Enable `--warn-redundant-casts` option in mypy (#14671)

* Enable `--warn-redundant-casts` option in mypy

Doesn't do much but helps me sleep better at night.

* Changelog

* Fix name of the ignore

* Fix one more missed cast

Not sure why I didn't see this one locally, maybe I needed a poetry update

* Remove old comment

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

changelog.d/14671.misc

@@ -0,0 +1 @@
+Improve type hints.

mypy.ini

@@ -12,6 +12,7 @@ local_partial_types = True
 no_implicit_optional = True
 disallow_untyped_defs = True
 strict_equality = True
+warn_redundant_casts = True
 
 files =
   docker/,

scripts-dev/release.py

@@ -27,7 +27,7 @@ import time
 import urllib.request
 from os import path
 from tempfile import TemporaryDirectory
-from typing import Any, List, Optional, cast
+from typing import Any, List, Optional
 
 import attr
 import click
@@ -174,9 +174,7 @@ def _prepare() -> None:
         click.get_current_context().abort()
 
     # Switch to the release branch.
-    # Cast safety: parse() won't return a version.LegacyVersion from our
+    parsed_new_version = version.parse(new_version)
-    # version string format.
-    parsed_new_version = cast(version.Version, version.parse(new_version))
 
     # We assume for debian changelogs that we only do RCs or full releases.
     assert not parsed_new_version.is_devrelease

synapse/storage/database.py

@@ -667,7 +667,8 @@ class DatabasePool:
                 )
         # also check variables referenced in func's closure
         if inspect.isfunction(func):
-            f = cast(types.FunctionType, func)
+            # Keep the cast for now---it helps PyCharm to understand what `func` is.
+            f = cast(types.FunctionType, func)  # type: ignore[redundant-cast]
             if f.__closure__:
                 for i, cell in enumerate(f.__closure__):
                     if inspect.isgenerator(cell.cell_contents):

synapse/storage/engines/postgres.py

@@ -77,7 +77,7 @@ class PostgresEngine(
         # docs: The number is formed by converting the major, minor, and
         # revision numbers into two-decimal-digit numbers and appending them
         # together. For example, version 8.1.5 will be returned as 80105
-        self._version = cast(int, db_conn.server_version)
+        self._version = db_conn.server_version
         allow_unsafe_locale = self.config.get("allow_unsafe_locale", False)
 
         # Are we on a supported PostgreSQL version?