I am new to Django and pretty new to Ajax. I am working on a project where I need to integrate the two. I believe that I understand the principles behind them both, but have not found a good explanation of the two together.
Could someone give me a quick explanation of how the codebase must change with the two of them integrating together?
For example, can I still use the HttpResponse with Ajax, or do my responses have to change with the use of Ajax? If so, could you please provide an example of how the responses to the requests must change? If it makes any difference, the data I am returning is JSON.
Even though this isn't entirely in the SO spirit, I love this question, because I had the same trouble when I started, so I'll give you a quick guide. Obviously you don't understand the principles behind them (don't take it as an offense, but if you did you wouldn't be asking).
Django is server-side. It means, say a client goes to a URL, you have a function inside views that renders what he sees and returns a response in HTML. Let's break it up into examples:
views.py:
def hello(request):
return HttpResponse('Hello World!')
def home(request):
return render_to_response('index.html', {'variable': 'world'})
index.html:
<h1>Hello {{ variable }}, welcome to my awesome site</h1>
urls.py:
url(r'^hello/', 'myapp.views.hello'),
url(r'^home/', 'myapp.views.home'),
That's an example of the simplest of usages. Going to 127.0.0.1:8000/hello means a request to the hello() function, going to 127.0.0.1:8000/home will return the index.html and replace all the variables as asked (you probably know all this by now).
Now let's talk about AJAX. AJAX calls are client-side code that does asynchronous requests. That sounds complicated, but it simply means it does a request for you in the background and then handles the response. So when you do an AJAX call for some URL, you get the same data you would get as a user going to that place.
For example, an AJAX call to 127.0.0.1:8000/hello will return the same thing it would as if you visited it. Only this time, you have it inside a JavaScript function and you can deal with it however you'd like. Let's look at a simple use case:
$.ajax({
url: '127.0.0.1:8000/hello',
type: 'get', // This is the default though, you don't actually need to always mention it
success: function(data) {
alert(data);
},
failure: function(data) {
alert('Got an error dude');
}
});
The general process is this:
The call goes to the URL 127.0.0.1:8000/hello as if you opened a new tab and did it yourself.
If it succeeds (status code 200), do the function for success, which will alert the data received.
If fails, do a different function.
Now what would happen here? You would get an alert with 'hello world' in it. What happens if you do an AJAX call to home? Same thing, you'll get an alert stating <h1>Hello world, welcome to my awesome site</h1>.
In other words - there's nothing new about AJAX calls. They are just a way for you to let the user get data and information without leaving the page, and it makes for a smooth and very neat design of your website. A few guidelines you should take note of:
Learn jQuery. I cannot stress this enough. You're gonna have to understand it a little to know how to handle the data you receive. You'll also need to understand some basic JavaScript syntax (not far from python, you'll get used to it). I strongly recommend Envato's video tutorials for jQuery, they are great and will put you on the right path.
When to use JSON?. You're going to see a lot of examples where the data sent by the Django views is in JSON. I didn't go into detail on that, because it isn't important how to do it (there are plenty of explanations abound) and a lot more important when. And the answer to that is - JSON data is serialized data. That is, data you can manipulate. Like I mentioned, an AJAX call will fetch the response as if the user did it himself. Now say you don't want to mess with all the html, and instead want to send data (a list of objects perhaps). JSON is good for this, because it sends it as an object (JSON data looks like a python dictionary), and then you can iterate over it or do something else that removes the need to sift through useless html.
Add it last. When you build a web app and want to implement AJAX - do yourself a favor. First, build the entire app completely devoid of any AJAX. See that everything is working. Then, and only then, start writing the AJAX calls. That's a good process that helps you learn a lot as well.
Use chrome's developer tools. Since AJAX calls are done in the background it's sometimes very hard to debug them. You should use the chrome developer tools (or similar tools such as firebug) and console.log things to debug. I won't explain in detail, just google around and find out about it. It would be very helpful to you.
CSRF awareness. Finally, remember that post requests in Django require the csrf_token. With AJAX calls, a lot of times you'd like to send data without refreshing the page. You'll probably face some trouble before you'd finally remember that - wait, you forgot to send the csrf_token. This is a known beginner roadblock in AJAX-Django integration, but after you learn how to make it play nice, it's easy as pie.
