When I call url_for('index') it will generate '/' but there are times where I'd like it to generate 'domain.tld/' instead. I can't find in the documentation where I would specify this. Do I just need to do 'domain.tld/%s' % url_for('index')?
url_for takes an _external keyword argument that will return an absolute (rather than relative) URL. I believe you will need to set a SERVER_NAME config key with your root domain to make it work correctly.
来源：https://stackoverflow.com/questions/12162634/where-do-i-define-the-domain-to-be-used-by-url-for-in-flask