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Welcome back.

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One type of expression that you make right
in Python is what's called the function

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call expression.

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So you can think of
functions as boxes that

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contain some machinery inside of them.

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So I'm going to visually represent
them here just using this gray box.

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And you can think of a function as
something that takes in some inputs.

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So here,
the inputs feed into the top here and

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then these inputs go
inside of this machinery.

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This machinery does something
with the inputs, and

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then after it's done its work with
the inputs, it spits out an output.

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So visually again, you can think of
it as working something like this.

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So here we have a function,
it takes in three inputs or arguments and

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then it does work, and then it spits
out an output or return value.

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So let's watch that again.

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So again, we have the function, and it
takes in three inputs here or arguments,

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then it does work, and then it spits
out an output or a return value.

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Now, let's translate this visual metaphor
into actual working Python code.

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Here's an example of how we would actually
write a function call expression in

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Python.

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Suppose that we have
a function named square and

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our square function takes in one
argument which is going to be a number.

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And then it returns, or outputs,
whatever that argument is squared.

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So for example, if we say square and
then open parenthesis and

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we put our argument in between
the parentheses here, so

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this is called calling the square
function with 4 as an argument.

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So this is how we actually write that
in Python again, we say square, or

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whatever the name of the function is.

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Then inside of parentheses immediately
after the name of the function,

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we pass in our arguments.

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So the arguments go here.

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And then the value of this
overall expression, so

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whatever the value of this expression is,
is going to be the return value.

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And so, in the case of square,
if square squares whatever input or

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arguments you give it,
then the return value is going to be 16.

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And this is what it looks like
when we translate this Python

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code into that visual representation
that I just mentioned.

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So we have the square function,
again represented as a box, and the square

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function in this case we suppose that
it takes in one input or argument.

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And if we pass in 4, it does some work,
and it spits out an output or

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return value of 16.

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To watch that again, we pass in 4,
the square function does some work,

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and it computes an output or
return value, in this case of 16.

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And again,
this is how we write that in Python code.

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We say the name of the function, square,
then open parentheses to say that we

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want to call that function and
then in between parentheses,

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then we pass in whatever arguments
we want our functions to take.

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Now, Python functions can take one,
two or any number of arguments or

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inputs, but
there's always exactly one output.

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So, for example, here this function
takes in three inputs, or arguments, but

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it spits out one return value.

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So again, functions always take
any number of arguments, but

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there's always one value
of that function call.

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So, let's look at a little bit more
Python code and calling functions.

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So, here on the first line we're actually
calling two functions technically because

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here print is the function
that we're calling and

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we're passing in as an argument to print,
an expression which just so

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happens to be another function call.

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And this function call uses the square
function that I just described.

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So, again,
the square function that I am describing,

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this isn't a built-in with Python.

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This is something that we defined for the
purpose of the explaining function calls.

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But we defined it in a way
that it takes in one argument,

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in this case square(3), and
then it spits out that number squared.

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So if we say,
print out the value of square(3),

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then this is going to
print out the integer 9.

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So let's see that in action.

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So I'm going to comment out the rest
of these lines and run my code.

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And I can see that the value of
this expression, square(3) is 9.

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If we call a function, but we don't
have a print statement, so let's suppose

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that I say square(5), the value of
this expression is going to be 10.

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But you'll notice when I save and

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run my code, it's not going to
actually affect the output.

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And that's because we never actually
printed the value of square(5).

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We just computed the value of square(5).

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Excuse me, it's not 10, it's 25 of course.

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So here the value of this is 25 but
we never print out its value.

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Now, let's suppose that we have
another function, called sub.

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So sub takes in two arguments.

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So I can represent that
visually like this.

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So it takes in one and two arguments.

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And just like every other function,
it's going to spit out one returned value.

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Now, let's suppose that sub subtracts the
second argument from the first argument.

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So, in other words,
suppose if I passed in 6,

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If I passed in 6 and 4,

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Then my sub function is going to spit

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out 2 because 6 minus 4 is 2.

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So we would call the sub function
by saying print out and then sub.

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And then here you'll notice
that we have two arguments and

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we separate those arguments by a comma.

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So you print out the value of sub(6, 4).

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And we see that the value of
this expression, sub(6, 4) is 2.

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If we call it again, so
if we call sub with arguments 5 and

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9, then this is going to give
us -4 because 5- 9 is -4.

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We can also combine function calls with
other operators including function calls.

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So here if we print out
the value of square(3) + 2.

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Then Python is going to first compute
the value of this overall expression by

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evaluating the value of square(3) and then
it's going to add the result of that to 2.

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So Python is going to compute
the value of square(3) and

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it's going to get 9 and then it's going
to compute the value of this expression,

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which has a value, 2,
because it's a literal expression.

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And then it's going to add these
numbers together to give us 11.

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Now this expression on line
2 is much more complicated.

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So here we can see that we're
calling the sub function.

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We're calling it with two arguments.

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So the first argument is square(3),
so this is arg 1,

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the second argument is square(1 + 1),
so this is arg 2.

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Now, whenever Python calls
a function call expression,

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it needs to figure out what
the values of the arguments are.

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So Python kind of computes the values
of a function call expression

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from the inside out.

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So it's going to go from
the value of 1 + 1.

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To the value of square(1+1) and
then the value of square(3), and

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then after it has the values for
square(3) and square(1+1),

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it's going to subtract those two
values by calling the sub function.

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So again, we're kind of going from
the inside out to the next layer.

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And then out to the next
layer beyond that.

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So when Python computes this,

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it's going to first compute
the value of square(1+1), or 2.

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So it's going to compute the value of
square(2), and that's going to give us 4.

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It's going to compute the value of
square(3), and that's going to give us 9.

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And then it's going to compute
the value of sub when called on 9 and

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4, and that's going to give us 5.

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So if I run my code,
you'll see that we get

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11 from line 1 and 5 from line 2.

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Just to run through the exact steps that
Python takes when computing the code on

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line 2, Python first computes,
what's the value of square(3)?

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It gets 9, then it computes what's 1 + 1,
and it gets 2.

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Then it computes what's square(2),
and it gets 4..

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Then it subtracts 9 and 4 and gets 5 and
that's what it ultimately prints out.

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Now, one thing to note is that
functions are objects in Python.

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So if I print out what's
the value of square,

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then Python is going to tell
me that square is a function.

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So if I run my code, you can see that
line 1 prints out function square.

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In order to actually call the function or
run that machinery,

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we need to have the name of the function,
and then open parenthesis and

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pass in our arguments inside
of those parentheses.

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So in other words, we need to have open
parentheses right after the name of

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the function in order to say that
we want to call the function,

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we don't want to just reference
that function object itself.

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That's all for now, until next time.