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[MUSIC PLAYING]

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SPEAKER: OK, so we've
seen so far that functions

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are these building blocks of programs
that take input and produce output.

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Now, it turns out as
we look more closely,

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functions can produce
two kinds of outputs.

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One kind is a return value.

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And one kind is a side effect.

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So in this short, we'll look
more closely at return values

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and what they can do to make our
programs more flexible as you go off

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and write more complex
programs over time.

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Now, here, I have a function called
Greet, where Greet takes an input,

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as you can see on line one.

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Now, the function does
something very simple.

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It basically asks the
question, is "hello,"

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this text "hello," is that in the input?

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And if it is, it's going to print on
the terminal "hello, there" in response.

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Otherwise, it's going to say
"I'm not sure what you mean."

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So it is if "hello" is
not in the input, it's

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going to say "I'm not sure what
you're talking about," which is fine.

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This is a very simple
chat bot, if you will.

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So I'll go down to line eight and let
me try to actually run this function

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to call it with some input.

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So I'll type Greet down below and
I'll say hello to the computer.

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I'll say "hello, computer."

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I'll save it.

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And I'll say python of greeting.py
in my terminal, hit Enter,

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and now I should see "hello,
there," which is really nice.

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So this is an example of a side effect.

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I'm seeing something in my terminal.

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But a return value is
slightly different.

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A return value is some value a
function can pass to my program

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to use later on in my code.

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And actually, right now,
I haven't explicitly

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said that Greet returns anything.

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To do that, I need to use this
keyword called Return in Python.

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So instead of printing
here where printing

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produces some side effect
of text on the screen,

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let me try returning the actual text.

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I'll say why don't I
return "hello, there"

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and why don't I return "I'm
not sure what you mean"?

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So with this change, now I've said that
Greet has an explicitly-defined return

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value.

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If hello is in the input, the
return value will be "hello, there."

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If "hello" is not in the
input, the return value will be

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"I'm not sure what you mean."

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So let's try this again.

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I'll say python of greeting.py,
and I'll hit enter.

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And I don't see anything anymore, but
that doesn't mean things are broken.

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So what happens now is that
Greet is returning some value,

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but I'm not really capturing it
or using it in my program yet.

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To capture it and to
use it later on, I need

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to make sure I store the return
value inside of some variable.

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And I can do that, let's say,
in a variable called Greeting.

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So I'll say Greeting equals
the result, or the return

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value of calling Greet with this
input called "hello, computer."

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So now line-by-line what will
happen is I will first run Greet

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and I'll give the input
"hello, computer."

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"Hello" is in that input, so Greet
will return to me the value "hello,

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there" and it will then
store it using this Greeting

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variable in this assignment
operator, this equal sign

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here, inside this
variable called Greeting.

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And now, I could actually go ahead
and print the greeting itself.

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So on line nine here,
I'll print Greeting.

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Now, I'll try this.

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I'll say python of greeting.py
and I'll hit Enter,

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and now I see "hello, there."

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What if I did something like,
how's the weather, like this?

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I'll say python of greeting.py, and
now I see "I'm not sure what you mean."

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So, kind of nice.

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Now I'll say "hello, computer" again.

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And notice I can take this variable
and modify it in all kinds of ways.

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Before, when I just had these
simple print statements in Greet,

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going back to something like this,
print "hello there" and print

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"I'm not sure what you mean," those
were the only things I could print.

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But now with the return
value, I get access

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to all kinds of ways I could modify
the return value later on in my code.

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Maybe I could do something like this.

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If maybe I could say--

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why don't I print not just Greeting, but
why don't I print something like "hm"

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and then add the greeting on later.

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So now everything will begin with
"hm" followed by the greeting.

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So I'll say python greeting.py, and
now it should say "hm, hello, there."

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Or, I could say something
like "how's the weather?"

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"How's the weather."

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Now I could say python greeting.py,
and it's a little more thoughtful.

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"Hm, I'm not sure what you mean."

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And I could even go further.

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I could say "hello, computer" again.

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And why don't I try to have
different kinds of greetings?

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I could say a greeting and then I could
say, space, "Carter," just like this.

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And now I could say python of
greeting.py, and it says, oh, "hello,

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there, Carter."

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Or, I could do greeting plus David,
and now it says python greeting.py,

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and then "hello, there,
David," or Rongxin.

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I could type any name after this and
I could kind of add on people's names

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at the end.

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So as you can see, the
value of return value

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is that it allows us to modify
things throughout our code

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and to use the output of
some function later on.

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A side effect is slightly different.

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It simply produces some change
immediately, whereas a return value,

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you can use throughout our code as
you go off and build new programs.

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