WEBVTT

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<v ->Alright, so continuing on with our project.</v>

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We're almost done at this point,

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but what if our data listBox isn't linked to another one?

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So we're going to need some way

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of linking two listBoxes together

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and this on_select method you can see here on line 59.

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Well, that should only try to requery

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another listBox if there actually is one.

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Now in a minute, what we're going to do

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is add another couple of fields to the class

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to handle the link.

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We're gonna finish off this method first, though.

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We're gonna get it to check

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that there is a link to this box

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before trying to requery one, so let's do that.

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We're going to put if self.

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so that's linked, and it's called box:

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we're gonna add that field shortly as I said,

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and then we're going to indent everything else.

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I'm gonna remove this extra commented out code now,

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which we no longer need.

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I'm also going to indent this code.

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And I'll just put another tab or space in there

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because IntelliJ was complaining

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that the hash was too close to the code,

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you need to put at least two spaces

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when you're actually adding a comment.

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So I've got rid of that warning.

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The other thing we're going to do here

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is change this albumList.requery,

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let's change that self.linked_box is what it should be.

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So that's self.linked_box.requery link id,

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so we haven't got the aristList hard coded anymore.

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So before we can see the new class working

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we need to add a way to link

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one data listBox to another.

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Now, not all listBoxes will be linked.

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Our songs list doesn't cause another box to be updated

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and we may want to use a single data listBox

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to display a database table

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without linking it to anything else.

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So rather than insisting that another data listBox

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is provided in the class init method,

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we're going to add a link method

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that can be used if required.

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Now we're going to use the link method

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by passing it a widget that we wanna link to

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and a database column that forms that link,

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and that means we're going to need

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another couple of data attributes in our class.

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Let's go back up to the top and add those data attributes,

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and we're going to put them just after

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the super call here in line 30.

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So first one, self. and it's going to be linked_box,

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we set that to none initially,

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and likewise for self.link_field

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we're going to set that to none,

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so that's our two attributes.

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And again, we're defaulting to none

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so that there's no link to start with.

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Now, if we need to link two listBoxes

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then we're going to call that link method,

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so let's add that method,

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and I'm gonna add that just after the clear method here,

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so it's gonna be def link,

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we've got self as an argument,

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, widget , link_field:

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and we're gonna put self.linked_box = widget

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and then we're gonna do widget.link_field

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gonna set that to link_field,

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so we're basically saving the value

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that was passed in the argument.

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So at this point whenever we want

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to link two listBoxes together,

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we can call this new link method of the master listBox,

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then we're passing in a reference

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to the data listBox that we wanna link to

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as well as the name of the column

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in the linked data listBoxes table.

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I'm gonna go through what this method's doing again

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once we've seen it working

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and can refer to the actual widgets.

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So that's fixed some of the warnings,

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if we have a look at our own query method,

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that's looking a lot better now,

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we fixed some of those warnings,

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but we've still got one more error to fix,

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so we come over here and have a look

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in our right margin and click on that,

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and you can see I've got a problem on line 115,

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and you can see with the code there

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that that's where we downed the listBox select event

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to the get_albums function.

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Now we've moved that function

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into the class and renamed it,

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which is why we're now, of course, getting that error.

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Now there's no reason why we have to perform

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the bind in the main programme.

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Our data listBox class knows

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that it's got a nonselect method,

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so we can bind to the method itself.

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So I'm gonna cut that line of code out,

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and I'm going to paste it to the data listBox init method,

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so I'm gonna go right up to the top,

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the init method up here,

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and I'm going to paste it after our data attributes

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before the sql select statement,

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paste it in there like that.

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Now we're no longer binding to the get_albums function

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so we have to change that in our on_select method,

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and we also wanna use self, though,

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instead of referring to the artistList widget,

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so I'm gonna change that to

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remove the artistList call there, that should be self.bind.

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The listBox select is still correct,

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but instead of the get_albums function

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it's going to be self, or method I should say,

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it's going to be self.on_select,

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and be careful when you're making that change

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because you can see here that IntelliJ tried to be helpful

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and added the parenthesis after the on_select.

