Door opener for a door having a door leaf which is moveably mounted in or on a door frame, wherein the door opener is able to be mounted in or on the fixed door frame or a fixed leaf for interaction with a lock latch on the door leaf, or wherein the door opener is able to be mounted on the door leaf for interaction with a counter element on the fixed door frame or fixed leaf, wherein the door opener comprises: - a moveably mounted door opener latch, - an electrically switchable locking device having an electrical actuator, e.g. electromagnets and a locking member which is able to be controlled by the actuator, e.g. an armature, - an unlocking switch means which interacts with the locking member of the locking device and is able to be switched into a first switch position and a second switch position by means of a manual actuating element, wherein it is provided that - when the unlocking switch means is in the "off' switch position, the locking device is able to be electrically switched into a locked position and a release position, wherein the locking device locks the door opener latch in the locked position under the influence of the locking member in order to lock the door leaf in the closed position and releases the door opener latch in the release position in order to reverse this locking of the door leaf, - when the unlocking switch means is in the "on" switch position, the locking member and/or the locking device are or is permanently switched into a release position and the door opener latch is thereby released, and - the unlocking switch means is formed as a retrofittable assembly, by providing that the unlocking switch means is able to be inserted in a receiving recess of the door opener forming an assembly and has a switch housing, in which the manual actuating element is mounted moveably, and the receiving recess has an opening for the insertion of the unlocking switch means, and the opening of the receiving recess is arranged in a position on a free front side of the door opener, characterised in that the unlocking switch means is formed as a purely mechanical switching device, - a moveably mounted door opener latch, - an electrically switchable locking device having an electrical actuator, e.g. electromagnets and a locking member which is able to be controlled by the actuator, e.g. an armature, - an unlocking switch means which interacts with the locking member of the locking device and is able to be switched into a first switch position and a second switch position by means of a manual actuating element, - when the unlocking switch means is in the "off' switch position, the locking device is able to be electrically switched into a locked position and a release position, wherein the locking device locks the door opener latch in the locked position under the influence of the locking member in order to lock the door leaf in the closed position and releases the door opener latch in the release position in order to reverse this locking of the door leaf, - when the unlocking switch means is in the "on" switch position, the locking member and/or the locking device are or is permanently switched into a release position and the door opener latch is thereby released, and - the unlocking switch means is formed as a retrofittable assembly, by providing that the unlocking switch means is able to be inserted in a receiving recess of the door opener forming an assembly and has a switch housing, in which the manual actuating element is mounted moveably, and the receiving recess has an opening for the insertion of the unlocking switch means, and the opening of the receiving recess is arranged in a position on a free front side of the door opener,