A key lock mechanism which can be made entirely of molded plastic parts having (1) a lock shell that can be snap fit in a mounting opening, wherein the shell is a predominantly cylindrical casing having a front end, a rear end, and an interior longitudinal channel; and (2) a lock plug, having a front end and a rear end, longitudinally housed within and rotatable about the longitudinal axis of the shell. The front end of the lock plug has a keyway allowing the insertion of a key to operate a double-acting, bar tumbler having a rocker body with a front peg and rear peg; an integral axle perpendicular to and connecting the center rocker body to a left and a right pivot pad upon which the bar tumbler pivots. The insertion of the key pivots the bar tumbler within the longitudinal shell channel and enables the key to turn the plug within the shell to operate the lock. The rear end of the plug receives a snap on actuation means such as a cam. Another embodiment of the invention is a plastic switch lock which combines the same tumbler means with a rotatable, electrically conductive contact within the plug effective for electrically communicating with a least two electrically conductive terminals.