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We actually don't want that, we wanna remove that,

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and again, the reason we're moving it is

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that if we left it with the parenthesis

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then that would result in binding

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to the result of calling the method

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and our method returns none in this case,

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which, by the way, is the default

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for all functions and methods

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if you don't explicitly return anything,

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and we wanna bind to the method,

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not to the value that calling it returns,

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so watch out for that.

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I'm saying, I know we've talked about that in the past,

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but it's a very easy mistake to make,

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especially with IntelliJ's, you know,

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quote unquote helpfulness here.

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Alright, so I fixed that,

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now I'm going to delete one of those lines

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to keep IntelliJ happy.

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Okay, so once our main code's created the albumList

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we can actually link it to the artistList,

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so let's go ahead and do that.

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We're gonna go down here

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and the moment we've got this albumList.bind

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to get_songs, so I'm actually gonna comment that out.

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What we're going to do instead

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is do an artistList.link

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and it's going to be to albumList,

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and then the field artist.

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Moving down we can do the same

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to link the songList to the albumList,

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so let's go ahead and do that.

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Then we'll do, this case albumList.link

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and that's going to be to songList in this case

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and then album, that's the column name, or the field.

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Alright, so let's run the programme.

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It should allow the album and songLists to be updated

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when items are selected in the list that they're linked to,

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so we'll actually run that and see.

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Go ahead and have a look at the code,

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and we can select, should be able to select an artist,

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we can see that's updating okay,

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and if I come over and click on an album

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we actually get a crash,

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so it's almost working.

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So selecting the artists you saw worked fine

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and the correct albums are displayed,

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but soon as I actually selected an album

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we actually had a crash.

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And if you come down here and look at the error,

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and I'll just close that down for now,

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and the error's saying here

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that there's no such column artists.

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Well the songs table doesn't actually have

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an artists column, so we shouldn't be referring

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to the artists column in the songs table,

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so obviously at this point something's not right.

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So if you click on this link to here in the error trace,

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that actually takes us to the line where the problem is.

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Well, that's actually the line

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that's actually executing the sql,

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but if you look above on line 56

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that's actually where the problem is.

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So I've still got the artists column hard coded

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as you can see here, into the sql where clause,

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and what we should be doing is using the link field,

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the one that was set when we call the link method,

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so let's go ahead and change that.

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So I'm going to delete the artist in the double quotes

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and we're going to replace that with self.link_field

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so we get the right column for the right table.

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But that raises another question, now,

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and that question is, what happens

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if we don't have a link field,

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because the listBox hasn't been linked to another one?

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Well in that cause, calling requery

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should populate the list with all the rows

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so we wanna execute the code

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in the else block in that scenario.

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So we can fix this by adding a test for the link_field

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as well as the link_value.

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So if we go ahead and do that,

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so I've got if link_value here on line 55,

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if we do or add the words "and" and self.link_field

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we're now only gonna execute that

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if there is a link that has been established

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between the two listBoxes.

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Otherwise the else code is gonna be executed,

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and reflectively going to get the entire contents

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of the table added to the listBox

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and that's actually what we want in this scenario.

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Alright, so we're going to also make a change down here

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after the call to self.clear,

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then we're actually through and adding the code there

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so you can see that adds the entries to the listBox.

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What we need to do also is down here

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put in some code if self.linked_box

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then we wanna do self.linked_box.clear.

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Now the reason we're doing that,

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that's to clear out the data in any linked data listBox

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so that way when we select a new artist

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the songs for the previous album aren't still displayed,

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and that's the reason, by the way,

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that we added a clear method in the first place

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rather than just deleting the list items earlier.

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So we need a method on the linked listBox

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to tell it to delete its contents.

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Alright, so let's actually try running this again,

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and hopefully things will actually work properly now.

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Alright, so if I select an artist,

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okay, so we've got an error there,

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let's just have a look.

