Ensure cache is set every time instead of branching

Checking if the cached value exists every time is costly and an
unnecessary branch.

sipa/babel.py

@@ -81,9 +81,10 @@ def iter_preferred_locales(request: Request) -> t.Iterator[str]:
     yield from request.accept_languages.values()
 
 
-def preferred_locales(request: Request) -> list[str]:
-    pl = g.get("preferred_locales")
-    if not pl:
-        pl = g.preferred_locales = list(iter_preferred_locales(request))
+def preferred_locales() -> list[str]:
+    return g.preferred_locales
 
-    return pl
+
+def cache_preferred_locales(*a, **extra):
+    """Store the preferred locales on the `g` object."""
+    g.preferred_locales = list(iter_preferred_locales(request))

sipa/flatpages.py

@@ -155,7 +155,7 @@ def localized_page(self) -> Page:
         :returns: The localized page
         """
         negotiated_locale = negotiate_locale(
-            preferred_locales(request),
+            preferred_locales(),
             self.available_locales,
             sep="-",
         )

sipa/initialization.py

@@ -5,14 +5,19 @@
 from datetime import datetime
 
 import sentry_sdk
-from flask import g
+from flask import g, request_started
 from flask_babel import Babel, get_locale
 from werkzeug import Response
 from werkzeug.middleware.proxy_fix import ProxyFix
 from flask_qrcode import QRcode
 from sentry_sdk.integrations.flask import FlaskIntegration
 
-from sipa.babel import possible_locales, save_user_locale_setting, select_locale
+from sipa.babel import (
+    possible_locales,
+    save_user_locale_setting,
+    select_locale,
+    cache_preferred_locales,
+)
 from sipa.backends import Backends
 from sipa.base import IntegerConverter, login_manager
 from sipa.blueprints.usersuite import get_attribute_endpoint
@@ -51,6 +56,7 @@ def init_app(app, **kwargs):
     babel = Babel()
     babel.init_app(app)
     babel.localeselector(select_locale)
+    request_started.connect(cache_preferred_locales)
     app.before_request(save_user_locale_setting)
     app.after_request(ensure_csp)
     app.session_interface = SeparateLocaleCookieSessionInterface()