Execute task when: "'Key' not in object"

[Rob Wilkerson]

I have a playbook that's creating _n_ EC2 instances using exact_count. I'd like to tag those instances with a semi-meaningful Name value. I can get a list of all of my tagged instances from the ec2 module return value using {{ servers.tagged_instances }}, but...

I only want to apply a Name tag to those that don't already have one.

- name: Instances | Tag each instance with a name

  ec2_tag:

    aws_access_key: "{{ awscli.access_key }}"

    aws_secret_key: "{{ awscli.secret_key }}"

    region: "{{ aws.region }}"

    resource: "{{ item.1.id }}"

    state: present

    tags:

      Name: "{{ env_short }}-{{ 'ws%02d' | format(item.0) }}"

  with_indexed_items: webservers.tagged_instances

  when: "'Name' not in item.tags.keys()"  # Untagged instances only

I've tried several variations of the `when` stanza, but nothing's worked. Likewise, I haven't found any documentation with an example like this. Is it possible?