# How to use apply_constraint method in autotest

Best Python code snippet using autotest_python

singlepointcalc.py

Source:singlepointcalc.py

`1"""This test makes sure that the forces returned from a2SinglePointCalculator are immutable. Previously, successive calls to3atoms.get_forces(apply_constraint=x), with x alternating between True and4False, would get locked into the constrained variation."""5from ase.lattice.surface import fcc1116from ase.calculators.emt import EMT7from ase.io import read8from ase.constraints import FixAtoms9def check_forces():10 """Makes sure the unconstrained forces stay that way."""11 forces = atoms.get_forces(apply_constraint=False)12 funconstrained = float(forces[0, 0])13 forces = atoms.get_forces(apply_constraint=True)14 forces = atoms.get_forces(apply_constraint=False)15 funconstrained2 = float(forces[0, 0])16 assert funconstrained2 == funconstrained17atoms = fcc111('Cu', (2, 2, 1), vacuum=10.)18atoms[0].x += 0.219atoms.set_constraint(FixAtoms(indices=[atom.index for atom in atoms]))20# First run the tes with EMT and save a force component.21atoms.set_calculator(EMT())22check_forces()23f = float(atoms.get_forces(apply_constraint=False)[0, 0])24# Save and reload with a SinglePointCalculator.25atoms.write('singlepointtest.traj')26atoms = read('singlepointtest.traj')27check_forces()28# Manually change a value.29forces = atoms.get_forces(apply_constraint=False)30forces[0, 0] = 42.31forces = atoms.get_forces(apply_constraint=False)...`

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