Support nested variables

I have been searching through documentation and various online articles and could not seem to find any mention of having nested variables so I am opening up a feature request. This could help benefit many projects by allowing more complex and automatic variable calls.

A normal variable can be defined like this:

{{ variable }}

It would be useful to allow dynamic information from another variable to be used to populate another variables name.

{{ variable{{ generated_var }} }}

My exact use case is with Ansible. I am grabbing a list of network interface devices from one variable (ansible_interfaces). Then I can use this to reference the information from another variable (ansible_eth0, for example). Here is a basic framework of what is trying to accomplish.

{% for interface in ansible_interfaces %}
IPADDR{{ loop.index }}={{ ansible_{{ interface }}.ipv4.address }}
NETMASK{{ loop.index }}={{ ansible_{{ interface }}.ipv4.netmask }}
{% endfor %}

Jinja2 does not like that when it renders as it complains about the extra brackets existing. For options of how to implement this I was thinking along the lines of one of these two different ideas.

(1) Allow variable expansion. This is exactly what I was showing earlier; allowing variables to be parsed from the inside-out.

(2) Add a filter to convert a string into a variable name.

{% for interface in ansible_interfaces %}
{% set interface_string="ansible_%s.ipv4.address"|format(interface) %}
IPADDR{{ loop.index }}={{ interface_string|variable }}
NETMASK{{ loop.index }}={{ interface_string|variable }}
{% endfor %}

Here is some rough (non-working) code showing the second idea.

# vim jinja2/jinja2/filters.py
@environmentfilter
def do_variable(environment, s):
    string_to_variable = "{{ %s }}" % s
    return environment.from_string(string_to_variable).render()

Ideally option 1 would be less complex in terms of end-user use. Let me know your thoughts on the matter. This is my first time ever looking into the code for Jinja2 but I would love to contribute back if any help is needed with the code.

[ThiefMaster]

http://serverfault.com/questions/762079/how-to-loop-through-interface-facts

Sounds like you can do this:

{{ hostvars[inventory_hostname]['ansible_%s' | format(interface)].ipv4.address }}

---

I’m very strong 👎 on variable variable names. It’s usually a sign of bad architecture if an application doesn’t provide a proper dict/list of data if you need to access it by dynamic key or iterate over it. FWIW, I think this hostvars dict is somewhat ugly compared to a proper dict mapping interface names to interface data. I’d open an issue with Ansible, suggesting to change that list to a dict. Since iterating over a dict yields you its keys it might not even be backwards incompatible if they changed it to a dict…

[davidism]

Please see http://stackoverflow.com/q/1373164/400617.