This is the web2py template language described here made available as stand alone package so it can be used anywhere.
Basically it is pure python within "{{" ... "}}" delimiters and blocks are terminated with "pass" if termination is not obvious. For example
from yatl import render, SPAN
example = """
<div>
{{for k in range(num):}}
<span>{{=SPAN(k, _class='number')}} is {{if k % 2 == 0:}}even{{else:}}odd{{pass}}</span>
{{pass}}
</div>
"""
print(render(example, context=dict(num=10, SPAN=SPAN), delimiters="{{ }}"))
In the example SPAN is an optional helper. Output is escaped by default unless marked up with the XML helper as in {{=XML('1 < 2')}}.
Note that the helpers included here are similar but not identical to the web2py ones. They are 99% compatible but the implementation is different.