Simplest explanation is that you can't do a 'show run' from global
configuration mode....
try something like
#exit global configuration mode
tn.write('end\n')
print tn.read_until('#')
#disable pause after 24 lines
tn.write('term len 0\n')
tn.read_until('#')
#now show the entire running-config
tn.write('show run\n')
print tn.read_until('#')