That's everything that comes to my head. It's a vast subject, but yeah, there's probably not enough examples out there. Just work your way there, slowly, you'll get it eventually.
Further from yuvi's excellent answer, I would like to add a small specific example on how to deal with this within Django (beyond any js that will be used). The example uses AjaxableResponseMixin and assumes an Author model.
import json
from django.http import HttpResponse
from django.views.generic.edit import CreateView
from myapp.models import Author
class AjaxableResponseMixin(object):
"""
Mixin to add AJAX support to a form.
Must be used with an object-based FormView (e.g. CreateView)
"""
def render_to_json_response(self, context, **response_kwargs):
data = json.dumps(context)
response_kwargs['content_type'] = 'application/json'
return HttpResponse(data, **response_kwargs)
def form_invalid(self, form):
response = super(AjaxableResponseMixin, self).form_invalid(form)
if self.request.is_ajax():
return self.render_to_json_response(form.errors, status=400)
else:
return response
def form_valid(self, form):
# We make sure to call the parent's form_valid() method because
# it might do some processing (in the case of CreateView, it will
# call form.save() for example).
response = super(AjaxableResponseMixin, self).form_valid(form)
if self.request.is_ajax():
data = {
'pk': self.object.pk,
}
return self.render_to_json_response(data)
else:
return response
class AuthorCreate(AjaxableResponseMixin, CreateView):
model = Author
fields = ['name']
Source: Django documentation, Form handling with class-based views
The link to version 1.6 of Django is no longer available updated to version 1.11
I am writing this because the accepted answer is pretty old, it needs a refresher.
So this is how I would integrate Ajax with Django in 2019 :) And lets take a real example of when we would need Ajax :-
Lets say I have a model with registered usernames and with the help of Ajax I wanna know if a given username exists.
html:
<p id="response_msg"></p>
<form id="username_exists_form" method='GET'>
Name: <input type="username" name="username" />
<button type='submit'> Check </button>
</form>
ajax:
$('#username_exists_form').on('submit',function(e){
e.preventDefault();
var username = $(this).find('input').val();
$.get('/exists/',
{'username': username},
function(response){ $('#response_msg').text(response.msg); }
);
});
urls.py:
from django.contrib import admin
from django.urls import path
from . import views
urlpatterns = [
path('admin/', admin.site.urls),
path('exists/', views.username_exists, name='exists'),
]
views.py:
def username_exists(request):
data = {'msg':''}
if request.method == 'GET':
username = request.GET.get('username').lower()
exists = Usernames.objects.filter(name=username).exists()
data['msg'] = username
data['msg'] += ' already exists.' if exists else ' does not exists.'
return JsonResponse(data)
Also render_to_response which is deprecated and has been replaced by render and from Django 1.7 onwards instead of HttpResponse we use JsonResponse for ajax response. Because it comes with a JSON encoder, so you don’t need to serialize the data before returning the response object but HttpResponse is not deprecated.
Simple and Nice. You don't have to change your views. Bjax handles all your links. Check this out:
Bjax
Usage:
<script src="bjax.min.js" type="text/javascript"></script>
<link href="bjax.min.css" rel="stylesheet" type="text/css" />
Finally, include this in the HEAD of your html:
$('a').bjax();
For more settings, checkout demo here:
Bjax Demo
AJAX is the best way to do asynchronous tasks. Making asynchronous calls is something common in use in any website building. We will take a short example to learn how we can implement AJAX in Django. We need to use jQuery so as to write less javascript.
This is Contact example, which is the simplest example, I am using to explain the basics of AJAX and its implementation in Django. We will be making POST request in this example. I am following one of the example of this post: https://djangopy.org/learn/step-up-guide-to-implement-ajax-in-django
models.py
Let's first create the model of Contact, having basic details.
from django.db import models
class Contact(models.Model):
name = models.CharField(max_length = 100)
email = models.EmailField()
message = models.TextField()
timestamp = models.DateTimeField(auto_now_add = True)
def __str__(self):
return self.name
forms.py
Create the form for the above model.
from django import forms
from .models import Contact
class ContactForm(forms.ModelForm):
class Meta:
model = Contact
exclude = ["timestamp", ]
views.py
The views look similar to the basic function-based create view, but instead of returning with render, we are using JsonResponse response.
from django.http import JsonResponse
from .forms import ContactForm
def postContact(request):
if request.method == "POST" and request.is_ajax():
form = ContactForm(request.POST)
form.save()
return JsonResponse({"success":True}, status=200)
return JsonResponse({"success":False}, status=400)
urls.py
Let's create the route of the above view.
from django.contrib import admin
from django.urls import path
from app_1 import views as app1
urlpatterns = [
path('ajax/contact', app1.postContact, name ='contact_submit'),
]
template
Moving to frontend section, render the form which was created above enclosing form tag along with csrf_token and submit button. Note that we have included the jquery library.
