ansible "with_lines: cat somefile" fails in a skipped block - ansible

Question

I have a playbook with a block that has a when condition. Inside is a task with a loop. How can I change this loop so that when the condition is false the skipped task doesn't fail?
block:
- name: create a file
lineinfile:
line: "Hello World"
path: "{{my_testfile}}"
create: yes
- name: use the file
debug:
msg: "{{ item}}"
with_lines: cat "{{my_testfile}}"
when: false
TASK [create a file] ************************************************************************************************************************************************************
TASK [use the file] *************************************************************************************************************************************************************
cat: files/my/testfile: No such file or directory
fatal: [ipad-icpi01]: FAILED! => {"msg": "lookup_plugin.lines(cat \"files/mytestfile\") returned 1"}

Answer 1

Change your failing task to the following which will always be able to run, even if the file does not exists, and will not use the shell or command where there is no need to:
- name: use the file
debug:
msg: "{{ item }}"
loop: "{{ (lookup('file', my_testfile, errors='ignore') | default('', true)).split('\n') }}"
The key points:
use the file lookup plugin with errors='ignore' so that it returns the file content or None rather than an error when file does not exists.
use the default filter with second option to true so that it return default value if var exists but is null or empty.
split the result on new lines to get a list of lines (empty list if file does not exist).
Note: as reported by #Vladimir, I corrected your var name which is not valid in ansible.

Answer 2

Test the existence of the file. For example
- block:
- name: create a file
lineinfile:
line: "Hello World"
path: "{{ my_testfile }}"
create: yes
- name: use the file
shell: '[ -f "{{ my_testfile }}" ] && cat {{ my_testfile }}'
register: result
- name: use the file
debug:
msg: "{{ item }}"
loop: "{{ result.stdout_lines }}"
when: false
The lookup plugin file should be preferred.

Answer 3

I ended up with a mix of the provided answers. These tasks will be skipped without failing or creating a warning.
- block:
- name: create a file
lineinfile:
line: "Hello World"
path: "{{ my_testfile }}"
create: yes
- name: get the file
slurp:
src: "{{ my_testfile }}"
register: result
- name: use the file
debug:
msg: "{{ item }}"
loop: "{{ (result['content'] | b64decode).split('\n') }}"
when: false