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Hi guys, and welcome back.

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In this video, we're going to learn about

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first-class functions.

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In first-class function, just means that functions

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are just variables.

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And you can pass them in as arguments to functions and use

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them in the same way you would use any other variable.

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Let's get started.

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Here we have a divide function that raises a zero division

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error if the divisor is zero.

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We've seen this before.

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And then we have a rather weird looking function.

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Calculate.

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That takes in any number of arguments and collects them

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into this value stopple.

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We've seen this before and if you can't remember how this

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works, please look at the unpacking arguments lecture.

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Then it also takes a mandatory key word argument parameter.

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So that is operator.

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And then what it does, is it calls the operator function

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with the values.

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That's kinda weird.

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How does that work? How does it know it is a function?

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Well, the simple answer is, it doesn't.

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However, because we've got these brackets after the

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variable name, this here must be a function,

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since that is the syntax for calling a function.

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No where calling this operator variable as a function

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with the values passed in as individual argument.

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How would we use this calculate function?

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You could do something like result equal calculate

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and then notice that calculate takes in

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any number of arguments.

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So we're gonna type twenty and four

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and then were gonna type operator equals

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the divide function.

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But this will pass in the divide function

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as the operator

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and then we will only call it when we reach line nine.

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Notice that this divide function is not called here,

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it is only used as a value.

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If you wanted to call this function,

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you always have to put the brackets at the end.

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That's how you call a function in Python.

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So by not putting the brackets, you are passing in the value

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to which this name divide refers to, which is the function,

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as the value for this parameter.

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So the operator parameter now has the same value

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as the divide variable.

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They are both this function here.

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So you can call it as a function, passing in the values

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as individual values so that will give you twenty for the

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dividend and four for the divisor.

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This here is an example of using a first-class function

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because you can see that you can pass it in as an argument

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to another function.

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Functions in Python are just variables that happen to be

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callable. You can call them with the brackets at the end.

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But they're no different than any other value

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other than that.

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So now you can print the result if you like,

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and you'll see that that has [inaudible] called

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the divide function. And you get 5.0.

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If you change this to a zero though,

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you will end up with trace back because you tried to

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divide by zero.

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Something important that will be a little bit of problem in

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this code is that the calculate function

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takes in any number of values.

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But this operator expects two values.

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So if you put anymore, then you will also get an error.

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Because now you're gonna pass in too

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many positional arguments.

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See divide takes two positional arguments

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but three were given.

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So that is a small problem with this code here.

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But anyway the point of it was to show you

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how first-class functions work.

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Let's do another example.

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I'm going to define a search function that takes in

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a sequence of things to search through.

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It will also take what you expect to find

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and finally it will also take a function that will be used

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to extract information from each item in the sequence,

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match it against the expected value,

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and see if we find what we wanted.

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So we will iterate over the sequence,

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then we will run the finder function on the element

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and see if it is equal to the expected value.

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And if it is, we will return the element

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otherwise we will raise a runtime error,

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saying something like,

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could not find an element with expected.

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So this is a little bit of a confusing function there

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because it uses some parameters together with

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other parameters to find a third parameter.

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So a little bit weird but lets say now we have a list of

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friends and we want to find out which friend is called

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Bob Smith. Which here doesn't doesn't exist.

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The first thing we have to do is we have define a function

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that will run on one of these dictionary's,

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and give us back the name.

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So we will say, get friend name,

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and we will make it take one parameter

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and we'll say return friend, name.

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So here we have a function that, if we run it on an element

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in this sequence, we give us back the name property

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of that element.

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Be that what it may, remember this may fail if the element

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doesn't have a name property.

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Then we're going to print out the search,

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which is this function up here.

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We're gonna pass in the friends as our sequence

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so this search function is going to iterate

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over our friends.

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Then we're going to look for, Bob Smith.

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And finally how we're going to extract that value

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from each element is going to be by the

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get friend name function.

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So just to recap, get friend name

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will be the finder function.

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So that will run on each element.

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For example, for the first one, it will run on this

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dictionary here, and it will give you back the name

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property of that value. So you'll get back Rolf Smith.

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The Rolf Smith will be here, and you'll compare it with

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the expected value, which is Bob Smith.

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If they match, you will return that element,

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which is this dictionary.

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The same thing will happen for all the other ones.

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At this point, we don't have our friend called Bob Smith.

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So by running this we will get the run-time error.

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Could not find an element with expected.

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However, if we change this to Rolf Smith,

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then you will get it to work.

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The name is Rolf Smith and the age is twenty-four.

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This is quite a confusing bit of code but is a prime

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example of how first-class functions can be useful.

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So I would recommend giving it a go, playing around with it,

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see if it makes sense. And if it doesn't please ask a

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question in the course Q and A.

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Of course we're always happy to help.

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This is also a great place to use a lambda function.

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So instead of using that function, you can just define

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something like this,

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and just created it there where it is used.

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This is an alternative. No one is better than the other.

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You can do what you prefer. The other one is slightly longer

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but it does have a nicer name.

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Because you get the function name and it tells

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you what it does.

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So that's a good thing as well.

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Finally, you can use neither of these,

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do you can not define your own function or use a

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lambda function.

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You can just use a built-in from operator,

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import item getter.

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And here instead of this lambda function that is used to get

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a property of a object,

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you can use item getter

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with name.

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And that does exactly the same thing.

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This here, is a function that creates a function and

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lets you use it afterwards.

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So this is a little bit more advanced. We're going to learn

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more about functions that create other functions later on.

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So there you have it, that is the same thing.

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I set aside the operator built-in module,

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in fact quite a few of

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the Python built-in modules are amazing and really useful.

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All right, that's everything for this video.

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I hope you've enjoyed it, I hope you've learned something,

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and I'll see you in the next one.