meet it

A hands out values one at a time with .

Put yield in a function and it becomes a generator. Calling it runs nothing yet: it gives you an iterator. Each next() runs the code up to the next yield, hands out that value, then pauses right there.

Generators are , so they only work when asked. One can read a huge log line by line, or count forever, without ever holding everything in memory.

A is a comprehension in round brackets. sum(n * n for n in range(4)) adds up the squares without building a list first.

def countdown(start):
    while start > 0:
        yield start
        start = start - 1
    yield "Lift off!"

for step in countdown(3):
    print(step)
print(sum(n * n for n in range(4)))

When a generator's code reaches the end, it raises StopIteration for you.

guess first

Watch when each line runs. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

def lines():
    print("opening")
    yield "first"
    print("reading on")
    yield "second"

reader = lines()
print("ready")
print(next(reader))

your turn

Your turn. Stream only the error lines.

Fill the blank with the word that hands one value out of a generator. Add an ERROR line of your own to the log if you like.

Your code

press run to see what happens

fix it

You want every even number, but this crashes.

Broken code to fix

press run to see what happens

The function stops at the very first even number. Which word ends a function for good?
return hands back one value and finishes. You want to hand out many and keep going.
Change return to yield on line 4, then run.

nailed it

you made values on demand. :D

yield hands out a value and pauses the function.
Generators are lazy: no work happens until you ask.
sum(x for x in items) skips building a whole list.

new words in your collection

generator
yield
lazy
generator expression

This lesson isn’t saved. A free account remembers every lesson you finish.

A generator hands out values one at a time with yield.

Watch when each line runs. What will this show?

Your turn. Stream only the error lines.

You want every even number, but this crashes.

you made values on demand. :D

A hands out values one at a time with .