverFieldsT],
         zerver_stream: List[ZerverFieldsT]) -> List[ZerverFieldsT]:
 
-    subscriptions = []  # type: List[ZerverFieldsT]
+    subscriptions: List[ZerverFieldsT] = []
 
     stream_ids = {stream['id'] for stream in zerver_stream}
 
@@ -227,7 +227,7 @@ def build_huddle_subscriptions(
         zerver_recipient: List[ZerverFieldsT],
         zerver_huddle: List[ZerverFieldsT]) -> List[ZerverFieldsT]:
 
-    subscriptions = []  # type: List[ZerverFieldsT]
+    subscriptions: List[ZerverFieldsT] = []
 
     huddle_ids = {huddle['id'] for huddle in zerver_huddle}
 
@@ -252,7 +252,7 @@ def build_huddle_subscriptions(
 
 def build_personal_subscriptions(zerver_recipient: List[ZerverFieldsT]) -> List[ZerverFieldsT]:
 
-    subscriptions = []  # type: List[ZerverFieldsT]
+    subscriptions: List[ZerverFieldsT] = []
 
     personal_recipients = [
         recipient
@@ -586,7 +586,7 @@ def run_parallel_wrapper(f: Callable[[ListJobData], None], full_items: List[List
             if count % 1000 == 0:
                 logging.info("A download thread finished %s items" % (count,))
         return 0
-    job_lists = [full_items[i::threads] for i in range(threads)]  # type: List[List[ListJobData]]
+    job_lists: List[List[ListJobData]] = [full_items[i::threads] for i in range(threads)]
     return run_parallel(wrapping_function, job_lists, threads=threads)
 
 def process_uploads(upload_list: List[ZerverFieldsT], upload_dir: str,