#100DaysOfCode in Python Transcripts
Chapter: Days 19-21: Iteration with itertools
Lecture: Iteration Refresher

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0:00 All right, so let's discuss what iteration is in Python.
0:06 When we say something is iterable, we're saying
0:09 that you're able to iterate over it,
0:10 you're able to move through it one item by item, okay?
0:16 So the easiest way to demonstrate this
0:19 is just to get a simple list.
0:21 So we'll create a quick list here of numbers.
0:26 Let's go with the numbers 1 through 10.
0:31 Okay, oops, what am I doing?
0:35 1 through 10.
0:37 And we know that,
0:41 and we know that number is 10 digits, right?
0:44 Nice and easy.
0:45 Now if we're iterating over that,
0:49 we're going to run a full loop, so for i in numbers,
0:54 in number, I should say, print i.
0:59 This is something we've all done before, we all know
1:02 what this is, but what's happening behind the scenes?
1:05 Okay, yes we're running a full loop,
1:07 but really what is that full loop doing
1:09 to iterate over the number list
1:14 and give you the numbers?
1:17 Well, it's actually calling the iter dunder method,
1:21 or iter protocol, okay?
1:24 So we can see this if we actually
1:27 drill in to the number list.
1:30 We can see that it is actually calling iter
1:35 or it's capable of being iterated over.
1:38 Okay, so we've got iter and dur number
1:43 and we get true, so it's in there.
1:45 This is an iterable item, we can iterate over it, okay?
1:50 Now another way to demonstrate
1:52 an iterable item is to use next.
1:56 We've all seen next before, or hopefully you've seen next.
1:59 And what happens is when you run next on an iterator,
2:03 when it finishes iterating over the sequence,
2:07 over the list or the string or whatever it happens to be,
2:10 when it finishes this, it then gives you an error,
2:15 okay, it gives you a StopIteration error
2:18 because it's only going to iterate through it
2:20 up until the end and then it will stop.
2:24 So now that we know that iter is actually
2:26 what's being called in the background, we can use that,
2:30 okay, we can use that with next.
2:32 Now if you haven't heard of next,
2:34 next is a little function you can run against an iterator.
2:41 And what it will do is it will pass over,
2:44 it will iterate over that iterator,
2:47 that list or that string or whatever it is,
2:50 and it will continue through it until it hits the end.
2:53 When it hits the last item, when it hits the last character,
2:57 it will actually give you a StopIteration error, okay?
3:01 So we'll demonstrate that with a string called,
3:04 let's call it string, okay?
3:06 So it equals iter, we're actually
3:09 calling iter now over the word string.
3:14 Okay, so we know that when you iterate
3:16 over this word here, over this string,
3:19 you're going to get the letters one by one, right?
3:22 So if we call next on that, we'll get the letter S, okay?
3:29 Now let's copy and paste this a few times,
3:31 just so we can demonstrate.
3:34 We get the T, we get the R, I, N, G.
3:38 But then, when we run it one more time,
3:42 we get the stop iteration, okay?
3:45 And that is because we're calling next directly.
3:51 That's because it's hit the end and it's taken care of.
3:53 It gets to the end but then it's not going to go any further
3:57 because it knows it's already finished.
3:59 Now, when you run a full loop,
4:01 so for character in string,
4:07 print character,
4:09 we get the letters, but we don't get this error,
4:13 we don't get the StopIteration error,
4:15 and that's because it's actually built in to the full loop
4:18 so that it's not going to give you that error.
4:21 It's expected, okay, so it knows
4:25 that it's hit the end and it's not going to actually
4:28 sit there and give you the error.
4:30 And that's it, that's a basic coverage of iteration.
4:33 You see it's just going through each object,
4:36 each item, one by one, to get to the end.
4:39 Now, there is iter tools, a nice series of functions
4:44 that are just so cool and make iteration
4:48 a lot more interesting and a lot easier,
4:51 so that's what we're going to look at.