RESTful and HTTP APIs in Pyramid Transcripts
Chapter: Content negotiation
Lecture: Adding a negotiating renderer

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0:01 Now let's pick up where we left off with our app previously.
0:04 We have our csv, our image and json renderers,
0:07 we have our abstract base type that enforces some of the common functionality,
0:11 and notice, this green one, I've now added a new type a negotiating renderer factory,
0:16 and this is just a totally empty implementation here,
0:19 we've got our ability to have adapters, we've got our call
0:22 our renderer, our can serialize and add adapter,
0:25 this is just basically what we need to be a renderer abstract base type.
0:29 So what we're going to do is we're going to apply a design pattern here
0:32 we're going to apply something called the composite design pattern,
0:36 so let me just give you a quick background if that's new to you,
0:39 so the idea is we may work with fundamental types that kind of do individual things,
0:43 but if we can somehow put the same interface on a group of things
0:46 and then we could treat that group as a building block in our system as well,
0:51 so maybe you think of like a gui type of thing, right,
0:55 maybe you've got buttons, you've got windows, you've got text boxes,
1:00 things like that, and those all have a common api let's say;
1:04 well, what if you want to have a frame that can contain many of them;
1:07 well, if you could add on to that frame object to have basically
1:11 all the behaviors, buttons and whatnot,
1:13 but also has a collection of children and somehow could manage those,
1:16 well then you can treat a frame like you could a button
1:19 and move it around and position it and render it, things like that.
1:22 So we are going to apply the same basic idea here for renderers.
1:25 So you can see, the negotiating renderer factory
1:27 implements the renderer abstract base
1:29 but we're going to actually have a few more pieces of functionality,
1:32 so let's go down here and add a renderer,
1:35 so we are going to add this function where we can pass in a renderer
1:38 and it will be used when a certain content type is asked for,
1:42 so we'll ask for the content type, so we could say something like
1:45 for application/ json use this renderer,
1:48 for image/ png use this other renderer, and so on.
1:52 Now we'll just call this renderer, but we can enforce
1:54 that this is a render abstract base class.
1:56 Now of course, this does nothing right, this is a suggestion to the editor
1:59 by enforce I mean if not is instance, then we'll raise an exception.
2:04 Renderer must be abstract based, must be a based renderer or something like that
2:08 Ok, so from then on, we pretty much are guaranteed like hey
2:14 we're actually working with a known type, that we can call on,
2:19 we can serialize values, we can add adapters, all those kinds of things.
2:23 The next, let's do a little bit of normalization on our content type here,
2:26 so we'll say, so we're going to lower case it and strip it
2:32 we're going to use this for a key, and so if somebody says
2:35 capital json versus the system passing the lower case json or something like that
2:39 we don't really want to have that be a variation right,
2:41 it doesn't really matter, it's not case sensitive as far as we're concerned.
2:44 And now we need somewhere to put these renderers
2:46 and so well put them like so, in just another dictionary
2:49 and how are we going to find them, we're going to find them by content type,
2:52 so just like we looked up adapters by type, and then we got a function,
2:56 now we're going to look up renderers by the type that they can transform to,
3:01 like text/ csv, and then we'll have the type which is one of these,
3:06 this would be like a json renderer or a csv renderer, something like that.
3:10 You're going to see that this is all well and good, but depending on the situation
3:13 the client may not even pass a content type,
3:15 and so we might want to explicitly say well if the client just says
3:19 I'll take whatever, what renderer is supposed to handle that situation?
3:23 So let's add something like this, like default renderer,
3:27 or let's say accept all, because that's basically
3:32 alright, so I have a renderer that's going to come in here
3:36 and all we're going to do is say self.add renderer,
3:38 and the content type is going to be */* and this is going to be the renderer.
3:43 Okay, this is looking pretty good, now I think we have this kind of generalized thing
3:49 let's go ahead and integrate it into our system down here,
3:53 so we've got our json renderer, our csv renderer, an image renderer,
3:56 and let's go down here and have a negotiate renderer,
4:00 it's going to be one of these types here,
4:03 and now we need to basically say hey all of these renderers are available for you
4:07 in what particular content type are those is going to be;
4:10 now, you might say well, we still need to have these lines
4:14 we still want to register them independently, it's up to you,
4:17 but if you want to be able to say this one is not negotiable, this one is json
4:20 then you want to leave line 56 for example.
4:24 All right, so let's go over here to our negotiate renderer and say add renderer,
4:28 and let's say application/ json is going to be the json renderer,
4:33 and also, let's make json the default, accept all renderer,
4:38 and then for text/ csv, grab the csv renderer,
4:43 and for image/ png, we'll have the image renderer.
4:49 Now, this is configuring this object, but still, the last thing we have to do
4:53 of course is to register with pyramid.
4:56 So it's going to go like that, and let's pick a name that we can put here
5:00 we'll say negotiate, if we say negotiate, that means we're going to pick this renderer,
5:05 which actually has this functionality built into it. Well, it almost does.
