Jinja2 Recursive Rendering of a Deep Data Structure to YAML

Internal

• Jinja2

Overview

To render a recursive data structure to "flow style" YAML, there's not much to do, Jinja2 renders recursively by default to a flow style. See Render to Flow-Style YAML.

To render a recursive data structure to "block style" YAML, use a recursive macro, as shown below. See Render to Block-Style YAML.

The Template

{%- macro recursively_render_data_structure(d, overall_offset, recursion_level_indentation) -%}
{%- if d is boolean %}{{ d | lower }}
{%- elif d is number %}{{ d }}
{%- elif d is string %}"{{ d }}"
{%- elif d is mapping -%}
  {%- for key, value in d.items() %}
{{ overall_offset }}{{ ' ' * recursion_level_indentation }}{{ key }}: {{ recursively_render_data_structure(value, overall_offset, recursion_level_indentation + 2) }}
  {%- endfor %}
{%- elif d is iterable %}
  {%- for i in d %}
{{ overall_offset }}{{ ' ' * recursion_level_indentation }} - {{ recursively_render_data_structure(i, overall_offset, recursion_level_indentation + 4) }}
  {%- endfor %}
{%- else %}ERROR: UNKNOWN VALUE {{ d }}
{%- endif %}
{%- endmacro -%}

{{- recursively_render_data_structure(m, '      ', 0) }}

The Rendering Code

The code includes a test that the rendered structure is equivalent with the input:

import yaml
from jinja2 import Environment, FileSystemLoader
from pathlib import Path

tdir = Path('.')
template = Environment(loader=FileSystemLoader(tdir)).get_template('experimental-jinja-template.yaml.j2')
yaml_data = """
config:
  color: blue
  size: 1
  enabled: true
  samples:
    - name: venice
      neighborhoods:
        cannaregio: 1
        castello: 2
        giudecca: 3
    - name: genoa
      neighborhoods:
        molo: 1
        foce: 2
        marassi: 3
    - name: florence
      neighborhoods:
        santamaria: 1
        sanmarco: 2
        santacroce: 3
        no-such-neighborhood:
          - a1: v1
            a2: v2
          - b
          - c
"""
m = yaml.safe_load(yaml_data)
rendered_text = template.render(m=m)
print(rendered_text)

m2 = yaml.safe_load(rendered_text)
assert m == m2