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One of the most common reasons for using a loop in programming is what's

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called an iterator.

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This is where you need to step through some data structure, taking one object

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at a time, and doing something with each of those objects before going back for the next one.

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Let's take a look at how this works in Python with the 'for' loop.

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We'll make a working copy of for.py, and call it for-working.py, and we'll go ahead

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and open that working copy.

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Here, we are opening a file with lines of text and reading that file line-by-

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line and printing out each line on the screen.

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So, let's go ahead and run it and see what it does, and we see that it prints

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these lines of text, and there they are.

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If we open the file here, lines.txt, we see that it has these five lines of text.

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You'll notice that in the file the lines are right next to each other, and in our

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result, it's printing a blank line between them.

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The reason for that is that the print function prints a blank line after each

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object that it prints, and the line itself has a line ending at the end of it.

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So, we can make print not to do that by typing end = " " just a empty string. If we

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save that and run it, then we get the lines of text, just like we expect them.

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So, the readlines method, inside the file object, is what's called an iterator.

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What this does is it takes one object at a time from a sequence of objects, and

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it returns those, one at a time, in this case into the line variable.

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The for loop is designed to work with iterators, so the for keyword introduces

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the for loop, and then a variable name, which is used to contain each object in

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the iteration, and in keyword introduces the iterator.

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So, it reads like for line in iteration or for line in file object readlines, and

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then it prints each line, one at a time.

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So, this here, this file object, .readlines, is the iterator.

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In Python, all the container types are iterators.

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So, if I have a list, let's say 1, 2, 3, 4, 5, then it will work exactly the same.

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It will just print 1, 2, 3, 4, 5 all in one line because we are not ending our

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lines with a new line here in the print function.

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So, if I save that and run it, we just get 12345.

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In fact, a string is a container object, and this will work exactly the same way.

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It will just take each letter from the string, and of course that will just look

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like the word string.

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But instead of we take this out and print them one line at a time, we can

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see that we are getting each letter as separate object.

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So, the string is a container that contains individual characters.

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In this case, we are printing the lines of the file, and the readlines method,

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from the file object, is the iterator that gives us one line at a time.

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So, if I save this and run it, there we get the lines of the file.

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So, that's the for loop.

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The for loop is used for stepping through iterators, and virtually all container

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types in Python are iterators.

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So, you'll see the for loop use a lot.

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In fact, in my informal survey of Python code, as I was preparing for this course,

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I saw probably 20 or 30 for loops for every while loop that I saw.

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The for loop is by far the most common type of a loop in Python because

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iteration is such a common thing to need to do, and so many objects in Python are iterators.

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So, that's the for loop, and that's how it works for line in readlines, so it's

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for variable in container.

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Then that variable becomes available to the suite of the loop, and that's how a for

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loop works in Python.