Extendable templates?

In my local python/module.rst I only need to override one block of the default template.

{% extends "python/module.rst" %}

{% block functions scoped %}
{% endblock %}

That fails, somewhat expectedly, with an RecursionError. My idea for a workaround was to inject a custom filter to make the path for the extend absolute.

def _autoapi_prepare_jinja_env(jinja_env):
        jinja_env.filters["base_template"] = lambda value: f'{autoapi.settings.TEMPLATE_DIR}/{value}'

autoapi_prepare_jinja_env = _autoapi_prepare_jinja_env

and then make the child template

{% extends "python/module.rst" | base_template %}

{% block functions scoped %}
{% endblock %}

That however fails since jinja's FileSystemLoader (or Environment?) always appends given paths as relative to its preset basedir(s). So then I hacked the jinja env further, including a new loader that has the filesystem root as the last search path.

autoapi_template_dir = '/my/local/template_dir'

def _autoapi_prepare_jinja_env(jinja_env):
    import jinja2
    tpl_dir = autoapi.settings.TEMPLATE_DIR
    jinja_env = jinja_env.overlay(loader=jinja2.FileSystemLoader([tpl_dir, autoapi_template_dir, '/']))
    jinja_env.filters["base_template"] = lambda value: f'{tpl_dir}/{value}'

Jinja still doesn't find the base template with the absolute path though, so I used a custom loader. Also the overlay doesn't actually use MyLoader.

autoapi_template_dir = this_dir / '_templates' / 'autoapi'

import jinja2
from os.path import join, exists, getmtime

class MyLoader(jinja2.FileSystemLoader):

    def get_source(self, environment, template):
        try:
            return super().get_source(environment, template)
        except jinja2.TemplateNotFound:
            path = template
            print(f"LOAD {template}")
            mtime = getmtime(template)
            with file(template) as f:
                source = f.read().decode('utf-8')
            return source, template, lambda: mtime == getmtime(template)

def _autoapi_prepare_jinja_env(jinja_env):
    tpl_dir = autoapi.settings.TEMPLATE_DIR
    jinja_env.filters["base_template"] = lambda value: f'{tpl_dir}/{value}'
    jinja_env.loader = MyLoader([tpl_dir, autoapi_template_dir])

This "works" as in I get no error.
Problem is AFAICT my custom get_source is only called once. For index.rst.

So bottom line I'm looking for either the problem with my workaround, or how I could generally make this workflow possible in autoapi, less hackish.

Start by reproducing the recursion with the local python/module.rst override and inspect _autoapi_prepare_jinja_env, the Jinja Environment/FileSystemLoader setup, and the index.rst load path. Define done as allowing a local template to extend the default template and override one block without recursion, while correctly loading inherited templates without a custom loader hack.