At its most basic, this is nothing more than an array or hash lookup,
but when used in conjunction with map, it is very useful. For example,
while constructing an "ssh-keyscan …" command to update known_hosts on
all hosts in a group, one can get a list of IP addresses with:
groups['x']|map('extract', hostvars, 'ec2_ip_address')|list
This returns hostvars[a].ec2_ip_address, hostvars[b].ec2_ip_address, and
so on. You can even specify an array of keys for a recursive lookup, and
mix string and integer keys depending on what you're looking up:
['localhost']|map('extract', hostvars, ['vars','group_names',0])|first
== hostvars['localhost']['vars']['group_names'][0]
== 'ungrouped'
Includes documentation and tests.
8cf1815867