Mastering Flask Web Development - Second Edition

The default filter

If the passed variable is None, then replace it with a default value as follows:

{{ post.date | default('2015-01-01') }} 
2015-01-01 

If you wish to replace the variable with the default value, and if the variable evaluates to False, then pass True to the optional second parameter, as follows:

{{ '' | default('An empty string', True) }} 
An empty string