Is selectattr filter expecting all the dicts in the list to have the same keys?

More precisely, it is expecting all dicts in the list to have the attribute you are selecting on. If not all dict in the list have it, you will have to first filter out the items where it is not defined. This can be done with selectattr as well. (thanks @Randy for making this clearer since my initial answer).

In your situation, the json_query filter (which implements jmespath) can also do the job in sometimes a more compact manner. But it is not a core filter and requires to have the community.general collection installed.

Here are a few examples taken from your above requirements solved with both core filters and json_query solutions.

The playbook:

---
- name: "Filter data with core filters or json query"
  hosts: "localhost"
  gather_facts: false

  vars:
    # Your initial data on a single line for legibility
    test_var: [{"vm":"vm1","ip":"10.10.10.1"},{"vm":"vm2","ip":"10.10.10.2"},{"test_vm":"something","process_1":"X","process_2":"Y","process_3":"Z"},{"another_vm":"something_other"}]

  tasks:
    - name: Get objects having vm==vm1
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('vm', 'defined') | selectattr('vm', '==', 'vm1') | list }}
          With json_query: {{ test_var | json_query("[?vm=='vm1']") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

    - name: Get all objects having vm attribute
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('vm', 'defined') | list }}
          With json_query: {{ test_var | json_query("[?vm]") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

    - name: Get all objects having process_2 attribute
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('process_2', 'defined') | list }}
          With json_query: {{ test_var | json_query("[?process_2]") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

    - name: Get only a list of process_2 attributes
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('process_2', 'defined') | map(attribute='process_2') | list }}
          With json_query: {{ test_var | json_query("[].process_2") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

which gives:

PLAY [Filter data with core filters or json query] *********************************************************************

TASK [Get objects having vm==vm1] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: [{'vm': 'vm1', 'ip': '10.10.10.1'}]",
        "With json_query: [{'vm': 'vm1', 'ip': '10.10.10.1'}]"
    ]
}

TASK [Get all objects having vm attribute] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: [{'vm': 'vm1', 'ip': '10.10.10.1'}, {'vm': 'vm2', 'ip': '10.10.10.2'}]",
        "With json_query: [{'vm': 'vm1', 'ip': '10.10.10.1'}, {'vm': 'vm2', 'ip': '10.10.10.2'}]"
    ]
}

TASK [Get all objects having process_2 attribute] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: [{'test_vm': 'something', 'process_1': 'X', 'process_2': 'Y', 'process_3': 'Z'}]",
        "With json_query: [{'test_vm': 'something', 'process_1': 'X', 'process_2': 'Y', 'process_3': 'Z'}]"
    ]
}

TASK [Get only a list of process_2 attributes] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: ['Y']",
        "With json_query: ['Y']"
    ]
}

PLAY RECAP *********************************************************************
localhost                  : ok=4    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

It looks to me like you just need to drop the vars[] and it should work fine. Change vars[item.id] to just item.id.

Try:

- name: Set Filters
  set_fact:
    snapshot_list: "{{ ec2_snapshot_facts.snapshots | selectattr('volume_id', 'equalto', item.id ) | list }}"
  with_items: "{{ aws_ec2_volums_setting }}"

I tried this :

when: "{{ apps | selectattr('name', 'match', app_name) | list }}"

It seems that without the list filter, selectattr returning a generator object does not seem to allow converting to a boolean for the test evaluation. I'm not 100% sure about this interpretation though.

Note that having is defined or not would not change anything, it would be totally equivalent.