[antonsoo]

Custom spring laws currently disappear during copying or pickling, so a restored simulation silently uses the default force or torque function:

import pymunk

def nonlinear_force(spring, distance):
    return spring.stiffness * (spring.rest_length - distance) ** 3

a, b = pymunk.Body(1, 1), pymunk.Body(1, 1)
b.position = (3, 0)
spring = pymunk.DampedSpring(a, b, (0, 0), (0, 0), 1, 2, 0)
spring.force_func = nonlinear_force
print(spring.copy().force_func is nonlinear_force)  # Before: False; after: True

Add force_func and torque_func to their classes' general-state lists. Restoration then uses the existing setters to install the callback or the native default function. Pickle requires a pickleable callback, as with other serialized callbacks; it no longer silently discards a custom law.

Validation:

• Added copy/deepcopy/pickle cases that step the restored springs and verify their impulses, plus reset-to-default controls. All six custom-law cases fail on the original source and pass after the change. The full python -m pymunk.tests run passes (289 tests, two skipped).
• Ran 48 native linear/rotary oscillator continuations at three checkpoints. All 24 custom-law cases now match freshly constructed reference worlds; 24 default-law controls remain identical. An independent scalar integrator agrees within 5.6e-17 for these undamped setups.
• Ran the shipped constraints demo headlessly with custom laws and copy/pickle checkpoints. All six continuations match explicitly restored reference callbacks over 240 steps. Original-source pickles remain loadable.

Validation used Linux and Python 3.12. Existing unrelated mypy and whole-file formatting failures are unchanged; the edited test module passes Black.

[viblo]

Thanks for finding and fixing the bug!