I found a tiny error when use app.CharmmPsfFile() to parse psf files
for example:

from simtk.openmm import app
psf = app.CharmmPsfFile('example.psf')
print(len(psf.residue_list))
print(len(psf.atom_list))

output the same number

elif (lr.resname != resname or lr.idx != resname or (line 535 in simtk/openmm/app/internal/charmm/topologyobjects.py)
replaced by
elif (lr.resname != resname or lr.idx != resnum or
May fix this problem