Jinja templating with recursive in dict doesn't works

I'm stuck in a Jinja implementation problem.

Here is my little python script:

path = Path(__file__).parent

env = Environment(
    loader=FileSystemLoader(path / "templates")
)
template = env.get_template("template1.rst")

rendered = template.render(sample={"a": {"b": "c"}})

And here is my template for jinja:

.. toctree::
   :maxdepth: 3

{% for k, v in sample.items() recursive %}
- {{ k }}:

  {%- if v is string %}
    {{ v }}

  {%- else %}
    {{ loop(v) }}
  
  {%- endif -%}

{%endfor%}

The execution returns this error:

File "/home/jaja/Bureau/coding/bac_a_sable/sphinx_test/templates/template1.rst", line 5, in top-level template code
    {% for k, v in sample.items() recursive %}
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jaja/Bureau/coding/bac_a_sable/sphinx_test/templates/template1.rst", line 21, in template
    {{ loop(v) }}
^^^^^^^^^^^^^^^^^^
  File "/home/jaja/Bureau/coding/bac_a_sable/sphinx_test/templates/template1.rst", line 5, in template
    {% for k, v in sample.items() recursive %}
    ^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: not enough values to unpack (expected 2, got 1)

As the v value of first loop is {"b": "c"}, it must work, but it doesn't.

Is Jinja unable to recursively loop in dictionaries ?

immediate fix

When you start the loop, you use sample.items() iterator -

{% for k, v in sample.items() recursive %}
               ^^^^^^^^^^^^^^

When you recur, you are passing the dict itself -

  {%- else %}
    {{ loop(v) }}
            ^

Simply change this to -

  {%- else %}
    {{ loop(v.items()) }}
            ^^^^^^^^^

---

naive test

An improvement to the entire loop would be to change the test to mapping, instead of string.

{% for k, v in sample.items() recursive %}
- {{ k }}:

  {%- if v is mapping %}
    {{ loop(v.items()) }}

  {%- else %}
    {{ v }}
  
  {%- endif -%}

{%endfor%}

This ensures that you only recur on dicts. In the original code, the else will recur any non-string input. If the data included a number, you would've encountered a different error.

---

nested input, nested output

Preserving the levels of nesting in the output can be challenging. Consider using the loop.depth helper to create the correct whitespace -

{% for k, v in sample.items() recursive %}
{{ "  " * (loop.depth - 1) }}- {{ k }}: 
  {%- if v is mapping %} {{- loop(v.items()) }}
  {%- else %} {{ v }}
  {%- endif -%}
{%endfor%}

Given sample input -

sample = {
  'a': {
    'b': 1,
    'c': { 'd': 2 },
    'e': 'f',
  },
}

Output

- a: 
  - c: 
    - d: 2
  - b: 1
  - e: f