<form id = "contactForm" method= "POST">{% csrf_token %}
{{ contactForm.as_p }}
<input type="submit" name="contact-submit" class="btn btn-primary" />
</form>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
Javascript
Let's now talk about javascript part, on the form submit we are making ajax request of type POST, taking the form data and sending to the server side.
$("#contactForm").submit(function(e){
// prevent from normal form behaviour
e.preventDefault();
// serialize the form data
var serializedData = $(this).serialize();
$.ajax({
type : 'POST',
url : "{% url 'contact_submit' %}",
data : serializedData,
success : function(response){
//reset the form after successful submit
$("#contactForm")[0].reset();
},
error : function(response){
console.log(response)
}
});
});
This is just a basic example to get started with AJAX with django, if you want to get dive with several more examples, you can go through this article: https://djangopy.org/learn/step-up-guide-to-implement-ajax-in-django
Easy ajax calls with Django
(26.10.2020)
This is in my opinion much cleaner and simpler than the correct answer. This one also includes how to add the csrftoken and using login_required methods with ajax.
The view
#login_required
def some_view(request):
"""Returns a json response to an ajax call. (request.user is available in view)"""
# Fetch the attributes from the request body
data_attribute = request.GET.get('some_attribute') # Make sure to use POST/GET correctly
# DO SOMETHING...
return JsonResponse(data={}, status=200)
urls.py
urlpatterns = [
path('some-view-does-something/', views.some_view, name='doing-something'),
]
The ajax call
The ajax call is quite simple, but is sufficient for most cases. You can fetch some values and put them in the data object, then in the view depicted above you can fetch their values again via their names.
You can find the csrftoken function in django's documentation. Basically just copy it and make sure it is rendered before your ajax call so that the csrftoken variable is defined.
$.ajax({
url: "{% url 'doing-something' %}",
headers: {'X-CSRFToken': csrftoken},
data: {'some_attribute': some_value},
type: "GET",
dataType: 'json',
success: function (data) {
if (data) {
console.log(data);
// call function to do something with data
process_data_function(data);
}
}
});
Add HTML to current page with ajax
This might be a bit off topic but I have rarely seen this used and it is a great way to minimize window relocations as well as manual html string creation in javascript.
This is very similar to the one above but this time we are rendering html from the response without reloading the current window.
If you intended to render some kind of html from the data you would receive as a response to the ajax call, it might be easier to send a HttpResponse back from the view instead of a JsonResponse. That allows you to create html easily which can then be inserted into an element.
The view
# The login required part is of course optional
#login_required
def create_some_html(request):
"""In this particular example we are filtering some model by a constraint sent in by
ajax and creating html to send back for those models who match the search"""
# Fetch the attributes from the request body (sent in ajax data)
search_input = request.GET.get('search_input')
# Get some data that we want to render to the template
if search_input:
data = MyModel.objects.filter(name__contains=search_input) # Example
else:
data = []
# Creating an html string using template and some data
html_response = render_to_string('path/to/creation_template.html', context = {'models': data})
return HttpResponse(html_response, status=200)
The html creation template for view
creation_template.html
{% for model in models %}
<li class="xyz">{{ model.name }}</li>
{% endfor %}
urls.py
urlpatterns = [
path('get-html/', views.create_some_html, name='get-html'),
]
The main template and ajax call
This is the template where we want to add the data to. In this example in particular we have a search input and a button that sends the search input's value to the view. The view then sends a HttpResponse back displaying data matching the search that we can render inside an element.
