For alignment template structures (i.e., sequences for which a structure is also available) see the Structure class in Section 6.17.
# Example for alnsequence objects
from modeller import *
env = environ()
aln = alignment(env, file='../commands/toxin.ali')
print "Alignment contains %d sequences:" % len(aln)
for seq in aln:
print " Sequence %s from %s contains %d residues" \
% (seq.code, seq.source, len(seq))