How Exactly Does Ansible Parse Boolean Variables? - boolean

Question

In Ansible, there are several places where variables can be defined: in the inventory, in a playbook, in variable files, etc. Can anyone explain the following observations that I have made?
When defining a Boolean variable in an inventory, it MUST be capitalized (i.e., True/False), otherwise (i.e., true/false) it will not be interpreted as a Boolean but as a String.
In any of the YAML formatted files (playbooks, roles, etc.) both True/False and true/false are interpreted as Booleans.
For example, I defined two variables in an inventory:
abc=false
xyz=False
And when debugging the type of these variables inside a role...
- debug:
msg: "abc={{ abc | type_debug }} xyz={{ xyz | type_debug }}"
... then abc becomes unicode but xyz is interpreted as a bool:
ok: [localhost] => {
"msg": "abc=unicode xyz=bool"
}
However, when defining the same variables in a playbook, like this:
vars:
abc: false
xyz: False
... then both variables are recognized as bool.
I had to realize this the hard way after executing a playbook on production, running something that should not have run because of a variable set to 'false' instead of 'False' in an inventory. Thus, I'd really like to find a clear answer about how Ansible understands Booleans and how it depends on where/how the variable is defined. Should I simply always use capitalized True/False to be on the safe side? Is it valid to say that booleans in YAML files (with format key: value) are case-insensitive, while in properties files (with format key=value) they are case-sensitive? Any deeper insights would be highly appreciated.

Answer 1

Variables defined in YAML files (playbooks, vars_files, YAML-format inventories)
YAML principles
Playbooks, vars_files, and inventory files written in YAML are processed by a YAML parser first. It allows several aliases for values which will be stored as Boolean type: yes/no, true/false, on/off, defined in several cases: true/True/TRUE (thus they are not truly case-insensitive).
YAML definition specifies possible values as:
y|Y|yes|Yes|YES|n|N|no|No|NO
|true|True|TRUE|false|False|FALSE
|on|On|ON|off|Off|OFF
Ansible docs confirm that:
You can also specify a boolean value (true/false) in several forms:
create_key: yes
needs_agent: no
knows_oop: True
likes_emacs: TRUE
uses_cvs: false
Variables defined in INI-format inventory files
Python principles
When Ansible reads an INI-format inventory, it processes the variables using Python built-in types:
Values passed in using the key=value syntax are interpreted as Python literal structure (strings, numbers, tuples, lists, dicts, booleans, None), alternatively as string. For example var=FALSE would create a string equal to FALSE.
If the value specified matches string True or False (starting with a capital letter) the type is set to Boolean, otherwise it is treated as string (unless it matches another type).
Variables defined through --extra_vars CLI parameter
All strings
All variables passed as extra-vars in CLI are of string type.

Answer 2

The YAML principles define the possible Boolean values that are accepted by Ansible. However after parsing only two values remain (true and false), these are valid in JSON too, so if you do some things with these values in Ansible, then true and false are good choices. Also the Ansible documentation states
Use lowercase ‘true’ or ‘false’ for boolean values in dictionaries if
you want to be compatible with default yamllint options.
#!/usr/bin/env ansible-playbook
---
- name: true or false?
hosts: all
gather_facts: false
tasks:
- name: "all these boolean inputs evaluate to 'true'"
debug:
msg: "{{ item }}"
with_items:
- true
- True
- TRUE
- yes
- Yes
- YES
- on
- On
- ON
- name: "all these boolean inputs evaluate to 'false'"
debug:
msg: "{{ item }}"
with_items:
- false
- False
- FALSE
- no
- No
- NO
- off
- Off
- OFF

Answer 3

The YAML principles define the possible Boolean values that are accepted by Ansible. However after parsing only two values remain (true and false), these are valid in JSON too, so if you do things with JSON in Ansible, then true and false are good choices. The best IMHO.
#!/usr/bin/env ansible-playbook
---
- name: true or false?
hosts: all
gather_facts: false
tasks:
- name: "all these boolean inputs evaluate to 'true'"
debug:
msg: "{{ item }}"
with_items:
- true
- True
- TRUE
- yes
- Yes
- YES
- on
- On
- ON
- name: "all these boolean inputs evaluate to 'false'"
debug:
msg: "{{ item }}"
with_items:
- false
- False
- FALSE
- no
- No
- NO
- off
- Off
- OFF