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Can't convert DataListBox object to string implicitly,

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okay just click on that,

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and I've made a typo here,

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I've actually typed in self.linked_box,

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but it actually should be link_field,

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because the field is ultimately gonna be

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the column name for the database

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that we're trying to add,

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so that's just a typo, so I'll fix that.

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Link_field, that should fix that up,

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so let's just try running that again.

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Let's see how we go this time.

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Alright so, we'll try running it again,

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this time we haven't got an error there, that's good,

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so we've got artists showing,

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albums you saw was updating automatically,

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so each time we're clicking we're getting

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the list of albums, so that's good,

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and multiple albums are being shown.

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Now if we click on an album

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we're correctly getting the list of songs,

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and that's updating as well.

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So that's good, everything's now working great,

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and you can see that as I was selecting items

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in any one odd data listBox,

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the corresponding data is displayed

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in the linked data listBox,

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and I think at this point that's pretty impressive.

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So we've taken the basic tkinter listbox

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and now created our own scrollable version of it,

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and with that extended that further

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to produce a data listBox that can

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load its own data from a database table

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as well as triggering loading in another data listBox.

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So the code in the main programme becomes a lot cleaner

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and easier to instal as a result of this,

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but with that said we've still got a bit

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of tidying up to do there in the code

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before I end this video,

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and I'm just gonna close this down.

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Now to start with, we don't need

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this get_songs function anymore

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'cause that's now all handled by our data listBox class,

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and we also don't need to connect to the database

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right at the start of the programme.

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Now, we did that originally because our functions

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were using the global database connexion,

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but now that we're passing the connexion

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to our data listBox objects

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that code can come down to the main programme.

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We should also add a condition

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so that the main code isn't executed

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when the module's imported by another programme,

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and that'll then let other programmes

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use our scroll box and data listBox classes.

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Let's go ahead and do that,

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so I'm actually going to delete the get_songs function

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because that's existed in quite a few

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other versions of the video prior to this,

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or videos prior to this, so we don't need to preserve that,

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and clean up some of this.

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Let's also add that code there

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to make sure that the main code

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isn't executed when the module's imported.

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So if __name__ is equal to '__main__' :

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and from there we want to indent the code

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but we gotta start by moving this connexion,

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I'll comment that out and we'll delete this,

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clear up some spaces there,

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then I'm gonna come down and put that code in here,

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and then we're going to indent this other code,

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then go right down to the bottom

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and indent all of that, like so.

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Now we also were loading data

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into the albums and songLists when the programme starts,

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and if you recall we did that when testing earlier

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but we can now go ahead and delete those lines now.

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So in terms of that code you've got

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this requery here, firstly for albums,

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so let's, I'll just comment that out.

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You could just as easily delete that if you prefer.

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We've also got another one in the songs,

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the songlist.requery, which we no longer need

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to have that code there anymore.

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But of course we do wanna leave

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the original artists in there,

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so that all the artists are initially loaded,

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so that's good, that's now fixed.

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And down here for the main loop

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we can also get rid of the testList

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and the albumLV.set tuple,

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you don't actually want that to be populated either,

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so let's actually get rid of that as well,

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comment that out.

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Which really only leaves the code for the main loop

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in printing, the fact that we're closing

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the database connexion and then the connection.close,

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so let's just run this to make sure that it still works.

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So I've now only got the artists populated initially.

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We can click on a particular artist.

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By the way, we can click on songs as well,

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and because that's not associated with any other listBox

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we're not getting any errors,

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and that's working as well as we think it should be working,

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and we're still getting our other selects working correctly

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and the various data listBoxes being updated on the screen.

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So that's actually it, that's our data ListBox finished.

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I think you'd agree that the main program's now

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a lot tidier than what it was,

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and we haven't got any code copied and pasted,

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more to the point.

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We've also now got two completely reusable classes

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that we can import into other projects

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whenever we need a scrolling listBox

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or one that can display data from a database table.

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Now, there is a bug in the data listBox class, though.

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Let's see if you can figure out

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and try and spot what that is.

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In the next video I'm gonna show you what the bug is

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and then set you a challenge to fix that bug,

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so I will see you in the next video.