Invalid filter: length_is Error in Django Template

Verified FixPython 3.10+Django 4.2+ / 5.0+Silo: django

Quick Fix / Solution Rapide

Replace {% if items|length_is:5 %} with the modern comparison syntax {% if items|length == 5 %} using Django's built-in length filter.

Root Cause Analysis

This error occurs when Python tries to parse a Django template containing the obsolete length_is filter, but modern versions of Django (Django 4.0+) have permanently removed this filter from the default template engine tags library.

Deprecation Timeline of `length_is`

In Django 1.x and early 2.x, template conditional tags ({% if %}) did not support flexible comparison operators like ==, !=, <, or >. To check the length of a list, Django provided the dedicated helper filter |length_is:N.

When Django introduced full expression parsing in the {% if %} tag (allowing operators such as == and mathematical comparisons), length_is became redundant. It was formally deprecated in Django 2.2 and completely removed in Django 4.0.

How the Bug Surfaces

Upgrading Legacy Codebases: Migrating older Django 2.2 or 3.2 LTS projects to Django 4.2 or 5.0.
Copying Outdated Tutorials: Reusing template snippets from legacy documentation or older blog posts.
Third-Party Reusable Apps: Installing unmaintained Django packages that retain deprecated template filters.

Reproduction Code (MCVE)

Example: Bug Reproduction

import django
from django.conf import settings
from django.template import Template

if not settings.configured:
    settings.configure(TEMPLATES=[{'BACKEND': 'django.template.backends.django.DjangoTemplates'}])
    django.setup()

# In modern Django, 'length_is' is no longer a valid filter and raises TemplateSyntaxError
invalid_template = '{% if users|length_is:3 %}Found 3 users{% endif %}'
Template(invalid_template)

Solution 1: Use `length` Filter with Comparison Operator `==`

Replace |length_is:N with |length == N directly inside the standard {% if %} template block.

Example: Recommended Solution

import django
from django.conf import settings
from django.template import Template, Context

if not settings.configured:
    settings.configure(TEMPLATES=[{'BACKEND': 'django.template.backends.django.DjangoTemplates'}])
    django.setup()

# Modern idiom: use length filter with standard == comparison
valid_template = Template('{% if users|length == 3 %}Match: {{ users|length }} users found.{% endif %}')
rendered = valid_template.render(Context({'users': ['Alice', 'Bob', 'Carol']}))

print(f'Successfully rendered: {rendered.strip()}')

Solution 2: Compute Length in the View or Custom Template Tag

For complex business logic, calculate counts in the Python view function before passing context to the template.

Example: Alternative Solution

import django
from django.conf import settings
from django.template import Template, Context

if not settings.configured:
    settings.configure(TEMPLATES=[{'BACKEND': 'django.template.backends.django.DjangoTemplates'}])
    django.setup()

# Pre-calculate boolean conditions in Python views for cleaner templates
items = [10, 20, 30, 40, 50]
context_data = {
    'items': items,
    'is_exact_count': len(items) == 5
}

template = Template('{% if is_exact_count %}Inventory count verified: 5 items.{% endif %}')
print(template.render(Context(context_data)).strip())

Common Pitfalls & Error Contrasts

A subtle mistake when migrating templates is writing {% if items|length = 5 %} (single equals sign), which triggers TemplateSyntaxError: Could not parse the remainder: '= 5'. Always use double equals == for comparison in template tags.

Contrasting TemplateSyntaxError with runtime template errors:

TemplateSyntaxError: Invalid filter: 'length_is': Parsing-time error indicating an unrecognized filter name.
TemplateDoesNotExist: Loader error when the HTML template file cannot be found in TEMPLATES['DIRS'].
VariableDoesNotExist: Handled silently in Django templates by defaulting to an empty string (string_if_invalid).

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