Comment 4 for bug 244681

Loading all the timezone objects is definitely the most thorough check, but it takes at least 300 milliseconds. Checking that the timezone files exist with os.path.isfile only takes 4 milliseconds.

>>> for timezone_name in pytz.common_timezones:
... if not os.path.isfile(os.path.join(dir, timezone_name)):
... pytz.common_timezones.remove(timezone_name)