{% extends 'base.html' %}
{% load static %}
{% block content %}
<input id="search-input" placeholder="Type something..." value="">
<button id="add-html-button" class="btn btn-primary">Add Html</button>
<ul id="add-html-here">
<!-- This is where we want to render new html -->
</ul>
{% end block %}
{% block extra_js %}
<script>
// When button is pressed fetch inner html of ul
$("#add-html-button").on('click', function (e){
e.preventDefault();
let search_input = $('#search-input').val();
let target_element = $('#add-html-here');
$.ajax({
url: "{% url 'get-html' %}",
headers: {'X-CSRFToken': csrftoken},
data: {'search_input': search_input},
type: "GET",
dataType: 'html',
success: function (data) {
if (data) {
console.log(data);
// Add the http response to element
target_element.html(data);
}
}
});
})
</script>
{% endblock %}
I have tried to use AjaxableResponseMixin in my project, but had ended up with the following error message:
ImproperlyConfigured: No URL to redirect to. Either provide a url or define a get_absolute_url method on the Model.
That is because the CreateView will return a redirect response instead of returning a HttpResponse when you to send JSON request to the browser. So I have made some changes to the AjaxableResponseMixin. If the request is an ajax request, it will not call the super.form_valid method, just call the form.save() directly.
from django.http import JsonResponse
from django import forms
from django.db import models
class AjaxableResponseMixin(object):
success_return_code = 1
error_return_code = 0
"""
Mixin to add AJAX support to a form.
Must be used with an object-based FormView (e.g. CreateView)
"""
def form_invalid(self, form):
response = super(AjaxableResponseMixin, self).form_invalid(form)
if self.request.is_ajax():
form.errors.update({'result': self.error_return_code})
return JsonResponse(form.errors, status=400)
else:
return response
def form_valid(self, form):
# We make sure to call the parent's form_valid() method because
# it might do some processing (in the case of CreateView, it will
# call form.save() for example).
if self.request.is_ajax():
self.object = form.save()
data = {
'result': self.success_return_code
}
return JsonResponse(data)
else:
response = super(AjaxableResponseMixin, self).form_valid(form)
return response
class Product(models.Model):
name = models.CharField('product name', max_length=255)
class ProductAddForm(forms.ModelForm):
'''
Product add form
'''
class Meta:
model = Product
exclude = ['id']
class PriceUnitAddView(AjaxableResponseMixin, CreateView):
'''
Product add view
'''
model = Product
form_class = ProductAddForm
When we use Django:
Server ===> Client(Browser)
Send a page
When you click button and send the form,
----------------------------
Server <=== Client(Browser)
Give data back. (data in form will be lost)
Server ===> Client(Browser)
Send a page after doing sth with these data
----------------------------
If you want to keep old data, you can do it without Ajax. (Page will be refreshed)
Server ===> Client(Browser)
Send a page
Server <=== Client(Browser)
Give data back. (data in form will be lost)
Server ===> Client(Browser)
1. Send a page after doing sth with data
2. Insert data into form and make it like before.
After these thing, server will send a html page to client. It means that server do more work, however, the way to work is same.
Or you can do with Ajax (Page will be not refreshed)
--------------------------
<Initialization>
Server ===> Client(Browser) [from URL1]
Give a page
--------------------------
<Communication>
Server <=== Client(Browser)
Give data struct back but not to refresh the page.
Server ===> Client(Browser) [from URL2]
Give a data struct(such as JSON)
---------------------------------
If you use Ajax, you must do these:
Initial a HTML page using URL1 (we usually initial page by Django template). And then server send client a html page.
Use Ajax to communicate with server using URL2. And then server send client a data struct.
Django is different from Ajax. The reason for this is as follows:
The thing return to client is different. The case of Django is HTML page. The case of Ajax is data struct.
Django is good at creating something, but it only can create once, it cannot change anything. Django is like anime, consist of many picture. By contrast, Ajax is not good at creating sth but good at change sth in exist html page.
In my opinion, if you would like to use ajax everywhere. when you need to initial a page with data at first, you can use Django with Ajax. But in some case, you just need a static page without anything from server, you need not use Django template.
If you don't think Ajax is the best practice. you can use Django template to do everything, like anime.
