76 – Built-in next with a default value

The built-in next can be given a second argument which is the default return value used when the given iterator is empty. A default value (of None, for example) tends to be more useful in idioms that use next. (For example, see this tip about the first element of an iterable that satisfies a condition.)

As an example, consider the generator expression assigned to squares and that is exhausted by calling the built-in next repeatedly:

squares = (x ** 2 for x in range(3))

print(next(squares))  # 0
print(next(squares))  # 1
print(next(squares))  # 4

The next time that the built-in next is called, Python raises a StopIteration:

print(next(squares))
# StopIteration

However, if you pass a second argument to the built-in next, that’s the result you get:

print(next(squares, "hey"))
# hey