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362 lines
16 KiB
Markdown
362 lines
16 KiB
Markdown
# Tutorial 1: Serialization
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## Introduction
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This tutorial will cover creating a simple pastebin code highlighting Web API. Along the way it will introduce the various components that make up REST framework, and give you a comprehensive understanding of how everything fits together.
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The tutorial is fairly in-depth, so you should probably get a cookie and a cup of your favorite brew before getting started. If you just want a quick overview, you should head over to the [quickstart] documentation instead.
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---
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**Note**: The code for this tutorial is available in the [tomchristie/rest-framework-tutorial][repo] repository on GitHub. The completed implementation is also online as a sandbox version for testing, [available here][sandbox].
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---
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## Setting up a new environment
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Before we do anything else we'll create a new virtual environment, using [virtualenv]. This will make sure our package configuration is kept nicely isolated from any other projects we're working on.
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:::bash
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virtualenv env
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source env/bin/activate
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Now that we're inside a virtualenv environment, we can install our package requirements.
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pip install django
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pip install djangorestframework
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pip install pygments # We'll be using this for the code highlighting
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**Note:** To exit the virtualenv environment at any time, just type `deactivate`. For more information see the [virtualenv documentation][virtualenv].
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## Getting started
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Okay, we're ready to get coding.
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To get started, let's create a new project to work with.
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cd ~
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django-admin.py startproject tutorial
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cd tutorial
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Once that's done we can create an app that we'll use to create a simple Web API.
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python manage.py startapp snippets
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We'll need to add our new `snippets` app and the `rest_framework` app to `INSTALLED_APPS`. Let's edit the `tutorial/settings.py` file:
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INSTALLED_APPS = (
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...
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'rest_framework',
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'snippets',
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)
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We also need to wire up the root urlconf, in the `tutorial/urls.py` file, to include our snippet app's URLs.
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urlpatterns = [
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url(r'^', include('snippets.urls')),
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]
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Okay, we're ready to roll.
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## Creating a model to work with
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For the purposes of this tutorial we're going to start by creating a simple `Snippet` model that is used to store code snippets. Go ahead and edit the `snippets/models.py` file. Note: Good programming practices include comments. Although you will find them in our repository version of this tutorial code, we have omitted them here to focus on the code itself.
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from django.db import models
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from pygments.lexers import get_all_lexers
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from pygments.styles import get_all_styles
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LEXERS = [item for item in get_all_lexers() if item[1]]
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LANGUAGE_CHOICES = sorted([(item[1][0], item[0]) for item in LEXERS])
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STYLE_CHOICES = sorted((item, item) for item in get_all_styles())
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class Snippet(models.Model):
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created = models.DateTimeField(auto_now_add=True)
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title = models.CharField(max_length=100, blank=True, default='')
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code = models.TextField()
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linenos = models.BooleanField(default=False)
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language = models.CharField(choices=LANGUAGE_CHOICES,
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default='python',
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max_length=100)
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style = models.CharField(choices=STYLE_CHOICES,
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default='friendly',
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max_length=100)
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class Meta:
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ordering = ('created',)
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We'll also need to create an initial migration for our snippet model, and sync the database for the first time.
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python manage.py makemigrations snippets
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python manage.py migrate
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## Creating a Serializer class
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The first thing we need to get started on our Web API is to provide a way of serializing and deserializing the snippet instances into representations such as `json`. We can do this by declaring serializers that work very similar to Django's forms. Create a file in the `snippets` directory named `serializers.py` and add the following.
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from django.forms import widgets
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from rest_framework import serializers
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from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES
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class SnippetSerializer(serializers.Serializer):
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pk = serializers.IntegerField(read_only=True)
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title = serializers.CharField(required=False,
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max_length=100)
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code = serializers.CharField(style={'type': 'textarea'})
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linenos = serializers.BooleanField(required=False)
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language = serializers.ChoiceField(choices=LANGUAGE_CHOICES,
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default='python')
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style = serializers.ChoiceField(choices=STYLE_CHOICES,
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default='friendly')
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def create(self, validated_attrs):
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"""
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Create and return a new `Snippet` instance, given the validated data.
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"""
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return Snippet.objects.create(**validated_attrs)
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def update(self, instance, validated_attrs):
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"""
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Update and return an existing `Snippet` instance, given the validated data.
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"""
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instance.title = validated_attrs.get('title', instance.title)
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instance.code = validated_attrs.get('code', instance.code)
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instance.linenos = validated_attrs.get('linenos', instance.linenos)
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instance.language = validated_attrs.get('language', instance.language)
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instance.style = validated_attrs.get('style', instance.style)
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instance.save()
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return instance
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The first part of the serializer class defines the fields that get serialized/deserialized. The `create()` and `update()` methods define how fully fledged instances are created or modified when calling `serializer.save()`
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A serializer class is very similar to a Django `Form` class, and includes similar validation flags on the various fields, such as `required`, `max_length` and `default`.
