An observable, is a function that creates an observer and attaches it to the source where values are expected, for example, clicks, mouse events from a dom element, etc.

The topics mentioned below will be studied in detail in this chapter.

• Create Observables

• Subscribe and Execute an Observable

Create observables

To create an observable we will use create() method and pass the function to it that has the following items.

• on_next() − This function gets called when the Observable emits an item.

• on_completed() − This function gets called when the Observable is complete.

• on_error() − This function gets called when an error occurs on the Observable.

To work with create() method first import the method as shown below −

from rx import create

Here is a working example, to create an observable −

testrx.py

from rx import create
deftest_observable(observer, scheduler):
   observer.on_next("Hello")
   observer.on_error("Error")
   observer.on_completed()
source = create(test_observable).

Subscribe and Execute an Observable

To subscribe to an observable, we need to use subscribe() function and pass the callback function on_next, on_error and on_completed.

Here is a working example −

testrx.py

from rx import create
deftest_observable(observer, scheduler):
   observer.on_next("Hello")
   observer.on_completed()
source = create(test_observable)
source.subscribe(
   on_next = lambda i: print("Got - {0}".format(i)),
   on_error = lambda e: print("Error : {0}".format(e)),
   on_completed = lambda: print("Job Done!"),
)

The subscribe() method takes care of executing the observable. The callback function on_next, on_error and on_completed has to be passed to the subscribe method. Call to subscribe method, in turn, executes the test_observable() function.

It is not mandatory to pass all three callback functions to the subscribe() method. You can pass as per your requirements the on_next(), on_error() and on_completed().

The lambda function is used for on_next, on_error and on_completed. It will take in the arguments and execute the expression given.

Here is the output, of the observable created −

E:pyrx>python testrx.py
Got - Hello
Job Done!