The defaults variable is a tuple of the default values for any keyword arguments the macro explicitly accepts. Let's change our macro to display the default values as well as the arguments:

{% macro test(var_a='a string') -%} 
{{ test.arguments }} 
{{ test.defaults }} 
{%- endmacro -%} 
{{ test() }}

Rendering this version of the template will result in the following output:

Here, we can see that the template rendered with both the names, and this time, the default values of the arguments were accepted by the macro.