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Generator functions are an incredibly useful pattern in Python.

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What a generator function does is it actually creates an iterator.

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Let's take a look at how this works.

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We can make a working copy of generator.py.

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I'm going to call it generator-working.py.

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Go ahead and open that up and take a look.

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This is the generator here.

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Now it calls this isprime, which is very similar to the one that we used in

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our functions example, except instead of the Print statements, it returns

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False if it's not prime, and it returns True if it is prime. So that's a

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little utility function.

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This one here, it looks, at first glance, like a normal function, and in fact, it

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is, except it has this yield statement.

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Yield is like return.

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It returns a value.

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You see here we have return False, return True. Those return those Boolean values.

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This one here returns a number,

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and it returns this number only if isprime returns true, so it returns the next

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prime number, but it uses yield instead of return.

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And what yield does is it returns a value, but then the next time the function

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is called, it continues execution after the yield.

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So in this case, we have this while loop.

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And it checks to see if the number is prime.

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If it's not, it increments and checks the next one.

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And if it is, it yields.

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And that will return a value.

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And then the next time this function is called, it'll just continue here,

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incrementing and looking for the next one.

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And so this actually, because of these of yield, it returns an iterator object,

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that is suitable for use in a for loop.

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And here we have a for loop, and for n in primes,

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and so it calls this primes function.

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There is the primes function.

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And for each iteration, it'll put the value in n. And it'll print it out, and it

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tests to see if it's greater than 100. Then it breaks out of the loop.

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And so this uses this generator function as an iterator.

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And it will print a list of prime numbers.

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Let's go ahead and take a look at this as it works. We'll run it.

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And here we have a complete list of all the prime numbers up to 97.

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So you can see that this is incredibly useful.

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And we'll some more examples of it as we go through the course.

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So this is a generator function, and what it generates is an iterator that's

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suitable for use in a for loop, just like any iterator in the Python language.