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You have completed A Social Network with Flask!
You have completed A Social Network with Flask!
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Eventually, someone is going to type in a URL that doesn't exist, or a link is going to expire. Flask can handle these on its own, but it's nice to provide a custom error page. Let's see how to do that.
New terms
-
abort()- Function to immediately end a request with a specified status code. -
errorhandler()- Decorator that marks a function as handling a certain status code. -
return render_template('template.html'), 404- The404on the end specifies the status code for the response.
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In every web app, at some point, someone
is going to go to a bad URL.
0:00
We have some places where that may happen
more often than not, namely where we
0:05
follow and unfollow users, or viewing
user's stream, or viewing a single post.
0:08
Let's set up a view to handle these 404
errors and trigger them when we need to.
0:13
So we wanna be able to handle when a URL
is wrong.
0:18
So like for instance, this one.
0:21
Since this is generated by Flask,
0:23
we don't really have to worry about this
being wrong.
0:24
But we're putting in this post ID and it
ends up with the URL that looks like this.
0:27
And what if somebody was like, oh, they
just you know, fat-fingered it and
0:31
they wanted to go to post 15.
0:34
So it comes out like this, with nothing
there.
0:36
And that's okay, but it'd be nice if it
rendered like a 404,
0:38
or maybe we tried to go to a user, and we
typed their name just slightly wrong.
0:44
We'll get this user doesn't exist.
0:50
So it'd be nice to get something that we
can kind of control.
0:52
So, let's go over here to our app.py.
0:55
And we need to add an import up at the
top.
1:00
So where we're importing all this stuff
from Flask, we need to add one new item,
1:03
which is the word abort.
1:07
So what does abort let us do?
1:09
Abort lets us end a request.
1:12
So post, I think is fine.
1:16
Cuz that's where we're posting a new
thing.
1:19
Index is fine.
1:20
Stream we probably wanna add some testing,
too.
1:21
So, here we have this if username and
username ba, blah, and
1:25
we trying get this username.
1:28
So, let's add in, a try here.
1:29
And I'm gonna actually bump this to the
next line,
1:35
just cuz that line's getting a little
long.
1:38
So, what if that user doesn't exist?
1:41
We'd get a models.DoesNotExist.
1:45
And then we need to do something, so we're
going to abort, and
1:49
we're gonna abort with a 404.
1:51
That's the error [SOUND] code.
1:53
So, 404, if you know HTTP status codes,
404 is the file doesn't exist.
1:57
Technically, our user doesn't exist, so
that file doesn't exist.
2:04
The rest of that we can leave alone.
2:07
And let's see.
2:10
What other ones would we want to change
like this?
2:11
I think probably the follow would be a
good one.
2:13
So, here where the user doesn't exist,
this would be a place to do the abort 404,
2:17
and we're gonna wanna do that in unfollow
as well.
2:22
And, I think the other place we'd probably
want this would be in the view post.
2:27
Sorry, I'm kinda scrolling back and forth
a lot.
2:32
Here we have those view posts and I think
we probably wanna do like, well,
2:35
I almost wonder if instead of doing a try
except there, we'd really wanna
2:40
do something like, if posts.count single
to 0, as if nothing comes back.
2:45
Then we did the import 404.
2:52
So, they put in a number where there is
nothing that matches that number.
2:54
We won't get a models that does not exist
because we're doing a where,
2:58
we're doing this filter.
3:01
Where it's just like, hey, give me any
posts you have that match this id.
3:02
It could give me 20, it could give me 2,
it could give me none.
3:06
And peewee doesn't really care.
3:09
So we have do this kind of check for the
if posts.count.
3:12
'Kay.
So we've rendered these 404's.
3:15
And let's, let's see what happens there.
3:17
So this is easy, it doesn't exist.
3:19
I get a 404 and this is just a manual
standard everyday 404.
3:22
And that's fine.
3:26
We could leave it like this.
3:28
But let's provide one that's a little bit
nicer.
3:29
Let's go over to back to our app.py, and
3:31
then down at the very bottom just because
it seems cleaner.
3:35
We're gonna add a new view down here.
3:39
And we're gonna say not_found and it's
gonna take an error.
3:41
And we're gonna return render_template.
3:45
And we'll call it 404.html.
3:49
And then we're gonna do this comma 404.
3:52
So, what is this?
3:55
This is the status code to send back.
3:56
So we're rendering this template, and this
is rendered as a 404.
3:59
The one other thing we have to add to this
is a new decorator,
4:03
which is app.errorhandler.
4:07
And we specify 404.
4:10
So, any time a 404 is triggered.
4:11
Our app will use this function.
4:14
And then, of course, we need to go look at
our 404.html.
4:17
So let's generate that new template.
4:22
So 404.html.
4:27
And let's do extends layout.html,
4:30
and we'll do block content, do an h1 of
404.
4:36
We'll do a p, and we'll say, Wow, sorry,
that page doesn't exist.
4:43
And we'll do a new p, and
4:50
we'll do a a href with a url_for index,
4:53
and we'll say Try again.
4:59
All right?
And then we'll end our block.
5:02
Okay.
5:06
So.
5:07
Let's go see how this works.
5:08
Let's refresh this page.
5:10
So we'd probably wanna clean this up some.
5:12
But, it definitely gives us the start.
5:14
And let's see.
5:18
If we were to go here, and go to a post
that doesn't exist, same thing.
5:19
Awesome.
5:24
It's nice to see that we can get a
reliable and stylish 404 page.
5:25
Great.
5:30
Now our app is more useful to visitors and
5:30
let's them know what's going on if
something goes wrong.
5:32
We should probably add views for errors in
the 500 range as well, but
5:35
I'll leave that up to you.
5:38
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