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- [Narrator] Hi and
welcome back to the course.

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In this video we are going to be improving

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these things a bit by
using filter functions.

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Instead of this iteration,
which looks a bit ugly,

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we're going to do something

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which is item equals filter.

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And now what we're gonna do is

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we're going to filter the list of items

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that is our in memory database.

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And we are going to just get the one

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that matches the name.

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Now if you've looked at the lecture

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in lambda functions,

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we discussed briefly the

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filter function there and

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you may be able to
implement this on your own.

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I would recommend that
you try that just now.

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It'll improve your learning if you try

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this on your own first.

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Now that you've done that,

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the solution is to say filter

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and then we're going to write a lambda

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function for what we're
going to filter by.

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So what we're gonna do is say

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lambda x: x name equal equal name

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and then comma items.

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So the filter function
takes two arguments.

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First a filtering function

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and then the list of items
that we're filtering.

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Essentially what's going to happen is

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exactly the same thing that happened

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in the foreloop.

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We're going to go through each item,

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execute this function and

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see if the item's name x matches

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the name which is the parameter.

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And if it does, then
we're gonna return it.

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So how do we do it essentially?

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How do we return the item?

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The filter function
doesn't return an item.

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Potentially there could be many items

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generated by this filter function.

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So it cannot possibly return a single one.

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However it also doesn't return a list.

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It returns a filter object and

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a filter object is an object that you can

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call a couple of methods on,

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such as list.

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And if you call the list method

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on this filter object, guess what?

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It returns a list of all the items

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that matched the filter function.

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Now we should only have one item

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with a given name,

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so we know that this filter function

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will most naturally only

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return a single item.

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So instead of list, which gives

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us a list of items and

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that would give us a
list of a single item,

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we can use next and that gives us

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the first item found by
this filter function.

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Do notice that we can call next on

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this filter function and that would

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give us the first item matched

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by this filter function,

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but we can call next again if there are

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more items and that would give us the

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second item and then the third item

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and so on.

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In our filter function, there is only

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goin to be one item so we're never gonna

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want to call next more than once.

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And also do remember, next can raise

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an error and it can break our programme

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if there are no items left.

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For example, if we don't have any items

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in our database and we try to

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filter it looking for an item

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and then we call next on that.

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It would break because there are

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no items that match the name.

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So what we can do is say comma none

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and then what happens is that

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if the next function doesn't find an item,

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it will just return none.

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So that's kind of a default value there.

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So what does that mean?

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Well, that we can say return item is item

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and now this item variable which we

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declared up here is going to be either

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the item that matches the name

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or it is going to be none

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if none of them matched.

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We don't want to return 404 though,

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so what we'll do is use
a ternary if statement.

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200 if item is not none

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else 404.

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So we're going to return a code 200

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if the item is not none

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and if the item was none,

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then we're going to return 404.

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Now item is not none can be shorthanded

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and simplified to be just item.

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So 200 if the item exists essentially

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and if not, 404.

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Okay?

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Now we've decided that we're normally

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only gonna have unique names for items,

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so let's ensure that.

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So I'm gonna once again use this piece

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of code here and I'm gonna say

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If the sort of next item

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in the filter function,

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so the only item that matches this name,

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is not none.

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So what does this mean?

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If we found an item matching this name

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and that item is not none,

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it means there is an item
that matches the name.

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And if there is, we don't want to

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create a new item.

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All we want to do is return.

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And we're gonna return
a message something like

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An item with name blah already exists.

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Make sure I spell things right.

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Dot format, delete that, name.

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Okay?

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So to recap, if we found an item

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and that item is not none,

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then we're going to
return this message here.

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Remember is not None can be omitted

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because that is the default.

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So if this thing here exists,

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that means it is not none.

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Then we're gonna return this message.

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Now what is the cause for

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the item already existing?

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That is a bit of trick question

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because the cause for the item name

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already existing is not that we've

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added it before.

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This is not our application's fault.

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This is a bad request because

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the client should have verified

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that this name already exists or

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at least we can tell them that

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this name already exists but

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it is their fault because they

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made a request with an invalid name.

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It's invalid because
it's already in this app.

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So what should the code be?

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The status code for this message.

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You got it, it is 400.

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400 is bad request and that is essentially

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what happened here.

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Well, that's everything for this video.

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Hopefully you learned something and

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we have now improved the code here

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to have some error control and

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also to just be slightly nicer

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to read and to edit.

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Thanks for everything and

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I'll see you in the very next video.