A custom Jinja tag required: ignore enclosed block of content if any of the variables is empty

app.jinja_env.add_extension('jinja2_ext_required.RequiredVariablesExtension')

jinja_env = Environment(extensions=['jinja2_ext_required.RequiredVariablesExtension'])

While working in Flask, I always encounter this pattern in template files:

{% if a.complicated_function() %}
    A has a complicated value :{{a.complicated_function()}}
{% endif %}

The complicated_function is called twice. Most of the time, complicated_function makes some calls to the database. This is obviously not very efficient.

There for I created a custom tag extension. The parser is a combination of an if block and a with block. When the parser encounters the tag it evaluates all the variables in the assignment line and only process the body codes between required and endrequired tags if all variables are not empty.

NOTE: 0(zero), ‘’(empty string), [](empty list) and {}(empty dictionary) are all considered False, which follows Python’s convention.

{% required value=a.complicated_function() %}
    A has a complicated value : {{value}}
{% endrequired %}

{% required va=a.complicated_function(), vb=b.complicated_function(), vc=c.complicated_function()%}
    A has a complicated value : {{va}}
    B has a complicated value : {{vb}}    
    C has a complicated value : {{vc}}
{% endrequired %}

ANOTHER NOTE: The values must be assigned to a variable in order for the code to work. This is because of how with block behaves.

More details can be found in this blog post