Ansible - Compare dictionaries works but will not accept an addition - ansible

Question

I am trying to compare two dictionaries and create a 3rd dictionary if an item has changed or is not present. I have this playbook
---
- hosts: localhost
connection: local
gather_facts: no
vars:
host_list1:
host1: 1.1.1.1
host2: 2.2.2.2
host3: 3.3.3.3
host_list2:
host1: 1.1.1.1
host2: 2.2.2.2
host3: 3.3.3.3
tasks:
- name: Define Dict
set_fact:
diff_list: {}
- name: Compare lists
set_fact:
diff_list: "{{ diff_list|combine({item: host_list1[item]}) }}"
loop: "{{ host_list1.keys()|list }}"
when: host_list1[item] != host_list2[item]
- name: Display Results
debug:
msg: "{{ diff_list }}"
If I change the IP in host_list1 it has the desired result and will update the diff_list.
PLAY [localhost] **************************************************************************************************************************************************************
TASK [Define Dict] ************************************************************************************************************************************************************
ok: [localhost]
TASK [Compare lists] **********************************************************************************************************************************************************
ok: [localhost] => (item=host3)
ok: [localhost] => (item=host2)
skipping: [localhost] => (item=host1)
TASK [Display Results] ********************************************************************************************************************************************************
ok: [localhost] => {
"msg": {
"host2": "2.2.2.5",
"host3": "3.3.3.4"
}
}
PLAY RECAP ********************************************************************************************************************************************************************
localhost : ok=3 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
However if I try and add another host into the host_list1 dictionary I get the following error
PLAY [localhost] **************************************************************************************************************************************************************
TASK [Define Dict] ************************************************************************************************************************************************************
ok: [localhost]
TASK [Compare lists] **********************************************************************************************************************************************************
fatal: [localhost]: FAILED! => {"msg": "The conditional check 'host_list1[item] != host_list2[item]' failed. The error was: error while evaluating conditional (host_list1[item] != host_list2[item]): 'dict object' has no attribute u'host4'\n\nThe error appears to be in '/home/playbooks/test.yml': line 20, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n - name: Compare lists\n ^ here\n"}
PLAY RECAP ********************************************************************************************************************************************************************
localhost : ok=1 changed=0 unreachable=0 failed=1 skipped=0 rescued=0 ignored=0
I have tried updating the when statement
when: host_list1[item] != host_list2[item] or not defined
However this does not work and I am unable to get the new host appended to the diff_list dictionary

Answer 1

The problem is your conditional statement:
when: host_list1[item] != host_list2[item]
If item is key that is present in only host_list1, then host_list2[item] is an error. There are (at least!) two ways of fixing this.
Use a default value
You can use the default filter to provide a default value when the result of an item lookup is undefined:
when: host_list1[item] != host_list2[item]|default(none)
Explicitly check for membership
You can check that the key exists in host_list2 before attempting to get its value:
when: item not in host_list2 or host_list1[item] != host_list2[item]

Answer 2

You can get your result using a single jinja2 expression without using any tasks.
The following playbook:
---
- hosts: localhost
connection: local
gather_facts: no
vars:
hosts_current:
host1: 1.1.1.1
host2: 2.2.2.3
host3: 3.3.3.4
host4: 5.5.5.5
hosts_origin:
host1: 1.1.1.1
host2: 2.2.2.2
host3: 3.3.3.3
diff_dict: >-
{{
hosts_origin | dict2items
| symmetric_difference(hosts_current | dict2items)
| items2dict
}}
tasks:
- name: Show the calculated var
debug:
var: diff_dict
gives:
PLAY [localhost] **************************************************************************************************************************************************************************************************************
TASK [Show the calculated var] ************************************************************************************************************************************************************************************************
ok: [localhost] => {
"diff_dict": {
"host2": "2.2.2.3",
"host3": "3.3.3.4",
"host4": "5.5.5.5"
}
}
PLAY RECAP ********************************************************************************************************************************************************************************************************************
localhost : ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0