Simplified versions of the pybonjour examples. These use a single event loop for the chained asynchronous callbacks and include some extra comments to explain what's going on.

browse_and_resolve.py

import select
import sys
from pybonjour import (kDNSServiceErr_NoError,
                       kDNSServiceFlagsAdd,
                       DNSServiceResolve,
                       DNSServiceBrowse,
                       DNSServiceProcessResult)

regtype  = sys.argv[1]
timeout  = 5
references = []

def active(ref):
    """Determine whether or not the reference is still open."""
    return bool(ref) and ref.fileno() != -1

def on_discover(sdRef, flags, interface, error, name, regtype, domain):
    """Handle discovery of a service (or its disappearance)."""
    if error != kDNSServiceErr_NoError:
        return
    elif not (flags & kDNSServiceFlagsAdd):
        print 'Service removed'
        return

    print 'Service added; resolving'
    resolve_sdRef = DNSServiceResolve(
            0,
            interface,
            name,
            regtype,
            domain,
            on_resolve)

    references.append(resolve_sdRef)

def on_resolve(sdRef, flags, interface, error, fullname, target, port, txtRecord):
    """Handle resolution of a service."""
    if error != kDNSServiceErr_NoError:
        raise Exception('Unexpected pybonjour error code: %r' % error)

    print 'Resolved service:'
    print '  fullname   =', fullname
    print '  hosttarget =', target
    print '  port       =', port
    sdRef.close()

if __name__ == '__main__':
    # Get a reference to browse for services.
    browse_sdRef = DNSServiceBrowse(regtype=regtype, callBack=on_discover)
    references.append(browse_sdRef)

    try:
        # Our event loop. Any DNSService references will be polled for updates
        # here. When results are available the reference is passed along to
        # DNSServiceProcessResult to do whatever internal work is required,
        # and then trigger the appropriate callback.
        while True:
            # Make sure closed references are removed before calling select()
            references = filter(active, references)

            r, w, x = select.select(references, [], [])
            map(DNSServiceProcessResult, r)

    except KeyboardInterrupt:
        pass

    finally:
        for ref in references:
            ref.close()

browse_resolve_query.py

import select
import socket
import sys
from pybonjour import (kDNSServiceErr_NoError,
                       kDNSServiceFlagsAdd,
                       kDNSServiceType_A,
                       DNSServiceBrowse,
                       DNSServiceProcessResult,
                       DNSServiceQueryRecord,
                       DNSServiceResolve)

regtype  = sys.argv[1]
timeout  = 5
references = []

def active(ref):
    """Determine whether or not the reference is still open."""
    return bool(ref) and ref.fileno() != -1

def on_discover(sdRef, flags, interface, error, name, regtype, domain):
    """Handle discovery of a service (or its disappearance)."""
    if error != kDNSServiceErr_NoError:
        return
    elif not (flags & kDNSServiceFlagsAdd):
        print 'Service removed'
        sdRef.close()
        return

    print 'Service added; resolving'
    resolve_sdRef = DNSServiceResolve(
            0,
            interface,
            name,
            regtype,
            domain,
            on_resolve)

    references.append(resolve_sdRef)

def on_resolve(sdRef, flags, interface, error, fullname, target, port, txtRecord):
    """Handle resolution of a service."""
    if error != kDNSServiceErr_NoError:
        raise Exception('Unexpected pybonjour error code: %r' % error)

    print 'Resolved service:'
    print '  fullname   =', fullname
    print '  hosttarget =', target
    print '  port       =', port
    sdRef.close()

    query_sdRef = DNSServiceQueryRecord(
        interfaceIndex=interface,
        fullname=target,
        rrtype=kDNSServiceType_A,
        callBack=on_query)

    references.append(query_sdRef)

def on_query(sdRef, flags, interface, error, fullname, rrtype, rrclass, rdata, ttl):
    """Handle query results from a record."""
    if error != kDNSServiceErr_NoError:
        raise Exception('Unexpected pybonjour error: %r' % error)

    print '  IP         =', socket.inet_ntoa(rdata)
    print '  rrtype     =', rrtype
    print '  rrclass    =', rrclass
    print '  rdata      =', rdata
    print '  ttl        =', ttl
    sdRef.close()

if __name__ == '__main__':
    # Get a reference to browse for services.
    browse_sdRef = DNSServiceBrowse(regtype=regtype, callBack=on_discover)
    references.append(browse_sdRef)

    try:
        # Our event loop. Any DNSService references will be polled for updates
        # here. When results are available the reference is passed along to
        # DNSServiceProcessResult to do whatever internal work is required,
        # and then trigger the appropriate callback.
        while True:
            # Make sure closed references are removed before calling select()
            references = filter(active, references)

            r, w, x = select.select(references, [], [])
            map(DNSServiceProcessResult, r)

    except KeyboardInterrupt:
        pass

    finally:
        for ref in references:
            ref.close()