5:10 Okay, let's go to our API and focus on let's do this all cars thing here,
5:16 and here we have a bunch of different versions let’ s get rid of this,
5:19 let's go over here and say okay this one if you do a get
5:25 we're going to let you negotiate what it is, and we're just going to return the cars.
5:28 Now, remember, we've already taught all three of these to deal with cars
5:32 and so we don't have to worry about anything here,
5:34 this should just delegate to those other pieces and you'll get cars
5:37 so if you ask for application/ json, it'll find its way over to here
5:42 which knows how to deal with the car;
5:44 if you ask for text/ csv, it will find its way over here,
5:46 which knows how to deal with the car.
5:48 So, as far as what we do in our code it's totally simple, it just return cars.
5:51 Now, this is not quite perfect yet,
5:54 because we haven't finished implementing the negotiate renderer,
5:57 but let's go and just make a request and see what we get.
6:00 All right, do ascend and you can see we get nothing,
6:02 although it does come back ok, so that's kind of cool, sort of,
6:05 and why did it come back ok, well, because we just return none.
6:09 So now, the question is what are we going to do when we render this,
6:12 well first thing, let's go ahead and get a hold of the request,
6:15 the next, we need to figure out what are they asking for, right,
6:20 so what they're asking for is sent over in the headers, here.
6:25 So in this case, I said accept is application json
6:28 so let's go and actually get the headers and pull those out
6:32 so we'll say accept headers and this should be accept, like this okay.
6:36 You can print out really quick, found accept header,
6:39 and who knows what format that is,
6:43 so let's say accept headers like this, so if we run this again,
6:47 we go over here and click this, what are we going to get,
6:49 you can see we found accept headers this,
6:51 so let's take a first pass and say all right, we're just going to like use this as the key
6:55 it turns out that that's insufficient, but let's go ahead and just say
6:58 we are going to use that as the key,
7:01 so let's go ahead and apply this lower case normalization to it
7:04 and we'll say something like this, we'll say if not accept header,
7:07 so maybe if they don't pass anything here, what are we going to do
7:10 we're going to return, we want to get the renderer,
7:13 so self.renderer, that has */*
7:16 now this assumed somebody's called that function
7:18 so you want to be a little careful but it's okay,
7:21 so we're going to have it render with a value and system
7:24 who knows whatever that means, okay.
7:27 Next, we need to find the renderer, so renderer, let's do it like this
7:30 we had to come over here and we'll say accept,
7:33 self.renderer.get, and we'll just pass it the accept header.
7:37 This is not good enough, but for the simple request it's going to work,
7:43 so we'll say if not renderer we'll raise an exception like could not
7:49 alright, so if we don't find one, we're going to complain and say hey
7:53 we didn't find one, you asked for say application/text or text/ I don't know,
7:59 some type that we have no idea about it,
8:02 there's many mime types and content types.
8:05 So then, once we have this, now we're just going to go down there and say
8:08 if we do have one, let's just return renderer.render,
8:13 pass the value, pass the system, okay.
8:16 This is looking good, let's see, I think we might be good
8:24 let's go ahead and run this and see if we get, if we got it working yet.
8:27 We don't want to see non down here, we want to see a list of json ;
8:30 and, it has no render, oh yes, yes, yes, okay I'm calling this incorrectly
8:35 so let's do something really quick here, let's say and down here
8:41 let's store this, okay, so instead of doing this renderer
8:45 we need to do the call to get a hold of the object,, and then
8:50 we're going to get back the render function, and then we'll do this,
8:55 so yes, that was a little bit off, wasn't it.
8:57 Alright, remember these renderers are render factories
9:00 and so this call right here creates an instance of it
9:03 by calling this function which returns this render function
9:08 and then we're calling value, passing value system to the function return there.
9:12 All right, try again— there we go, perfect, json.
9:16 I think that other code that I wrote would have worked fine for images, for csvs,
9:22 but because we're using just the built in json 1
9:25 for this type with a little adaptation, it doesn't have that _render,
9:29 let's ask for something else, let's go have a different accept type, and we want csv,
9:34 look at that, that is awesome, that is great,
9:40 so we're going to this one function right here, let's look up with api,
9:44 we're going to this one function, and we just said,
9:47 I don't care what renderer you get me, you do your best that you can
9:50 to provide the type that they're asking for,
9:53 so if we come over here and ask for text/ csv, we get this
9:56 if we come over here and ask for json, we get this;
9:59 now if we ask for something that can't handle like image/ png
10:03 which the individual one can handle, but the list like I said
10:08 would you make a composite image, what the heck would this be,
10:11 it'll say error, could not convert type list to an image basically,
10:16 which I think that's reasonable, I can't imagine we could do much better than that.
10:20 So it doesn't mean that it's always going to have a good answer
10:24 but I think it makes a lot of sense.
10:26 Put this negotiation thing in there, and let the client ask.
