create a bridge, create 20 veth interfaces, put veth peers to namespaces (and rename as eth0), add veth interfaces as ports to the bridge

functional.py

from pyroute2 import IPDB
from pyroute2 import netns

with IPDB() as ip:
    ip.create(ifname='v0', kind='bridge').commit()

    map(lambda x: ip.create(ifname='v0p%02i' % x,
                            peer='p0p%02i' % x,
                            kind='veth').commit(), range(20))

    map(lambda x: netns.create('host-%02i' % x), range(20))
    map(lambda x: ip.interfaces['p0p%02i' % x]
        .set('net_ns_fd', 'host-%02i' % x)
        .set('ifname', 'eth0')
        .commit(), range(20))

    reduce(lambda x, y: x.add_port(y),
           map(lambda x: ip.interfaces[x],
               sorted(filter(lambda x: x.startswith('v0'),
                             ip.by_name.keys())))).commit()