Code style: here and in other places, always add a space after , and :.

frame_number -> frame_end

Code style: no empty line right after def line.

Avoid using operators when possible, this can probably be scene.frame_set or similar?

It's not even clear to me what this line does, which frame is it supposed to jump to?

Code style: use only one empty line as separator inside functions.

Easier to just pass the physics_spec entirely to this function.

For consistency, file should be lower case softbody_test.blend.

modifier -> physics modifier

Modifier -> Physics Modifier

Two empty lines still here.

This can also just call scene.frame_set?