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The field flags can also control how the serializer should be displayed in certain circumstances, such as when rendering to HTML. The `style={'type': 'textarea'}` flag above is equivelent to using `widget=widgets.Textarea` on a Django `Form` class. This is particularly useful for controlling how the browsable API should be displayed, as we'll see later in the tutorial.
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We can actually also save ourselves some time by using the `ModelSerializer` class, as we'll see later, but for now we'll keep our serializer definition explicit.
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## Working with Serializers
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Before we go any further we'll familiarize ourselves with using our new Serializer class. Let's drop into the Django shell.
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python manage.py shell
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Okay, once we've got a few imports out of the way, let's create a couple of code snippets to work with.
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from snippets.models import Snippet
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from snippets.serializers import SnippetSerializer
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from rest_framework.renderers import JSONRenderer
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from rest_framework.parsers import JSONParser
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snippet = Snippet(code='foo = "bar"\n')
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snippet.save()
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snippet = Snippet(code='print "hello, world"\n')
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snippet.save()
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We've now got a few snippet instances to play with. Let's take a look at serializing one of those instances.
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serializer = SnippetSerializer(snippet)
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serializer.data
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# {'pk': 2, 'title': u'', 'code': u'print "hello, world"\n', 'linenos': False, 'language': u'python', 'style': u'friendly'}
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At this point we've translated the model instance into Python native datatypes. To finalize the serialization process we render the data into `json`.
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content = JSONRenderer().render(serializer.data)
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content
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# '{"pk": 2, "title": "", "code": "print \\"hello, world\\"\\n", "linenos": false, "language": "python", "style": "friendly"}'
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Deserialization is similar. First we parse a stream into Python native datatypes...
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# This import will use either `StringIO.StringIO` or `io.BytesIO`
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# as appropriate, depending on if we're running Python 2 or Python 3.
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from rest_framework.compat import BytesIO
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stream = BytesIO(content)
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data = JSONParser().parse(stream)
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...then we restore those native datatypes into to a fully populated object instance.
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serializer = SnippetSerializer(data=data)
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serializer.is_valid()
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# True
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serializer.object
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# <Snippet: Snippet object>
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Notice how similar the API is to working with forms. The similarity should become even more apparent when we start writing views that use our serializer.
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We can also serialize querysets instead of model instances. To do so we simply add a `many=True` flag to the serializer arguments.
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serializer = SnippetSerializer(Snippet.objects.all(), many=True)
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serializer.data
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# [{'pk': 1, 'title': u'', 'code': u'foo = "bar"\n', 'linenos': False, 'language': u'python', 'style': u'friendly'}, {'pk': 2, 'title': u'', 'code': u'print "hello, world"\n', 'linenos': False, 'language': u'python', 'style': u'friendly'}]
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## Using ModelSerializers
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Our `SnippetSerializer` class is replicating a lot of information that's also contained in the `Snippet` model. It would be nice if we could keep our code a bit more concise.
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In the same way that Django provides both `Form` classes and `ModelForm` classes, REST framework includes both `Serializer` classes, and `ModelSerializer` classes.
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Let's look at refactoring our serializer using the `ModelSerializer` class.
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Open the file `snippets/serializers.py` again, and edit the `SnippetSerializer` class.
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class SnippetSerializer(serializers.ModelSerializer):
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class Meta:
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model = Snippet
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fields = ('id', 'title', 'code', 'linenos', 'language', 'style')
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Once nice property that serializers have is that you can inspect all the fields an serializer instance, by printing it's representation. Open the Django shell with `python manange.py shell`, then try the following:
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>>> from snippets.serializers import SnippetSerializer
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>>> serializer = SnippetSerializer()
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>>> print repr(serializer) # In python 3 use `print(repr(serializer))`
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SnippetSerializer():
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id = IntegerField(label='ID', read_only=True)
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title = CharField(allow_blank=True, max_length=100, required=False)
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code = CharField(style={'type': 'textarea'})
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linenos = BooleanField(required=False)
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language = ChoiceField(choices=[('Clipper', 'FoxPro'), ('Cucumber', 'Gherkin'), ('RobotFramework', 'RobotFramework'), ('abap', 'ABAP'), ('ada', 'Ada')...
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style = ChoiceField(choices=[('autumn', 'autumn'), ('borland', 'borland'), ('bw', 'bw'), ('colorful', 'colorful')...
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It's important to remember that `ModelSerializer` classes don't do anything particularly magically, they are simply a shortcut to creating a serializer class with:
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* An automatically determined set of fields.
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* Simple default implementations for the `create()` and `update()` methods.
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## Writing regular Django views using our Serializer
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Let's see how we can write some API views using our new Serializer class.
