Jinja is one of powerful template engine in python.

Template Engine is exists for rendering certain formats(e.g. HTML, XML) with data. So you can reuse it once you make a template. Also, It is used for separating view from logic. You don't have to change view(template) although you change the logic.

I think, that's the reason it is not allowed to write pure python code in jinja.

Variables, Conditional Inclusion like if-then clause, Looping are allowed.

You can find out it here -> Link

But you can call functions to get output that you want from jinja.

from jinja2 import Template

template = "Example Function Call
            Function1 {{custom_function1(name)}}
            Function2 {{custom_function2(name)}}"
jinja_html_template = Template(template)

def template_function(func):
    jinga_html_template.globals[func.__name__] = func
    return func

@template_function
def custom_function1(name):
    return name + " custom_function1"

@template_function
def custom_function2(name):
    return name + " custom_function2"

fields = {'name': 'Sam'}
print(jinga_html_template.render(**fields))

I hope this answer helps you!!

Ciao