(My English is not good)
I'm working with a raspberry pi zero and Python to send and recieve sensor data with Azure IoT. I've already created an endpoint and message routing to the storage container. But when I check the JSON-Files in the container, I've got two problems:
The file include various general data which i don't need
My message body is in Base24-format
My message looks like this:
{"EnqueuedTimeUtc":"2021-06-25T13:03:25.7110000Z","Properties":{},"SystemProperties":{"connectionDeviceId":"RaspberryPi","connectionAuthMethod":"{"scope":"device","type":"sas","issuer":"iothub","acceptingIpFilterRule":null}","connectionDeviceGenerationId":"637555519600003402","enqueuedTime":"2021-06-25T13:03:25.7110000Z"},"Body":"eyJ0ZW1wZXJhdHVyZSI6IDI4Ljk1LCAicHJlc3N1cmUiOiA5ODEuMDg2Njk1NDU5MzMyNiwgImh1bWlkaXR5IjogNDYuMjE0ODE3NjkyOTEyODgsICJ0aW1lIjogIjIwMjEtMDYtMjUgMTQ6MDM6MjUuNjMxNzk1In0="}
The body included my sensor data in Base64-format. I've already read about contentType = application/JSON and contentEncoding = UTF-8 so that Azure can work with correct JSON files. But where do i apply these settings? When I apply it to the routing query, I get the following error:
Routing Query Error (The server didn't understand your query. Check your query syntax and try again)
I just want to get the body-message in correct JSON Format.
Thank you all for any kind of help! Since it's my first experience with this kind of stuff, I'm a little helpless.
Zero clue if this helps, but here is my code for sending data from Raspberry Pi Python to AWS - Parse Server using base64/JSON. The only reason I use base64 is to send pictures. You should only have to use JSON to send your other data.
import requests
import random, time
import math
import json
import Adafruit_DHT
import base64
from Adafruit_CCS811 import Adafruit_CCS811
from picamera import PiCamera
from time import sleep
DHT_SENSOR = Adafruit_DHT.DHT22
DHT_PIN =4
ccs = Adafruit_CCS811()
camera = PiCamera()
while True:
time.sleep(5)
camera.start_preview()
sleep(5)
camera.capture('/home/pi/Desktop/image.jpg')
camera.stop_preview()
with open('/home/pi/Desktop/image.jpg', 'rb') as binary_file:
binary_file_data = binary_file.read()
base64_encoded_data = base64.b64encode(binary_file_data)
base64_message = base64_encoded_data.decode('utf-8')
humidity, temperature = Adafruit_DHT.read_retry(DHT_SENSOR, DHT_PIN)
ccs.readData()
parseServer = {
"temp": temperature,
"humid": humidity,
"co2": ccs.geteCO2(),
"pic": base64_message
}
resultJSON = json.dumps(parseServer)
headers = {
'X-Parse-Application-Id': 'myappID',
'Content-Type': 'application/json',
}
data = resultJSON
response =
requests.put('http://1.11.111.1111/parse/classes/Gamefuck/TIuRnws3Ag',
headers=headers, data=data)
print(data)
If you're using the Python SDK for Azure IoT, sending the message as UTF-8 encoded JSON is as easy as setting two properties on your message object. There is a good example here
msg.content_encoding = "utf-8"
msg.content_type = "application/json"
Furthermore, you don't need to change anything in IoT Hub for this. This message setting is a prerequisite to be able to do message routing based on the body of the message.
I trying to retrieve the information from my API via Y/views.py from another app in the same project. I am receiving the following error ([WinError 10061]). Although, i am able to perform get/post/put via my json_main.js. Thanks to all who support me in this journey. Please read what is already working. And as additional question I would ask why it is not possible to retrieve the data directly via my model/sqlite db.
I already created the following:
X/Views + X/Template.html. X/Views renders information from the backend to the X/template.html. Within this template there is a JSON script(working) that performs a POST/GET/PUT to the API(working) on image click.
The function ultimately results in a new record in API/Bestellingen. The information stored via this JSON function (incl. token authentication; csrf) should now be retrieved in the views.py of Y/view. I already created an example dict. that is rendered to Y/template.html
I tried several JSON request methods (coreapi, urllib2, urllib3, requests), but keep receiving the error as mentioned before. As already stated: JSON.js script does work. Doing the same via POSTMAN also works.
Since i am performing the same via .js and postman, I am quite sure that the variables (token, header and the request) should be ok.
I will show some short snippets of already working code. Herafter i will show the code that doesn't seem to work.
