Writing migration scripts (and manipulating VyOS config files outside VyOS) just got easier

Long story short

VyOS 1.2.0-rolling (starting with the next nightly build) includes a library for parsing and manipulating config files without loading them into the system config. It can be used for automatically converting configs from old versions in case an incompatible change was made, and for standalone utilities. Motivation and history are discussed below.

Here is an example of interacting with the new library: 
>>> from vyos import configtree

>>> c = configtree.ConfigTree("system { host-name vyos \n } interfaces { dummy dum0 { address 192.0.2.1/24 \n address 192.0.2.20/24 \n disable \n } }  /* version: 1.2.0 */")

>>> print(c.to_string())
system {
    host-name vyos
}
interfaces {
    dummy dum0 {
        address 192.0.2.1/24
        address 192.0.2.20/24
        disable { }
    }
}

 /* version: 1.2.0 */

>>> c.set(['interfaces', 'dummy', 'dum0', 'address'], value='293.0.113.3/32', replace=False)

>>> c.delete_value(['interfaces', 'dummy', 'dum0', 'address'], '192.0.2.1/24')

>>> c.delete(['interfaces', 'dummy', 'dum0', 'disable'])

>>> c.is_tag(['interfaces', 'dummy'])
True

>>> c.exists(['interfaces', 'dummy', 'dum0', 'disable'])
False

>>> c.list_nodes(['interfaces', 'dummy'])
['dum0']

>>> print(c.to_string())
system {
    host-name vyos
}
interfaces {
    dummy dum0 {
        address 192.0.2.20/24
        address 293.0.113.3/32
    }
}

 /* version: 1.2.0 */

As you can see, it largely mimics the API you get for the running config. The only notable differences are that the "set" method requires that you specify the path and the value separately, and to have nodes formatted as tag nodes (i.e. "ethernet eth0 { ..." as opposed to "ethernet { eth0 { ..." you need to mark them as such with "set_tag", unless they were originally formatted that way in the config you parsed.

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