Ansible tags with 'include'

When using the include directive things do not quite work as I would expect when I also add a tag to the include section.

Note: this was tested with Ansible 2.5.2 and python 2.7.15

Scenario

I split the playbook of the previous example into two:
The tasks (tagged in the same fashion as before) have been moved to a new file include_this.yml:

---

- name: Debug No Tag
  debug:
    msg: "No tag"

- name: Debug Tag A
  debug:
    msg: "Tag A"
  tags:
    - tagA

- name: Debug Tag B
  debug:
    msg: "Tag B"
  tags:
    - tagB

- name: Debug Tag A and B
  debug:
    msg: "Tag A and B"
  tags:
    - tagA
    - tagB

The playbook playbook.yml has been reduced to two tasks:

• one debug task
• one include task

I am testing two versions: the second one has an additional tag for the include directive.

Version 1	Version 2
- name: Debug Playbook hosts: localhost tasks: - name: Debug No Tag in Playbook debug: msg: "No tag in playbook" - name: Include include: include_this.yml	- name: Debug Playbook hosts: localhost tasks: - name: Debug No Tag in Playbook debug: msg: "No tag in playbook" - name: Include include: include_this.yml tags: - tagA

The playbook is run as

ansible-playbook [--tags tagA] [--skip-tags tagB] playbook.yml

Results for version 1

If called with --tags or --skip-tags version 1 delivers the same results as described in my previous post when the include file was part of the playbook.
The additional 5th task in playbook.yml is executed in case of --skip-tags tagB or when no arguments are supplied.

no args	TASK [Debug No Tag in Playbook] TASK [Debug No Tag] TASK [Debug Tag A] TASK [Debug Tag B] TASK [Debug Tag A and B]
--tags tagA	TASK [Debug Tag A] TASK [Debug Tag A and B]
--tags tagA --skip-tags tagB	TASK [Debug Tag A]
--skip-tags tagB	TASK [Debug No Tag in Playbook] TASK [Debug No Tag] TASK [Debug Tag A]
Now I am reversing tagA and tagB in the call which leads to the expected results.
--tags tagB	TASK [Debug Tag B] TASK [Debug Tag A and B]
--tags tagB --skip-tags tagA	TASK [Debug Tag B]

Results for version 2

no args	TASK [Debug No Tag in Playbook] TASK [Debug No Tag] TASK [Debug Tag A] TASK [Debug Tag B] TASK [Debug Tag A and B]	Same as version 1. All five tasks are being executed for both version of the playbook. This works as expected. No restrictions of any sort apply.
--tags tagA	TASK [Debug No Tag] TASK [Debug Tag A] TASK [Debug Tag B] TASK [Debug Tag A and B]	All four task of the included files are being executed. No filtering by tagA for the included file takes place. This was a complete suprise to me. It seems that the tagging of the include step supersedes somehow the tags of the included file.
--tags tagA --skip-tags tagB	TASK [Debug No Tag] TASK [Debug Tag A]	The negative filter is applied to the result before and leaves two tasks for execution.
--skip-tags tagB	TASK [Debug No Tag in Playbook] TASK [Debug No Tag] TASK [Debug Tag A]	Same as version 1. The two tasks with tagB are skipped and the other three are executed.
--tags tagB	TASK [Debug Tag B] TASK [Debug Tag A and B]	Same as version 1. The two tasks with tagB are executed and the tag setting in the include directive is not taken into account.
--tags tagB --skip-tags tagA	... nothing ...	Now here is a surprise. The --skip-tags tagA has skipped the whole include file and no task is being executed at all.

An explanation using sets

Let's look at it from a set perspective.

--tags tagA	--skip-tags tagA
These two results are complementary and - if joined - build the complete set.
TASK [Debug No Tag] TASK [Debug Tag A] TASK [Debug Tag B] TASK [Debug Tag A and B]	TASK [Debug No Tag in Playbook]
Here the same complimentary sets for tagB.
--tags tagB	--skip-tags tagB
TASK [Debug Tag B] TASK [Debug Tag A and B]	TASK [Debug No Tag in Playbook] TASK [Debug No Tag] TASK [Debug Tag A]

Now any invocation of --skip-tags leads to an intersection with the respective sets.
Example: --tags tagB --skip-tags tagA: the sets have no task in common and thus nothing will be executed.

Conclusion

My idea was that a tag for the include directive would determine whether the include happens or not. Obviously wrong. It seems that it only determines which other tasks in the playbook.yml are executed i.e. it prohibits the untagged task from being run. The include takes place in any case (see the examples when using tagB either in --tags or --skip-tags) but in a strange way since only other tags than the supplied one are applied. I find this hard to remember or explain so my personal rule of mutual tag exclusion will be:

• tagged include section => no tags in the included file
• tags in the included file => no tag for the include section