10:29 Now, we can just set the default, like hey, if we don't know what it is,
10:32 then do json, so for example one adaptation we could make here
10:36 is right now I'm raising an exception, say look if you ask for a type
10:39 and we don't have a renderer that maps to it
10:42 we could just return the default type, right, we could say okay fine
10:46 give me the one for this, right, this will be the all, the accept all renderer,
10:51 so that would be a possibility, or we could just say just return json or whatever
10:56 but we chose to say look, we have no way to deal with this,
10:59 if it's a type that we haven't preset up, so you can be
11:03 more precise, more restrictive, less precise, less restrictive
11:08 by just falling back to json or something to that effect.
11:12 Ok, but that is how we work with our renderer,
11:15 now we don't have to do any of the adaptation stuff because it's already happening,
11:19 we may want to delegate here, like we might want to say something like this
11:24 for renderer in self.renderers.values, right, so if somebody calls
11:30 here is how you move a car over like, suppose I take this off here for now
11:35 and try, this should crash when I call this function,
11:39 don't have a converter right, but if I go and I add this adapter to the generalized one
11:44 now careful, it has to be done at the end, copied a little too much there didn't I,
11:50 there, now that should add it to all of them,
11:56 it looks like that little bit here— let's go and fix up that json renderer,
12:04 add adapter, it looks like I misspelled add adapter,
12:09 okay, so there we go, now let's just check if I remember this didn't convert
12:17 because we had no adapter for car, now we've added it back in.
12:21 All right, so this is probably a good implementation,
12:25 where we have this adapter add in for each one of those,
12:29 but we still need to be over here, if we're going to use it individually
12:35 we still need to add it on this one as well,
12:39 so how much value is that, you know it's up to you guys— let's put this back as well,
12:43 okay, so this is the composite pattern in action, right,
12:48 this thing really holds a bunch of these, it's made up of a bunch of renderers
12:51 but in fact acts as its own renderer, which means that we can just go here
12:55 and say render=negotiate, and however we've configured the system
12:59 it's just going to handle that for us, and that's quite a cool thing.
13:03 Let's go down here and fix up this other one,
13:06 we have single auto and single auto image, that's super annoying, isn't it,
13:09 let's apply a little fix here, for this as well;
13:12 so we have this, we have get, but this accept type can now go away
13:16 and this is now also going to be negotiate, all right,
13:19 how about post this one, it's your call,
13:23 does it make sense to put it here, maybe it does,
13:26 it doesn't, the renderer doesn't mean you can post like csv,
13:29 so you have to post json right, from here right, really from right there,
13:33 but the type you're going to get back does it make sense—
13:36 I'm thinking for the creating auto since you got to post json,
13:39 I am going to leave that one alone, the put, this one doesn't have a renderer,
13:43 it doesn't do anything, same with delete, it doesn't do anything.
13:45 So this one I think I'll go and leave this as json
13:48 like if you're creating an auto, maybe just it's json in, it's json out.
13:52 But everything else here is fine,
13:56 now notice, this one this responds 404
13:59 and this json body again, if there's an error, you're getting json back,
14:02 basically but if there's no errors, you get what you ask for.
14:05 I'm okay with that to keep it simple, we could do a lot of work
14:09 we could actually like somehow work with the renderer here
14:13 but let's not do that right, error cases, they can just look at the status code right
14:18 404 is going to be telling them everything they need to know
14:21 and that works with all the different formats.
14:23 But this one I think is really sweet, right
14:25 we've added this negotiate and now it can do whatever it wants,
14:28 let's get rid of this, I think I had copied that from duplicating it somewhere and so on
14:32 but we don't need the csv, just all autos, right,
14:34 let's see this one more time, we've added negotiate to the single auto so let's go to that;
14:37 so here, for all the autos let's start with json, do we have it, we have json for text/ csv
14:45 we have csv, now we don't have image because who knows how to do that right
14:49 error 500 internal server error, maybe that's what we want
14:53 it could be bad request, like we could catch that, determine that,
14:57 return a bad request, but we're not doing that.
14:59 Now, let's get one of these individually, and let's start out with json again,
15:02 remember my little trick, first, so if we go ask for first, now we get json
15:07 pretty cool, go up here and ask for text/ csv,
15:13 name is not defined, what have we done wrong here?
15:16 Oh, again, that's supposed to be none,
15:22 we'll get the bugs out of the system all right try again,
15:25 so we ask for text/ csv, we get text/ csv, it's just one car
15:27 but you know, maybe your parser is expecting csv,
15:30 so here's the one car right, again json works.
15:33 Now, if we ask for image, we should get which one do we have configured,
15:36 I think we have the direct one configured
15:39 and boom— there it is, status 200 ok.
15:43 Let's just double check which one we've got running here
15:46 we have the image direct factory running,
15:48 so when we hit this, it goes off to the server, right
15:51 and sucks it down and then resends it through our web server, pretty darn cool.
15:56 Now, again, we can do this against an individual car,
16:01 we can do not that, but we could do this against all of the cars
16:05 if I can put that together correctly, because we just said negotiate.
16:09 Now, there is one more little catch that we have to be aware of
16:12 and we'll come back to that next.