os specific pre_task in ansible - ansible

Question

I would like to run a set of raw commands (to install python) if a given
operating system on the target host is OpenBSD
I have to run these checks as pre_tasks (because to run anything as a task Python must already be present on the target system)
What I do not understand is, whether any OS characteristics variables are available in pre_task stage (like ansible_os_family)
- name: Check for Python
raw: test -e /usr/bin/python
changed_when: false
failed_when: false
register: check_python
- name: Install Python on OpenBSD
# raw: test -e /usr/bin/apt && (apt -y update && apt install -y python-minimal) || (yum -y install python libselinux-python)
raw: pkg_add -r python%3
when: check_python.rc != 0
when: ansible_os_family == "OpenBSD" # <-- problem here
I seem to get a problem when trying to use ansible_os_family
Is there a way to enable the pre_tasks without me writing my own os family checks?
PS. I used the above python installation code, following recommendation here:
[1] https://relativkreativ.at/articles/how-to-install-python-with-ansible

Answer 1

Following your latest comments, as said earlier by #Vladimir and myself, you will not be able to use ansible's facts variables to detect the OS on hosts without python since it is impossible to gather facts (i.e. play the setup module). You need to do that job on your own for this first step.
I don't have a BSD system I can access. So the below is totally untested for your platform. But I believe it should work, or at least put you on track. Check the content of /etc/os-release on your system to adapt the regex if needed. The one I wrote gave a correct result on Ubuntu, Debian, Centos, RHEL and on RedHat UBI and Alpine docker images.
---
- name: Fix hosts with no python at all
hosts: all
gather_facts: false
tasks:
- name: Perform a dirty OS detection if python is not installed
raw: |-
if which python > /dev/null || which python3 > /dev/null; then
echo installed
else
sed -n "s/^NAME=\"\(.*\)\"/\\1/p" /etc/os-release
fi
changed_when: false
register: dirty_detect
- name: Print message when already installed
debug:
msg: Host {{ inventory_hostname }} already has python. Next will skip.
when: dirty_detect.stdout_lines[0] == 'installed'
- name: Install python on openbsd if needed
raw: |-
pkg_add -r python%3
become: true
when: dirty_detect.stdout_lines[0] == 'OpenBSD'
- name: Install python on Ubuntu if needed
raw: |-
apt install -y python3
become: true
when: dirty_detect.stdout_lines[0] == 'Ubuntu'
- name: Back to normal life with ansible
hosts: all
tasks:
- name: Now we gathered facts and we can use all facts vars
debug:
msg: Host {{ inventory_hostname }} is running {{ ansible_os_familly }}
- name: We might have left hosts with python 2.7 only above, make sure python3 is installed
package:
name: python3
become: true

Answer 2

Q: "Is there a way to enable the pre_tasks without me writing my own os family checks?"
A: No. It is not. Python is needed to collect the facts.
See the verbose output. For example the output of connecting FreeBSD 12 remote host from the Ubuntu controller
shell> ansible --version
ansible 2.9.6
...
python version = 3.8.2 (default, Apr 27 2020, 15:53:34) [GCC 9.3.0]
shell> ansible test_01 -m setup -vvv
ESTABLISH SSH CONNECTION FOR USER: admin
SSH: EXEC ssh -C -o ControlMaster=auto -o ControlPersist=60s -o KbdInteractiveAuthentication=no -o PreferredAuthentications=gssapi-with-mic,gssapi-keyex,hostbased,publickey -o PasswordAuthentication=no -o 'User="admin"' -o ConnectTimeout=30 -o ControlPath=/export/home/admin.config/.ansible/cp/5a3ab05cf7 test_01 '/bin/sh -c '"'"'/usr/local/bin/python3.7 && sleep 0'"'"''
Without Python installed the module will crash
(127, b'', b'/bin/sh: /usr/bin/python: not found\n')
Failed to connect to the host via ssh: /bin/sh: /usr/bin/python: not found
[WARNING]: No python interpreters found for host test_01 (tried ['/usr/bin/python',
'python3.7', 'python3.6', 'python3.5', 'python2.7', 'python2.6', '/usr/libexec/platform-
python', '/usr/bin/python3', 'python'])
test_01 | FAILED! => {
"ansible_facts": {
"discovered_interpreter_python": "/usr/bin/python"
},
"changed": false,
"module_stderr": "/bin/sh: /usr/bin/python: not found\n",
"module_stdout": "",
"msg": "The module failed to execute correctly, you probably need to set the interpreter.\nSee stdout/stderr for the exact error",
"rc": 127
}