The take_while operator emits items from the source observable until a criterion is met. The following figure shows the marble diagram of this operator:

Its prototype is the following:

Observable.take_while(self, predicate)

Here, predicate is a function called each time an item is emitted form the source observable. When the predicate function returns True, then the take_while operator stops emitting items of the source observable.

Here is an example of the take_while operator:

numbers = Observable.from_([1, 2, 3, 4])

numbers.take_while(lambda i: i < 2).subscribe(
    on_next = lambda i: print("on_next {}".format(i)),
    on_error = lambda e: print("on_error: {}".format(e)),
    on_completed = lambda: print("on_completed")
)

This example gives the following result:

on_next 1
on_completed