Although this is somewhat of an old thread, I will jump in. I've bumped against the same challenge and I thought I would share the solution. So, basically the config file would populate the 'nameservers' instance variable of the dns.resolver.Resolver you are using. Hence, if you want to coerce your Resolver to use a particular nameserver, you can do it direcly like this:
import dns.resolver
my_resolver = dns.resolver.Resolver()
# 8.8.8.8 is Google's public DNS server
my_resolver.nameservers = ['8.8.8.8']
answer = my_resolver.query('google.com')
Hope someone finds it useful.
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