# Tutorial 1: Serialization

## Introduction

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.

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.

!!! note
    The code for this tutorial is available in the [encode/rest-framework-tutorial][repo] repository on GitHub. Feel free to clone the repository and see the code in action.

## Setting up a new environment

Before we do anything else we'll create a new virtual environment, using [venv]. This will make sure our package configuration is kept nicely isolated from any other projects we're working on.

```bash
python3 -m venv env
source env/bin/activate
```

Now that we're inside a virtual environment, we can install our package requirements.

```bash
pip install django
pip install djangorestframework
pip install pygments  # We'll be using this for the code highlighting
```

!!! tip
    To exit the virtual environment at any time, just type `deactivate`.  For more information see the [venv documentation][venv].

## Getting started

Okay, we're ready to get coding.
To get started, let's create a new project to work with.

```bash
cd ~
django-admin startproject tutorial
cd tutorial
```

Once that's done we can create an app that we'll use to create a simple Web API.

```bash
python manage.py startapp snippets
```

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:

```text
INSTALLED_APPS = [
    ...
    'rest_framework',
    'snippets',
]
```

Okay, we're ready to roll.

## Creating a model to work with

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.

```python
from django.db import models
from pygments.lexers import get_all_lexers
from pygments.styles import get_all_styles

LEXERS = [item for item in get_all_lexers() if item[1]]
LANGUAGE_CHOICES = sorted([(item[1][0], item[0]) for item in LEXERS])
STYLE_CHOICES = sorted([(item, item) for item in get_all_styles()])


class Snippet(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=100, blank=True, default="")
    code = models.TextField()
    linenos = models.BooleanField(default=False)
    language = models.CharField(
        choices=LANGUAGE_CHOICES, default="python", max_length=100
    )
    style = models.CharField(choices=STYLE_CHOICES, default="friendly", max_length=100)

    class Meta:
        ordering = ["created"]
```

We'll also need to create an initial migration for our snippet model, and sync the database for the first time.

```bash
python manage.py makemigrations snippets
python manage.py migrate snippets
```

## Creating a Serializer class

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.

```python
from rest_framework import serializers
from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES


class SnippetSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    title = serializers.CharField(required=False, allow_blank=True, max_length=100)
    code = serializers.CharField(style={"base_template": "textarea.html"})
    linenos = serializers.BooleanField(required=False)
    language = serializers.ChoiceField(choices=LANGUAGE_CHOICES, default="python")
    style = serializers.ChoiceField(choices=STYLE_CHOICES, default="friendly")

    def create(self, validated_data):
        """
        Create and return a new `Snippet` instance, given the validated data.
        """
        return Snippet.objects.create(**validated_data)

    def update(self, instance, validated_data):
        """
        Update and return an existing `Snippet` instance, given the validated data.
        """
        instance.title = validated_data.get("title", instance.title)
        instance.code = validated_data.get("code", instance.code)
        instance.linenos = validated_data.get("linenos", instance.linenos)
        instance.language = validated_data.get("language", instance.language)
        instance.style = validated_data.get("style", instance.style)
        instance.save()
        return instance
```

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()`

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`.

The field flags can also control how the serializer should be displayed in certain circumstances, such as when rendering to HTML. The `{'base_template': 'textarea.html'}` flag above is equivalent 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.

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.

## Working with Serializers

Before we go any further we'll familiarize ourselves with using our new Serializer class.  Let's drop into the Django shell.

```bash
python manage.py shell
```

Okay, once we've got a few imports out of the way, let's create a couple of code snippets to work with.

```pycon
>>> from snippets.models import Snippet
>>> from snippets.serializers import SnippetSerializer
>>> from rest_framework.renderers import JSONRenderer
>>> from rest_framework.parsers import JSONParser

>>> snippet = Snippet(code='foo = "bar"\n')
>>> snippet.save()

>>> snippet = Snippet(code='print("hello, world")\n')
>>> snippet.save()
```

We've now got a few snippet instances to play with.  Let's take a look at serializing one of those instances.

```pycon
>>> serializer = SnippetSerializer(snippet)
>>> serializer.data
{'id': 2, 'title': '', 'code': 'print("hello, world")\n', 'linenos': False, 'language': 'python', 'style': 'friendly'}
```

At this point we've translated the model instance into Python native datatypes.  To finalize the serialization process we render the data into `json`.

```pycon
>>> content = JSONRenderer().render(serializer.data)
>>> content
b'{"id":2,"title":"","code":"print(\\"hello, world\\")\\n","linenos":false,"language":"python","style":"friendly"}'
```

Deserialization is similar.  First we parse a stream into Python native datatypes...

```pycon
>>> import io

>>> stream = io.BytesIO(content)
>>> data = JSONParser().parse(stream)
```

...then we restore those native datatypes into a fully populated object instance.

```pycon
>>> serializer = SnippetSerializer(data=data)
>>> serializer.is_valid()
True
>>> serializer.validated_data
{'title': '', 'code': 'print("hello, world")', 'linenos': False, 'language': 'python', 'style': 'friendly'}
>>> serializer.save()
<Snippet: Snippet object>
```

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.

We can also serialize querysets instead of model instances.  To do so we simply add a `many=True` flag to the serializer arguments.

```pycon
>>> serializer = SnippetSerializer(Snippet.objects.all(), many=True)
>>> serializer.data
[{'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'}, {'id': 3, 'title': '', 'code': 'print("hello, world")', 'linenos': False, 'language': 'python', 'style': 'friendly'}]
```

## Using ModelSerializers

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.

In the same way that Django provides both `Form` classes and `ModelForm` classes, REST framework includes both `Serializer` classes, and `ModelSerializer` classes.

