Fibers are lightweight primitives for cooperative multitasking in Python. They provide means for running pieces of code that can be paused and resumed. Unlike threads, which are preemptively scheduled, fibers are scheduled cooperatively, that is, only one fiber will be running at a given point in time, and no other fiber will run until the user explicitly decides so. When a fiber is created it will not run automatically. A fiber must be 'switched' into for it to run. Fibers can switch control to other fibers by way of the switch or throw functions, which switch control or raise and exception in the target fiber respectively.