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So let's talk about functions calling
functions. In the last video, I mentioned

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that we already had the code on line 18.
This was copied from the is palindrome

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function. When you find yourself copying
and pasting code, stop and think if there

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isn't a better way. Functions allow you
to reuse a block of code, without copying

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and pasting. In our case, we copied the
code from a function that we'd already

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written. It might not have been obvious
at the time, but we can just call our

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function instead. So what I mean is, line
18 can be replaced by a call to the is

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palindrome function. So what I'm going to
do is comment out line 18, and on line 19

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what I'm going to do, is just type return
is_palindrome, and in

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parentheses I'm going to put string.

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And it would help if I typed return correctly. Okay.

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Line 19 is now calling out is_palindrome
function, then returns whatever value

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that returns. We've already tested the is
palindrome function and we know that it

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works. Using it with the palindrome
sentence function saves duplicating code.

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Of course, our testing has been quite
basic, but it's quite a simple function.

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We'll talk more about testing, later in
the course. That's another reason why

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calling the is_palindrome function is
a good idea. If we hadn't already noticed

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and fixed the uppercase bug, we'd have a
bug in two places. We'd need to add calls

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to .casefold on line 9, right at
the top of the screen there, and also on

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the old line 18 that I've now commented
out. But with the change we've just made,

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the bug only needs fixing in one place,
inside the is_palindrome function.

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So just to be clear, your functions can
call other functions that you've written.

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You're not restricted to just calling
the built-in Python functions. Alright,

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so let's end the video here, and in the
next one we'll look at the return

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keyword in more detail. See you in the
next video.