Add the force_locale utility function

It will be needed when sending emails and generating log messages.

calsocial/utils.py

@@ -0,0 +1,70 @@
+# Calendar.social
+# Copyright (C) 2018  Gergely Polonkai
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Utility functions for Calendar.social
+"""
+
+from contextlib import contextmanager
+
+@contextmanager
+def force_locale(locale):
+    """Temporarily overrides the currently selected locale.
+
+    Sometimes it is useful to switch the current locale to different one, do some tasks and then
+    revert back to the original one.  For example, if the user uses German on the web site, but
+    you want to send them an email in English, you can use this function as a context manager::
+
+        with force_locale('en_US'):
+            send_email(gettext('Hello!'), ...)
+
+    Shamelessly extracted from Flask-Babel.
+
+    :param locale: The locale to temporary switch to (ex: 'en_US').
+    :type locale: str
+    """
+
+    from flask import current_app, has_request_context, request
+
+    def _get_current_context():
+        if has_request_context():
+            return request
+
+        if current_app:
+            return current_app
+
+        return None
+
+    ctx = _get_current_context()
+    if ctx is None:
+        yield
+        return
+
+    babel = current_app.extensions['babel']
+
+    orig_locale_selector_func = babel.locale_selector_func
+    orig_attrs = {}
+    for key in ('babel_translations', 'babel_locale'):
+        orig_attrs[key] = getattr(ctx, key, None)
+
+    try:
+        babel.locale_selector_func = lambda: locale
+        for key in orig_attrs:
+            setattr(ctx, key, None)
+        yield
+    finally:
+        babel.locale_selector_func = orig_locale_selector_func
+        for key, value in orig_attrs.items():
+            setattr(ctx, key, value)