script_interface: fix time_step==-1 sentinel restore guard (bug-sweep #35)

src/script_interface/integrators/IntegratorHandle.cpp

@@ -108,7 +108,7 @@ void IntegratorHandle::on_bind_system(::System::System &system) {
               system.propagation->integ_switch == INTEG_METHOD_NVT and
               system.get_time_step() == -1. and
               is_type<double>(params.at(key)) and
-              get_value<double>(is_type<double>(params.at(key))) == -1.)) {
+              get_value<double>(params.at(key)) == -1.)) {
         do_set_parameter(key, params.at(key));
       }
     }

testsuite/python/CMakeLists.txt

@@ -452,6 +452,7 @@ python_test(FILE propagation_stokesian.py MAX_NUM_PROC 2)
 python_test(FILE integrator_symplectic_euler_langevin.py MAX_NUM_PROC 4)
 python_test(FILE integrator_steepest_descent.py MAX_NUM_PROC 4)
 python_test(FILE integrator_exceptions.py MAX_NUM_PROC 1)
+python_test(FILE integrator_time_step_restore.py MAX_NUM_PROC 1)
 python_test(FILE integrator_langevin_stats.py MAX_NUM_PROC 1 LABELS long)
 python_test(FILE integrator_brownian_stats.py MAX_NUM_PROC 1 LABELS long)
 python_test(FILE integrator_npt_stats.py MAX_NUM_PROC 4 LABELS long)

testsuite/python/integrator_time_step_restore.py

@@ -0,0 +1,54 @@
+#
+# Copyright (C) 2026 The ESPResSo project
+#
+# This file is part of ESPResSo.
+#
+# ESPResSo is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# ESPResSo is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+import unittest as ut
+import pickle
+
+import espressomd
+
+
+class IntegratorTimeStepRestore(ut.TestCase):
+    """
+    Regression test for the ``on_bind_system`` sentinel-skip guard in
+    ``IntegratorHandle``. A :class:`System` whose ``time_step`` was never set
+    keeps the ``-1.`` sentinel and the default Velocity Verlet integrator
+    (``INTEG_METHOD_NVT``). Serializing and deserializing such a system must
+    succeed: the guard is supposed to *skip* re-applying ``time_step == -1.``
+    during ``on_bind_system`` so that ``System::set_time_step(-1.)`` is never
+    called (it throws ``std::domain_error`` for non-positive time steps).
+    """
+
+    def test_restore_unset_time_step(self):
+        system = espressomd.System(box_l=[1., 1., 1.])
+        # time_step must still be the -1. sentinel (never set by the user)
+        self.assertEqual(system.integrator.time_step, -1.)
+
+        # round-trip via pickle: this triggers
+        #   System._restore_object -> do_construct (else-branch) ->
+        #   do_set_parameter("integrator", deserialized handle) ->
+        #   IntegratorHandle::on_bind_system -> sentinel-skip guard
+        state = pickle.dumps(system)
+        restored = pickle.loads(state)
+
+        # restore must succeed and preserve the unset sentinel
+        self.assertEqual(restored.integrator.time_step, -1.)
+
+
+if __name__ == "__main__":
+    ut.main()