Proteins structures often have more than one chain of residues present in a structure file. This command will fetch the names of each chain present. Because of the lack of standards, sometimes chain A will also be blank "". This isn't a problem for PyMOL, but be warned sometimes you may get back, [""].
# using get_chains for the object or selection called objSel
get_chains objSel
# examples
fetch 1cll
print "1CLL has the following chains:", cmd.get_chains("1cll")
# list all chains in all proteins loaded in PyMOL:
for x in cmd.get_names():
for ch in cmd.get_chains(x):
print x, " has chain ", ch
