/etc/network/interfaces parser

input.txt

# Comment lo
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
    address 162.242.220.169
    gateway 162.242.220.1
    netmask 255.255.255.0

iface eth0 inet6 static
    address 2001:4802:7801:0102:adbb:e94c:ff20:3cbe
    dns-nameservers 69.20.0.164 69.20.0.196
    gateway fe80::def
    netmask 64

auto eth1
iface eth1 inet static
    # Comment nested
    address 10.176.129.214
    dns-nameservers 69.20.0.164 69.20.0.196
    netmask 255.255.224.0
    post-up route add -net 10.208.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true
    pre-down route del -net 10.208.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true
    post-up route add -net 10.176.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true
    pre-down route del -net 10.176.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true

auto eth2
iface eth2 inet static
    address 172.20.0.3
    netmask 255.255.255.0

auto eth3
iface eth3 inet manual
    up ip link set $IFACE up
    down ip link set $IFACE down

output.txt

# Comment lo
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
    address 162.242.220.169
    gateway 162.242.220.1
    netmask 255.255.255.0

iface eth0 inet6 static
    address 2001:4802:7801:0102:adbb:e94c:ff20:3cbe
    dns-nameservers 69.20.0.164 69.20.0.196
    gateway fe80::def
    netmask 64

auto eth1
iface eth1 inet static
    # Comment nested
    address 10.176.129.214
    dns-nameservers 69.20.0.164 69.20.0.196
    netmask 255.255.224.0
    post-up route add -net 10.208.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true
    pre-down route del -net 10.208.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true
    post-up route add -net 10.176.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true
    pre-down route del -net 10.176.0.0 netmask 255.240.0.0 gw 10.176.128.1 || true

auto eth2
iface eth2 inet static
    address 172.20.0.3
    netmask 255.255.255.0

auto lxb-mgmt
iface lxb-mgmt inet manual
    up ip link set $IFACE up
    down ip link set $IFACE down
    pre-up ip link add name phy-lxb-mgmt type veth peer name ovs-lxb-mgmt || true
    bridge_ports eth3 phy-lxb-mgmt

test.py

#!/usr/bin/env python

from __future__ import print_function


class Interfaces():
    "Manage /etc/network/interfaces file"

    def __init__(self):
        # Open file
        fd = open("interfaces")

        # Make iterator of contents
        self.iterator = iter(
            [
                line.strip()
                for line in fd.read().splitlines()
                if line.strip()
            ]
        )
        # Close file
        fd.close()

        # Parsed directives
        self.directives = []

        # Do the needful
        self.parse()

    def parse(self):
        "Parse super- and sub-directives, and return nested results"

        # For each directive
        for directive in self.iterator:
            # If we're on a super, start sub loop
            while directive.startswith(("iface", "mapping")):
                sup = None  # Clear super-directive
                subs = []   # Clear sub-directives

                # For each sub-directive
                for sub in self.iterator:
                    # If sub is actually a super
                    if sub.startswith(
                        (
                            "auto",
                            "allow-",
                            "iface",
                            "mapping",
                            "source"
                        )
                    ):
                        sup = sub         # Set new super
                        break             # Exit sub loop
                    # Else it's just a sub, so add it
                    else:
                        subs.append(sub)

                # If we found subs, store them
                if subs:
                    self.directives.append([directive, subs])
                # Else just store directive
                else:
                    self.directives.append([directive])

                # If we didn't find a super, return
                if not sup:
                    return

                directive = sup  # Store super for next inner loop check

            # Not a super here so just add directive
            self.directives.append([directive])

        # End of iterator, return
        return

    # TODO: -> save
    def prettyPrint(self):
        "Pretty-print interface directives"

        for directive in self.directives:
            # Print directive
            print(directive[0])

            # If has subs
            if len(directive) > 1:
                # Print indented subs
                for sub in directive[1]:
                    print("    {}".format(sub))
            # If super, add a blank line for spacing
            if directive[0].startswith(("iface", "mapping")):
                print()

    def swapDirective(self, one, two):
        "Swap one directive with another"

        for index, directive in enumerate(self.directives):
            if one in directive[0]:
                self.directives[index][0] = directive[0].replace(one, two)

    def addSubs(self, sup, subs):
        "Add sub-directives to super-directive"

        for index, directive in enumerate(self.directives):
            if sup == directive[0] and len(directive) > 1:
                self.directives[index][1].extend(subs)


interfaces = Interfaces()
interfaces.swapDirective("eth3", "lxb-mgmt")
interfaces.addSubs(
    "iface lxb-mgmt inet manual",
    [
        "pre-up ip link add name phy-lxb-mgmt "
        "type veth peer name ovs-lxb-mgmt || true",
        "bridge_ports eth3 phy-lxb-mgmt"
    ]
)
interfaces.prettyPrint()