Jinja: NeverUndefined

End the lawless era of Jinja.

NeverUndefined is more restrictive than StrictUndefined. See this issue.

Usage

1. Install the package:

   pip3 install jinja2_neverundefined

2. Create a template file named template.j2 with the following content:

   {% macro test(a, b, c) %}
   a macro with three parameters but not used
   {%endmacro%}
   
   {{test()}}

3. Create a Python script named example.py with the following content:

   from jinja2 import Environment, FileSystemLoader
   from jinja2_neverundefined import NeverUndefined
   
   # Create Jinja2 environment with the extension
   env = Environment(
       loader=FileSystemLoader('.'),
       undefined=NeverUndefined
   )
   
   # Render the template
   template = env.get_template('template.j2')
   rendered = template.render()
   
   # Print the rendered output
   print(rendered)

4. Run the script:

   python3 example.py

The script will raise an exception because parameters a b c are not provided when calling macro test in the template.

Take another example:

{% macro mtest(a, b, c) %}
append undefined parameters to list
{% set s = [] %}
{% do s.append(a) %}
{{ s }}
{% endmacro %}
{{ mtest() }}

StrictUndefined will not throw error. It will produce:

append undefined parameters to list
[Undefined]

However, NeverUndefined will:

  File "template.j2", line 9, in top-level template code
    {{ mtest() }}
      └ <Macro 'mtest'>
  File "/usr/local/lib/python3.9/dist-packages/jinja2/runtime.py", line 777, in _invoke
    rv = self._func(*arguments)
         │           └ [missing, missing, missing]
         └ <Macro 'mtest'>
  File "template.j2", line 2, in template
    {% macro mtest(a, b, c) %}
  File "/usr/local/lib/python3.9/dist-packages/jinja2_neverundefined/__init__.py", line 14, in __init__
    raise Exception(info)
                    └ "parameter 'a' was not provided"
Exception: parameter 'a' was not provided

By using the NeverUndefined extension, the Jinja environment will raise an exception when encountering an undefined variable. This helps catch potential issues or missing variable bindings in your templates.