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For the moment we won't use any of REST framework's other features, we'll just write the views as regular Django views.
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We'll start off by creating a subclass of HttpResponse that we can use to render any data we return into `json`.
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Edit the `snippets/views.py` file, and add the following.
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from django.http import HttpResponse
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from django.views.decorators.csrf import csrf_exempt
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from rest_framework.renderers import JSONRenderer
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from rest_framework.parsers import JSONParser
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from snippets.models import Snippet
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from snippets.serializers import SnippetSerializer
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class JSONResponse(HttpResponse):
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"""
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An HttpResponse that renders its content into JSON.
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"""
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def __init__(self, data, **kwargs):
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content = JSONRenderer().render(data)
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kwargs['content_type'] = 'application/json'
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super(JSONResponse, self).__init__(content, **kwargs)
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The root of our API is going to be a view that supports listing all the existing snippets, or creating a new snippet.
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@csrf_exempt
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def snippet_list(request):
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"""
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List all code snippets, or create a new snippet.
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"""
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if request.method == 'GET':
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snippets = Snippet.objects.all()
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serializer = SnippetSerializer(snippets, many=True)
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return JSONResponse(serializer.data)
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elif request.method == 'POST':
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data = JSONParser().parse(request)
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serializer = SnippetSerializer(data=data)
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if serializer.is_valid():
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serializer.save()
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return JSONResponse(serializer.data, status=201)
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return JSONResponse(serializer.errors, status=400)
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Note that because we want to be able to POST to this view from clients that won't have a CSRF token we need to mark the view as `csrf_exempt`. This isn't something that you'd normally want to do, and REST framework views actually use more sensible behavior than this, but it'll do for our purposes right now.
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We'll also need a view which corresponds to an individual snippet, and can be used to retrieve, update or delete the snippet.
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@csrf_exempt
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def snippet_detail(request, pk):
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"""
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Retrieve, update or delete a code snippet.
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"""
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try:
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snippet = Snippet.objects.get(pk=pk)
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except Snippet.DoesNotExist:
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return HttpResponse(status=404)
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if request.method == 'GET':
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serializer = SnippetSerializer(snippet)
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return JSONResponse(serializer.data)
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elif request.method == 'PUT':
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data = JSONParser().parse(request)
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serializer = SnippetSerializer(snippet, data=data)
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if serializer.is_valid():
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serializer.save()
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return JSONResponse(serializer.data)
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return JSONResponse(serializer.errors, status=400)
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elif request.method == 'DELETE':
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snippet.delete()
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return HttpResponse(status=204)
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Finally we need to wire these views up. Create the `snippets/urls.py` file:
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from django.conf.urls import patterns, url
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from snippets import views
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urlpatterns = [
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url(r'^snippets/$', views.snippet_list),
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url(r'^snippets/(?P<pk>[0-9]+)/$', views.snippet_detail),
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]
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It's worth noting that there are a couple of edge cases we're not dealing with properly at the moment. If we send malformed `json`, or if a request is made with a method that the view doesn't handle, then we'll end up with a 500 "server error" response. Still, this'll do for now.
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## Testing our first attempt at a Web API
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Now we can start up a sample server that serves our snippets.
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Quit out of the shell...
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quit()
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...and start up Django's development server.
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python manage.py runserver
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Validating models...
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0 errors found
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Django version 1.4.3, using settings 'tutorial.settings'
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Development server is running at http://127.0.0.1:8000/
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Quit the server with CONTROL-C.
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In another terminal window, we can test the server.
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We can get a list of all of the snippets.
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curl http://127.0.0.1:8000/snippets/
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[{"id": 1, "title": "", "code": "foo = \"bar\"\n", "linenos": false, "language": "python", "style": "friendly"}, {"id": 2, "title": "", "code": "print \"hello, world\"\n", "linenos": false, "language": "python", "style": "friendly"}]
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Or we can get a particular snippet by referencing its id.
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curl http://127.0.0.1:8000/snippets/2/
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{"id": 2, "title": "", "code": "print \"hello, world\"\n", "linenos": false, "language": "python", "style": "friendly"}
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Similarly, you can have the same json displayed by visiting these URLs in a web browser.
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## Where are we now
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We're doing okay so far, we've got a serialization API that feels pretty similar to Django's Forms API, and some regular Django views.
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Our API views don't do anything particularly special at the moment, beyond serving `json` responses, and there are some error handling edge cases we'd still like to clean up, but it's a functioning Web API.
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We'll see how we can start to improve things in [part 2 of the tutorial][tut-2].
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[quickstart]: quickstart.md
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[repo]: https://github.com/tomchristie/rest-framework-tutorial
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[sandbox]: http://restframework.herokuapp.com/
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[virtualenv]: http://www.virtualenv.org/en/latest/index.html
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[tut-2]: 2-requests-and-responses.md
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