A lock with a programming device, includes a stator, a rotor rotatably inserted therein, a keyhole, a longitudinal groove, a set of key followers and a set of locking pins which, under control of a change bar, can be mutually engaged for normal operation and disengaged in a change condition, and can be moved or blocked under control of a stop bar provided with springs which stress the stop bar outwards. The change bar is arranged to keep or assume, even in the change condition, a position in which it retains the engagement among at least some of the key followers and locking pins. A change element displaces the change bar to the position in which it disengages all the key followers from the locking pins, when the user intends to proceed with the change of the key. A device for preventing a faulty programming is provided.