Mastering Flask Web Development - Second Edition

Using if statements

Using if statements in Jinja is similar to using them in Python. Anything that returns, or is, a Boolean determines the flow of the code, as shown in the following code:

{%if user.is_logged_in() %} 
  <a href='/logout'>Logout</a> 
{% else %} 
  <a href='/login'>Login</a> 
{% endif %} 

Filters can also be used in if statements, as follows:

{% if comments | length > 0 %} 
  There are {{ comments | length }} comments 
{% else %} 
  There are no comments 
{% endif %}