Most of the code of a ReactiveX application is composed of operators that are chained together. However, at some point, the source of these events must be exposed as observables. ReactiveX provides many ways to create an observable from any source of data. Many factory operators are already available to convert virtually any source of data or event to an observable, and, should none of these operators be applicable to a situation, it is possible to create an operator from custom code logic.

All of the factory operators in RxPY are implemented as static methods of the Observable class. This means that they are invoked by using Observable.xxx, where xxx is the factory operator, and they return an observable. Also, almost all of these operators take a scheduler as an optional parameter. A scheduler allows you to control how the items are emitted on the observable. They are only mentioned in this chapter briefly, and they will be covered in detail in Chapter 5, Concurrency and Paralellism in RxPY.