Ansible - default to everything when no arguments are specified - deployment

Question

I have a fairly large playbook that is capable of updating up to 10 services on a given host.
Let's say I have the services a b c d and I'd like to be to be able to selectively update the services by passing command line arguments, but default to updating everything when no arguments are passed. How could you do this in Ansible without being able to drop into arbitrary scripting?
Right now what I have is a when check on each service and define whether or not the service is true at playbook invocation. Given I may have as many as 10 services, I can't write boolean logic to accommodate every possibility.
I was hoping there is maybe a builtin like $# in bash that lists all arguments and I can do a check along the lines of when: $#.length = 0
ansible-playbook deploy.yml -e "a=true b=true d=true"
when: a == "true"
when: b == "true"
when: c == "true"
when: d == "true"

Answer 1

I would suggest to use tags. Lets say we have two services for example nginx and fpm. Then tag the play for nginx with nginx and for fpm with fpm. Below is an example for task level tagging, let say its named play.yml
- name: tasks for nginx
service: name=nginx state=reloaded
tags:
- nginx
- name: tasks for php-fpm
service: name=php-fpm state=reloaded
tags:
- fpm
Exeucting ansible-playbook play.yml will by default run both the tasks. But, If i change the command to
ansible-playbook play.yml --tags "nginx"
then only the task with nginx tag is executed. Tags can also be applied over play level or role level.
Play level tagging would look like
- hosts: all
remote_user: user
tasks:
- include: play1.yml
tags:
- play1
- include: play2.yml
tags:
- play2
In this case, all tasks inside the playbook play1.yml will inherit the tag play1 and the same for play2. While running ansible-playbook with the tag play1 then all tasks inside play1.yml are executed. Rather if we dont specify any tag all tasks from play1 and play2 are executed.
Note: A tasks is not limited to just one tag.

Answer 2

If you have a single play that you want to loop over the services, define that list in group_vars/all or somewhere else that makes sense:
services:
- first
- second
- third
- fourth
Then your tasks in start_services.yml playbook can look like this:
- name: Ensure passed variables are in services list
fail:
msg: "{{ item }} not in services list"
when: item not in services
with_items: "{{ varlist | default(services) }}"
- name: Start services
service:
name: "{{ item }}"
state: started
with_items: "{{ varlist | default(services) }}"
Pass in varlist as a JSON array:
$ ansible-playbook start_services.yml --extra-vars='{"varlist":[first,third]}'