diff --git a/dpgen/generator/lib/pwscf.py b/dpgen/generator/lib/pwscf.py
index c4a86fc80..3e1ac5e00 100644
--- a/dpgen/generator/lib/pwscf.py
+++ b/dpgen/generator/lib/pwscf.py
@@ -88,7 +88,7 @@ def _make_pwscf_02_species(sys_data, pps):
     assert ntypes == len(atom_masses)
     assert ntypes == len(pps)
     for ii in range(ntypes):
-        ret += "%s %d %s\n" % (atom_names[ii], atom_masses[ii], pps[ii])
+        ret += f"{atom_names[ii]} {atom_masses[ii]} {pps[ii]}\n"
     return ret