Let's look at refactoring our serializer using the `ModelSerializer` class.
Open the file `snippets/serializers.py` again, and replace the `SnippetSerializer` class with the following.

```python
class SnippetSerializer(serializers.ModelSerializer):
    class Meta:
        model = Snippet
        fields = ["id", "title", "code", "linenos", "language", "style"]
```

One nice property that serializers have is that you can inspect all the fields in a serializer instance, by printing its representation. Open the Django shell with `python manage.py shell`, then try the following:

```pycon
>>> from snippets.serializers import SnippetSerializer

>>> serializer = SnippetSerializer()
>>> print(repr(serializer))
SnippetSerializer():
    id = IntegerField(label='ID', read_only=True)
    title = CharField(allow_blank=True, max_length=100, required=False)
    code = CharField(style={'base_template': 'textarea.html'})
    linenos = BooleanField(required=False)
    language = ChoiceField(choices=[('Clipper', 'FoxPro'), ('Cucumber', 'Gherkin'), ('RobotFramework', 'RobotFramework'), ('abap', 'ABAP'), ('ada', 'Ada')...
    style = ChoiceField(choices=[('autumn', 'autumn'), ('borland', 'borland'), ('bw', 'bw'), ('colorful', 'colorful')...
```

It's important to remember that `ModelSerializer` classes don't do anything particularly magical, they are simply a shortcut for creating serializer classes:

* An automatically determined set of fields.
* Simple default implementations for the `create()` and `update()` methods.

## Writing regular Django views using our Serializer

Let's see how we can write some API views using our new Serializer class.
For the moment we won't use any of REST framework's other features, we'll just write the views as regular Django views.

Edit the `snippets/views.py` file, and add the following.

```python
from django.http import HttpResponse, JsonResponse
from django.views.decorators.csrf import csrf_exempt
from rest_framework.parsers import JSONParser
from snippets.models import Snippet
from snippets.serializers import SnippetSerializer
```

The root of our API is going to be a view that supports listing all the existing snippets, or creating a new snippet.

```python
@csrf_exempt
def snippet_list(request):
    """
    List all code snippets, or create a new snippet.
    """
    if request.method == "GET":
        snippets = Snippet.objects.all()
        serializer = SnippetSerializer(snippets, many=True)
        return JsonResponse(serializer.data, safe=False)

    elif request.method == "POST":
        data = JSONParser().parse(request)
        serializer = SnippetSerializer(data=data)
        if serializer.is_valid():
            serializer.save()
            return JsonResponse(serializer.data, status=201)
        return JsonResponse(serializer.errors, status=400)
```

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.

We'll also need a view which corresponds to an individual snippet, and can be used to retrieve, update or delete the snippet.

```python
@csrf_exempt
def snippet_detail(request, pk):
    """
    Retrieve, update or delete a code snippet.
    """
    try:
        snippet = Snippet.objects.get(pk=pk)
    except Snippet.DoesNotExist:
        return HttpResponse(status=404)

    if request.method == "GET":
        serializer = SnippetSerializer(snippet)
        return JsonResponse(serializer.data)

    elif request.method == "PUT":
        data = JSONParser().parse(request)
        serializer = SnippetSerializer(snippet, data=data)
        if serializer.is_valid():
            serializer.save()
            return JsonResponse(serializer.data)
        return JsonResponse(serializer.errors, status=400)

    elif request.method == "DELETE":
        snippet.delete()
        return HttpResponse(status=204)
```

Finally we need to wire these views up.  Create the `snippets/urls.py` file:

```python
from django.urls import path
from snippets import views

urlpatterns = [
    path("snippets/", views.snippet_list),
    path("snippets/<int:pk>/", views.snippet_detail),
]
```

We also need to wire up the root urlconf, in the `tutorial/urls.py` file, to include our snippet app's URLs.

```python
from django.urls import path, include

urlpatterns = [
    path("", include("snippets.urls")),
]
```

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.

## Testing our first attempt at a Web API

Now we can start up a sample server that serves our snippets.

Quit out of the shell...

```pycon
>>> quit()
```

...and start up Django's development server.

```bash
python manage.py runserver

Validating models...

0 errors found
Django version 5.0, using settings 'tutorial.settings'
Starting Development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
```

In another terminal window, we can test the server.

We can test our API using [curl][curl] or [httpie][httpie]. Httpie is a user friendly http client that's written in Python. Let's install that.

You can install httpie using pip:

```bash
pip install httpie
```

Finally, we can get a list of all of the snippets:

```bash
http GET http://127.0.0.1:8000/snippets/ --unsorted

HTTP/1.1 200 OK
...
[
    {
        "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"
    },
    {
        "id": 3,
        "title": "",
        "code": "print(\"hello, world\")",
        "linenos": false,
        "language": "python",
        "style": "friendly"
    }
]
```

Or we can get a particular snippet by referencing its id:

```bash
http GET http://127.0.0.1:8000/snippets/2/ --unsorted

HTTP/1.1 200 OK
...
{
    "id": 2,
    "title": "",
    "code": "print(\"hello, world\")\n",
    "linenos": false,
    "language": "python",
    "style": "friendly"
}
```

Similarly, you can have the same json displayed by visiting these URLs in a web browser.

## Where are we now

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.

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.

We'll see how we can start to improve things in [part 2 of the tutorial][tut-2].

[quickstart]: quickstart.md
[repo]: https://github.com/encode/rest-framework-tutorial
[venv]: https://docs.python.org/3/library/venv.html
[tut-2]: 2-requests-and-responses.md
[httpie]: https://github.com/httpie/httpie#installation
[curl]: https://curl.haxx.se/
