AnsibleByExample

Ansible List — No Attribute Length

BeginnerAnsible errorlist objectJinja2 filterlength attribute
2024-01-015By Luca Berton

Lesson 1 of 20 in Production Playbooks

Production Playbooks and Troubleshooting — full curriculum
  1. 1. Advanced Playbook Structure & Execution Strategies
  2. 2. Deep Dive: Variable Precedence & Best Practices
  3. 3. Jinja2 Templating Beyond Basics
  4. 4. Advanced Conditionals
  5. 5. Loops Masterclass
  6. 6. Blocks, Rescue, Always (Error Handling)
  7. 7. Handlers Advanced
  8. 8. Change Control: changed_when, failed_when
  9. 9. Tags and Selective Runs
  10. 10. Performance Tuning
  11. 11. Delegation Patterns
  12. 12. Local Actions and Remote Actions
  13. 13. Dynamic Inventory & Runtime Host Management
  14. 14. Includes & Imports (Structuring Large Codebases)
  15. 15. Role Design Patterns
  16. 16. Ansible Galaxy & Collections in Real Projects
  17. 17. Working with APIs (uri + JSON workflows)
  18. 18. Testing and Linting (Quality Gates)
  19. 19. Advanced Debugging Playbook Failures
  20. 20. Capstone: Multi-Stage Deployment
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Introduction

The error list object has no attribute length is one of the most common Jinja2 mistakes in Ansible. It occurs when you try to access length as a Python attribute (.length) instead of using Jinja2's length filter (| length). This guide explains why, shows the fix, and covers all the essential Jinja2 list operations you should know.

The Error

# WRONG — causes "list object has no attribute length"
- name: Display list length
  ansible.builtin.debug:
    msg: "{{ my_list.length }}"

Error output:

fatal: [localhost]: FAILED! => {"msg": "The task includes an option with an undefined variable. 
The error was: 'list object' has no attribute 'length'"}

The Fix

Use the | length Jinja2 filter:

# CORRECT
- name: Display list length
  ansible.builtin.debug:
    msg: "{{ my_list | length }}"

Why This Happens

Ansible uses Jinja2 for templating, not raw Python. In Jinja2:

  • Attributes (dot notation) access object properties: my_dict.key
  • Filters (pipe notation) transform values: my_list | length

Python lists have a len() function, not a .length attribute. Jinja2 provides the length filter as the equivalent:

PythonJinja2 (Ansible)
len(my_list){{ my_list | length }}
my_list.sort(){{ my_list | sort }}
", ".join(my_list){{ my_list | join(", ") }}

Practical Examples

Count Items in a List

vars:
  packages:
    - nginx
    - postgresql
    - redis
    - memcached

tasks:
  - name: Show package count
    ansible.builtin.debug:
      msg: "Installing {{ packages | length }} packages"
    # Output: "Installing 4 packages"

Conditional Based on List Length

- name: Warn if too many hosts
  ansible.builtin.debug:
    msg: "WARNING: {{ ansible_play_hosts | length }} hosts in play — consider using serial"
  when: ansible_play_hosts | length > 20

Check If List Is Empty

- name: Skip if no errors found
  ansible.builtin.debug:
    msg: "Processing {{ errors | length }} errors"
  when: errors | length > 0

# Alternative: use truthiness
- name: Skip if no errors (simpler)
  ansible.builtin.debug:
    msg: "Processing errors"
  when: errors  # Empty list is falsy

Use Length in Loops

- name: Show progress
  ansible.builtin.debug:
    msg: "Processing {{ ansible_loop.index }} of {{ my_list | length }}: {{ item }}"
  loop: "{{ my_list }}"
  loop_control:
    extended: true

Essential Jinja2 List Filters

Beyond length, here are the most useful list filters in Ansible:

Sorting

# Sort ascending
msg: "{{ my_list | sort }}"

# Sort descending
msg: "{{ my_list | sort(reverse=true) }}"

# Sort by attribute
msg: "{{ users | sort(attribute='name') }}"

Joining

# Join with comma
msg: "{{ packages | join(', ') }}"
# Output: "nginx, postgresql, redis"

# Join for command line
msg: "apt install {{ packages | join(' ') }}"

Filtering

# Select items matching criteria
msg: "{{ numbers | select('greaterthan', 5) | list }}"

# Reject items
msg: "{{ packages | reject('equalto', 'redis') | list }}"

# Select by attribute
msg: "{{ users | selectattr('active', 'equalto', true) | list }}"

Extracting

# Get unique values
msg: "{{ tags | unique }}"

# Get first/last
msg: "{{ my_list | first }}"
msg: "{{ my_list | last }}"

# Flatten nested lists
msg: "{{ nested_list | flatten }}"

# Extract attribute from list of dicts
msg: "{{ users | map(attribute='name') | list }}"

Set Operations

# Union (combine unique)
msg: "{{ list1 | union(list2) }}"

# Intersection (common items)
msg: "{{ list1 | intersect(list2) }}"

# Difference
msg: "{{ list1 | difference(list2) }}"

# Symmetric difference
msg: "{{ list1 | symmetric_difference(list2) }}"

Math on Lists

# Sum
msg: "{{ numbers | sum }}"

# Min/Max
msg: "{{ numbers | min }}"
msg: "{{ numbers | max }}"

Similar Attribute Errors

The same pattern applies to other common mistakes:

# WRONG                          # CORRECT
{{ my_list.sort() }}             {{ my_list | sort }}
{{ my_list.reverse() }}          {{ my_list | reverse | list }}
{{ my_string.upper() }}          {{ my_string | upper }}
{{ my_string.lower() }}          {{ my_string | lower }}
{{ my_string.strip() }}          {{ my_string | trim }}
{{ my_dict.keys() }}             {{ my_dict | dict2items | map(attribute='key') | list }}
{{ my_list.append('x') }}        # Not possible — use: {{ my_list + ['x'] }}

Real-World Patterns

Dynamic Inventory Sizing

- name: Calculate worker count
  ansible.builtin.set_fact:
    worker_count: "{{ [ansible_processor_vcpus, groups['workers'] | length] | min }}"

Validate Input Lists

- name: Ensure at least 3 DNS servers configured
  ansible.builtin.assert:
    that:
      - dns_servers | length >= 3
    fail_msg: "Need at least 3 DNS servers, got {{ dns_servers | length }}"

Batch Processing

- name: Process in batches of 10
  ansible.builtin.debug:
    msg: "Batch {{ ansible_loop.index }}: {{ item }}"
  loop: "{{ my_list | batch(10) | list }}"
  loop_control:
    extended: true

Conclusion

The list object has no attribute length error has a one-line fix: replace .length with | length. The underlying principle is simple — Jinja2 uses filters (pipe syntax) for operations that Python handles with methods (dot syntax). Once you internalize this pattern, you'll avoid an entire class of Ansible templating errors: use | sort not .sort(), | upper not .upper(), | join not .join().

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