Ansible中with_items与loop的区别是什么？

Great question! As someone who's worked with Ansible across multiple versions, I can break down the core distinctions between these two loop methods clearly:

1. Version Legacy vs. Modern Standard #

• with_items is the original loop syntax, predating Ansible 2.4. While it still works in newer releases, it's classified as legacy—Ansible's docs strongly recommend using loop for all new playbook development.
• loop was introduced in Ansible 2.4 as part of a unified loop system, and it's now the official, supported way to handle iterations in current Ansible versions. It's actively maintained and gets new features.

2. Implicit vs. Explicit List Handling #

This is one of the most noticeable differences in practice:

• with_items uses the older items lookup plugin under the hood, which automatically flattens nested lists. For example, if you pass [1, [2, 3], 4] to with_items, it will iterate over 1, 2, 3, 4 as separate items.
• loop iterates directly over the given iterable without any implicit flattening. The same nested list [1, [2, 3], 4] would result in three iterations: 1, [2,3], 4. If you want flattening with loop, you have to explicitly use the flatten filter, like loop: "{{ my_nested_list | flatten }}".

3. Feature Compatibility & Scoping #

• with_items has limited support for modern Ansible features. For example, it doesn't integrate as cleanly with advanced loop_control options (like labeling items to make output more readable, setting iteration pauses, or limiting loop runs).
• loop was built to handle variable scoping more reliably, especially in nested loop scenarios. It also works seamlessly with modern lookup functions like query() (the recommended replacement for most older lookup plugins) to generate dynamic iterables.

Quick Example to Illustrate Flattening #

Using with_items (automatic flattening): #

- name: Iterate with with_items
  debug:
    msg: "Current item: {{ item }}"
  with_items:
    - apple
    - [banana, cherry]

Output:

ok: [localhost] => (item=apple) => {
    "msg": "Current item: apple"
}
ok: [localhost] => (item=banana) => {
    "msg": "Current item: banana"
}
ok: [localhost] => (item=cherry) => {
    "msg": "Current item: cherry"
}

Using loop (no implicit flattening): #

- name: Iterate with loop
  debug:
    msg: "Current item: {{ item }}"
  loop:
    - apple
    - [banana, cherry]

Output:

ok: [localhost] => (item=apple) => {
    "msg": "Current item: apple"
}
ok: [localhost] => (item=['banana', 'cherry']) => {
    "msg": "Current item: ['banana', 'cherry']"
}

To get the same flattening as with_items with loop, adjust it to:

loop: "{{ ['apple', ['banana', 'cherry']] | flatten }}"

内容的提问来源于stack exchange，提问作者Mohd Abdul Mujib