Information from X/views.py -> X/template.html
return render(request, 'smaakjes/smaakjes.html', {'Drank': super_dict})
X/Template.html (see onclick ="")
{% for key, value_list in Drank.items %}
<img onclick="PostImageDetails(this)" style="margin: 0 auto;" src="{{ value }}" id="{{ value }}">
JSON.js (works), sends information to http:127:0:0:0:8000/api/bestellingen. I can see the stored information via the API view.
xmlhttp.send(JSON.stringify({'url':imageSrc, 'username': imageId}))
Y/views.py -> renders information to Y/template.html(works)
def BestellingenDashboard(request):
Data = {'iets': '1'}
return render(request, 'homepage.html', {'bestellingen' : Data})
Once information has been stored in the API/bestellingen, I would like to retrieve the whole json dict. via python Y/Views.py and store it in the above 'Data'(that's ofc easy :)). Does somebody know what i am doing wrong? Why is postman working? Am i Missing a header or something? In my opinion, retrieving data from your own API should be very easy :D
from django.http import HttpResponse
from django.shortcuts import render
from api.models import Bestellingen
import json
import secrets
import requests
URL = "http://127.0.0.1:8000/api/bestellingen/"
data_json = json.dumps(data)
payload = {'json_payload': data_json}
r = requests.get(url=URL, headers={'Content-Type': 'application/json', 'token' : secrets.token_hex(40)}, json=payload)
a = r.json()
print(a)
def BestellingenDashboard(request):
Data = {'iets': '1'}
return render(request, 'homepage.html', {'bestellingen' : Data})
As you can read, most is working (api, endpoints, apiviews, rendering info from X-X.html,X.html->API. I now want to get the information in Y.views, so that i can use this information in Y/template.html
I was successful in publishing (POST) a JSON file in Zapier and creating a Storage for it. However, I´d like to access the JSON in Zapier Storage using a Python code run locally. I am able to access the storage with Python3, see that is something written there, but I cannot access the JSON contents.
import urllib
import json
import codecs
reader = codecs.getreader("utf-8")
access_token = "password"
def GetStorage(page_id, access_token):
url = 'https://hooks.zapier.com/url/'
response = urllib.request.urlopen(url)
data = json.load(reader(response))
return data
a=GetStorage(url, access_token)
print(a)
All I get is:
{'attempt': '5a539a49-65eb-44f8-a30e-e171faf7a680',
'id': '1b38d21a-0150-46df-98c1-490a0d04b565',
'request_id': '5a539a49-65eb-44f8-a30e-e171faf7a680',
'status': 'success'}
When in fact I need:
{'Name':'value',
'Address': 'value'
}
Any ideas ?
David here, from the Zapier Platform team.
You're close! hooks.zapier.com is the url we use for incoming webhooks, so we always reply with a 200 and the response body you're seeing.
Instead, use store.zapier.com. You'll also want to make sure to include your secret. A full request URL will look like:
https://store.zapier.com/api/records?secret=test
which will return arbitrary json data:
{
"name": "david",
"job": "programmer"
}
The full docs are in json here: https://store.zapier.com/
I am using webapp2 for development in App Engine. What I would like to do is to send a custom JSON formatted response in case of an error. For example when the request length is larger that a threshold, to respond with HTTP 400 and response body
{'error':'InvalidMessageLength'}
In webapp2, there is the option to assign error handlers for certain exceptions. For example:
app.error_handlers[400] = handle_error_400
Where handle_error_400 is the following:
def handle_error_400(request, response, exception):
response.write(exception)
response.set_status(400)
When webapp2.RequestHandler.abort(400) is executed, the above code is executed.
How is it possible to have different response formats (HTML and JSON) dynamically based on the above setup? That is, how it is possible to call different versions of handle_error_400 function?
Here is a fully working example that demonstrates how to have the same error handler for all kind of errors and if your URL starts with /json then the response will be an application/json (use your imagination on how you could make a good use of the request object to decide what kind of response you should provide):
import webapp2
import json
def handle_error(request, response, exception):
if request.path.startswith('/json'):
response.headers.add_header('Content-Type', 'application/json')
result = {
'status': 'error',
'status_code': exception.code,
'error_message': exception.explanation,
}
response.write(json.dumps(result))
else:
response.write(exception)
response.set_status(exception.code)
app = webapp2.WSGIApplication()
app.error_handlers[404] = handle_error
app.error_handlers[400] = handle_error
In the above example you can easily test the different behaviours by visting the following URLs that will return a 404 which is the easiest error to test:
http://localhost:8080/404
http://localhost:8080/json/404