Template

synopsis:

template utilities in django-boost

Strict invalid template variables

StrictInvalidTemplateVariable raises an exception when a Django template tries to render an invalid or missing variable.

It provides Jinja2 StrictUndefined-like behavior for Django templates, which is useful when you want missing template variables to fail fast instead of rendering as empty strings.

Use it as Django’s string_if_invalid template option:

from django_boost.template import StrictInvalidTemplateVariable

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "DIRS": [],
        "APP_DIRS": True,
        "OPTIONS": {
            "string_if_invalid": StrictInvalidTemplateVariable(),
        },
    },
]

By default, missing variables raise ValueError with the missing template variable name in the message.

You can customize the message or exception class:

class TemplateVariableError(Exception):
    pass


TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "APP_DIRS": True,
        "OPTIONS": {
            "string_if_invalid": StrictInvalidTemplateVariable(
                message="Missing template variable: {name}",
                exception_class=TemplateVariableError,
            ),
        },
    },
]

Limitations

StrictInvalidTemplateVariable only raises on the resolution path Django routes through string_if_invalid % var. It does not cover every invalid lookup, so unlike Jinja2 StrictUndefined it is not strict everywhere:

• Tags that resolve variables while ignoring failures – {% if %}, {% for %}, {% firstof %} and the like – never reach it, so a missing variable there is still treated as falsy/empty instead of raising.

• A method marked alters_data or one that requires arguments yields Django’s invalid marker without the % step, so it renders the literal %s rather than raising.