Use flask.g as context

This is according to a recommendation on extension development in Flask
documentation. This also fixes an issue where force_locale context leaks
between two threads which are running in app context rather than request
context. `current_app` is a proxy to the application object, while
`flask.g` lives only for the duration of the context, either app context
or request context. `types.SimpleNamespace` is used with the extension
name as a prefix to avoid collisions.

Author: Atte Lautanala

flask_babel/__init__.py

@@ -9,11 +9,11 @@
 """
 from __future__ import absolute_import
 import os
+from types import SimpleNamespace
 
 from datetime import datetime
 from contextlib import contextmanager
-from flask import current_app, request
-from flask.ctx import has_request_context
+from flask import current_app, g
 from flask.helpers import locked_cached_property
 from babel import dates, numbers, support, Locale
 from pytz import timezone, UTC
@@ -661,11 +661,13 @@ def lazy_pgettext(self, context, string, **variables):
 
 
 def _get_current_context():
-    if has_request_context():
-        return request
+    if not g:
+        return None
+
+    if not hasattr(g, "_flask_babel"):
+        g._flask_babel = SimpleNamespace()
 
-    if current_app:
-        return current_app
+    return g._flask_babel
 
 
 def get_domain():

tests/test_force_locale.py

@@ -47,6 +47,33 @@ def first_request():
         thread.join()
 
 
+def test_force_locale_with_threading_and_app_context():
+    app = flask.Flask(__name__)
+    b = babel.Babel(app)
+
+    @b.localeselector
+    def select_locale():
+        return 'de_DE'
+
+    semaphore = Semaphore(value=0)
+
+    def first_app_context():
+        with app.app_context():
+            with babel.force_locale('en_US'):
+                assert str(babel.get_locale()) == 'en_US'
+                semaphore.acquire()
+
+    thread = Thread(target=first_app_context)
+    thread.start()
+
+    try:
+        with app.app_context():
+            assert str(babel.get_locale()) == 'de_DE'
+    finally:
+        semaphore.release()
+        thread.join()
+
+
 def test_refresh_during_force_locale():
     app = flask.Flask(__name__)
     b = babel.Babel(app)