analytics: Correctly type aggregate_table with Union.

Since mypy doesn't accept redefinition of the same variable within the
same scope, we need to use type annotations with Union to correctly
type aggregate_table. Note that the type cast is necessary for mypy to
narrow the type of aggregate_table.

analytics/views/stats.py

@@ -1,7 +1,7 @@
 import logging
 from collections import defaultdict
 from datetime import datetime, timedelta, timezone
-from typing import Any, Dict, List, Optional, Type
+from typing import Any, Dict, List, Optional, Tuple, Type, Union, cast
 
 from django.conf import settings
 from django.db.models.query import QuerySet
@@ -219,10 +219,16 @@ def get_chart_data(
     remote_realm_id: Optional[int] = None,
     server: Optional["RemoteZulipServer"] = None,
 ) -> HttpResponse:
+    TableType = Union[
+        Type[RemoteInstallationCount],
+        Type[InstallationCount],
+        Type[RemoteRealmCount],
+        Type[RealmCount],
+    ]
     if for_installation:
         if remote:
             assert settings.ZILENCER_ENABLED
-            aggregate_table = RemoteInstallationCount
+            aggregate_table: TableType = RemoteInstallationCount
             assert server is not None
         else:
             aggregate_table = InstallationCount
@@ -235,13 +241,15 @@ def get_chart_data(
         else:
             aggregate_table = RealmCount
 
+    tables: Union[Tuple[TableType], Tuple[TableType, Type[UserCount]]]
+
     if chart_name == "number_of_humans":
         stats = [
             COUNT_STATS["1day_actives::day"],
             COUNT_STATS["realm_active_humans::day"],
             COUNT_STATS["active_users_audit:is_bot:day"],
         ]
-        tables = [aggregate_table]
+        tables = (aggregate_table,)
         subgroup_to_label: Dict[CountStat, Dict[Optional[str], str]] = {
             stats[0]: {None: "_1day"},
             stats[1]: {None: "_15day"},
@@ -251,13 +259,13 @@ def get_chart_data(
         include_empty_subgroups = True
     elif chart_name == "messages_sent_over_time":
         stats = [COUNT_STATS["messages_sent:is_bot:hour"]]
-        tables = [aggregate_table, UserCount]
+        tables = (aggregate_table, UserCount)
         subgroup_to_label = {stats[0]: {"false": "human", "true": "bot"}}
         labels_sort_function = None
         include_empty_subgroups = True
     elif chart_name == "messages_sent_by_message_type":
         stats = [COUNT_STATS["messages_sent:message_type:day"]]
-        tables = [aggregate_table, UserCount]
+        tables = (aggregate_table, UserCount)
         subgroup_to_label = {
             stats[0]: {
                 "public_stream": _("Public streams"),
@@ -270,7 +278,7 @@ def get_chart_data(
         include_empty_subgroups = True
     elif chart_name == "messages_sent_by_client":
         stats = [COUNT_STATS["messages_sent:client:day"]]
-        tables = [aggregate_table, UserCount]
+        tables = (aggregate_table, UserCount)
         # Note that the labels are further re-written by client_label_map
         subgroup_to_label = {
             stats[0]: {str(id): name for id, name in Client.objects.values_list("id", "name")}
@@ -279,7 +287,7 @@ def get_chart_data(
         include_empty_subgroups = False
     elif chart_name == "messages_read_over_time":
         stats = [COUNT_STATS["messages_read::hour"]]
-        tables = [aggregate_table, UserCount]
+        tables = (aggregate_table, UserCount)
         subgroup_to_label = {stats[0]: {None: "read"}}
         labels_sort_function = None
         include_empty_subgroups = True
@@ -310,16 +318,20 @@ def get_chart_data(
         # should simply use the first and last data points for the
         # table.
         assert server is not None
-        if not aggregate_table.objects.filter(server=server).exists():
+        assert aggregate_table is RemoteInstallationCount or aggregate_table is RemoteRealmCount
+        aggregate_table_remote = cast(
+            Union[Type[RemoteInstallationCount], Type[RemoteRealmCount]], aggregate_table
+        )  # https://stackoverflow.com/questions/68540528/mypy-assertions-on-the-types-of-types
+        if not aggregate_table_remote.objects.filter(server=server).exists():
             raise JsonableError(
                 _("No analytics data available. Please contact your server administrator.")
             )
         if start is None:
-            first = aggregate_table.objects.filter(server=server).first()
+            first = aggregate_table_remote.objects.filter(server=server).first()
             assert first is not None
             start = first.end_time
         if end is None:
-            last = aggregate_table.objects.filter(server=server).last()
+            last = aggregate_table_remote.objects.filter(server=server).last()
             assert last is not None
             end = last.end_time
     else: