Where do I define the domain to be used by url

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')?

What is url_for function in flask?

Flask – URL Building Advertisements. The url_for() function is very useful for dynamically building a URL for a specific function. The function accepts the name of a function as first argument, and one or more keyword arguments, each corresponding to the variable part of URL.

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.