Ansible check if variables are set - ansible

Question

I want to automate the installation process of our software for our client. Therefore I wrote an Ansible playbook which has a task which should check if all the mandatory variables are set:
- name: Check environment variables.
hosts: all
vars_files:
- required_vars.yml
tasks:
- fail: msg="Variable '{{ item }}' is not defined"
when: item not in hostvars[inventory_hostname]
with_items:
- required_vars
The required_vars.yml looks like this:
required_vars:
- APPHOME: /home/foo/bar
- TMPDIR: /home/foo/bar/tmp
When I execute the playbook via ansible-playbook -i inventory/dev.yml playbook.yml I get the following error:
TASK [Gathering Facts] *************************************************************************************************************************************************************************************************************************ok: [localhost]
TASK [fail] ************************************************************************************************************************************************************************************************************************************failed:
[localhost] (item=required_vars) => {"changed": false, "failed": true, "item": "required_vars", "msg": "Variable 'required_vars' is not defined"}
It is obvious that I am doing something wrong, but I cannot point to the error. Can you help me please?
Edit: the accepted answer helped me out. Thank you.
But I have two more questions:
The executed task says:
TASK [fail]
skipping: [some_ip] => (item=/root)
skipping: [some_ip] => (item=TMPDIR: /home/foo/bar/tmp)
It is getting skipped because all variables are set, correct?
I think I figured out how to print the correct message, if the variable is not set:
- name: Check environment variables.
hosts: all
vars_files:
- required_vars.yml
tasks:
- fail:
msg: "Variable '{{ item }}' is not defined"
with_items: "{{ required_vars }}"
when: item is undefined
Correct? Or is there a better solution?

Answer 1

Two problems here:
You want to iterate over the value of required_vars variable value, so you need to provide it as an argument to with_items: "{{ required_vars }}":
with_items: "{{ required_vars }}"
Currently you are providing a list of a single element with a statically defined string required_vars.
You need to change the data type of the elements in your required_vars list to strings:
required_vars:
- "APPHOME: /home/foo/bar"
- "TMPDIR: /home/foo/bar/tmp"
Currently (because of : followed by space) you defined dictionaries, so for example in the first iteration item will have a value of { "APPHOME": "/home/foo/bar" }, which will then always fail on the when condition.
Bonus problem:
you defined a message in the form "Variable '{{ item }}' is not defined";
Ansible reports Variable 'required_vars' is not defined;
the above is not an error, as you think ("I get the following error"), but a correct result of the fail module with the message you defined yourself.

Answer 2

Since you have only one value for 'with_items' I think it should look like this:
with_items: "{{ required_vars }}"
On one line and with the brackets and quotation marks. Once you have more then one item, you can use the list like you did:
with_items:
- "{{ one }}"
- "{{ two }}"