Building Data-Driven Web Apps with Pyramid and SQLAlchemy Transcripts
Chapter: Routing and URLs
Lecture: Route constraints

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0:00 What we've seen so far is the basic routing
0:02 but there's a few advanced use cases that are
0:04 really, really helpful.
0:05 So, let's go over here for an example.
0:08 If I come over here and look at the
0:10 episodes that were recorded
0:11 I could click inside the new PyPI launch, awesome!
0:14 So actually if you listen to this
0:15 they'll talk about how they use Pyramid and various things
0:18 to build the thing that we're modeling for a course.
0:21 That was a lucky click.
0:23 Notice the URL is /episodes/show.
0:26 Really wish I'd have put details there
0:28 but that's in the past.
0:30 And then the number.
0:31 And then just for a little SEO
0:32 and helping people make sense of the URL, I put the actual
0:35 sort of friendly name of the episode as well.
0:40 And this is the main one.
0:41 But what I would like to do is
0:42 say for social media and stuff
0:44 let people just say, what is this 160, 159
0:48 like, let them just type
0:53 And notice that redirects over.
0:57 I'll just go here and just paste it.
0:58 It redirects back to the right site, the right page.
1:02 I want to add that functionality over here to our PyPI here.
1:06 This doesn't exist in the real one
1:07 but I want to be able to go like five
1:09 fifth most popular package.
1:13 That's pretty easy to do.
1:14 I could come over here and you could say
1:15 "Alright, well that just goes in the package thing here."
1:19 So let's go and just call this "popular".
1:25 And let's just say this is going to be {num}
1:28 as in 1, 2, so on.
1:30 Actually, I got to go ahead and add it over here, don't I?
1:33 Ah, make it listen somewhere; let's do this.
1:44 Now, we're going to show details 'cause it's
1:46 we're going to reuse this template here
1:48 because it's going to use a different route
1:51 but then it's going to effectively either just show that one
1:54 or probably what makes more sense
1:55 is what I do on TalkPython to do a redirect.
1:58 But let's just go over here, like this.
2:16 So we're going to convert this to an integer
2:18 and then let's do a little test that says
2:20 "If the number, if it's not, if it's not between 1 and 10
2:24 then we're not going to show the popular."
2:26 We're going to say
2:27 "You're not going to be able to ask for the 100th one."
2:29 I'll just say, "That's how it's working, alright?"
2:32 So then we'll say, "the whatever-eth popular package."
2:44 So we'll just drop that in here now
2:45 so we can use the template.
2:46 Really, we'd go to the database
2:47 and actually get what one that is going to be.
2:50 Okay, so, this is going to work, I think.
2:53 But not in the way that you are hoping it's going to work.
2:56 So, let's go over here, and we'll ask for 5.
3:00 The fifth most popular package. Awesome!
3:04 Well let's try to go home. That works good.
3:06 Let's try to go and see projects/requests again.
3:11 That works good.
3:13 What about, about?
3:16 Ah, so far we're getting lucky with the order.
3:18 Let's look over here
3:19 see if I can find something that'll break.
3:21 Yeah, let's try to go to account.
3:23 'Member, that worked before.
3:28 Account cannot be converted to 10.
3:30 Hm. Well, what's going on here?
3:34 So, there's a couple of things that work in this thing.
3:38 So, remember what I said is we start at the top
3:40 and you just go down into one of these matches.
3:42 Well this says, "I want to look just for
3:45 effectively / a single thing."
3:48 Well, account is a single thing.
3:51 So we need a way to tell it, "No, no no."
3:54 "That needs to be a number."
3:57 What we can do is we can actually add a little bit on here
4:01 to say, "No, we're going to check that
4:02 and this is going to be constrained in some way."
4:05 Now, this is not as wonderful as it could be
4:08 but let's go and do it. So we have custom predicates, is going to be an array.
4:12 But what we put in here
4:13 is we're going to put any function that
4:16 given some of the information that's in the route
4:19 whether or not it's matching this route, this popular route.
4:23 So we'll just create a lambda.
4:25 It's going to take info and some stuff we don't care about.
4:28 And then we're going to say, "Go to the info
4:29 which has the match data in it."
4:32 And we'll say, "Get me the num."
4:36 And if you don't have that, "Give me the match string there."
4:38 And on the string, I'm going to ask, isdigit().
4:41 And we're just going to return that.
4:42 So now if I rerun it, let's see how it works now.
4:46 First of all, account should not match, alright?
4:50 Account is not isdigit().
4:52 Here we go. So, account goes there.
4:55 But let's see if what we had before, like 5, still works?
4:58 The fifth most popular package. 8.
5:03 The first most popular package, first, and so on.
5:08 So now our system is working beautifully
5:11 and we're able to add that constraint right there.
5:15 Really, really nice.