Django’s serialization framework provides a mechanism for “translating” Django objects into other formats. Usually these other formats will be text-based and used for sending Django objects over a wire, but it’s possible for a serializer to handle any format (text-based or not).
See also
If you just want to get some data from your tables into a serialized form, you could use the dumpdata management command.
At the highest level, serializing data is a very simple operation:
from django.core import serializers
data = serializers.serialize("xml", SomeModel.objects.all())
The arguments to the serialize function are the format to serialize the data to (see Serialization formats) and a QuerySet to serialize. (Actually, the second argument can be any iterator that yields Django objects, but it'll almost always be a QuerySet).
You can also use a serializer object directly:
XMLSerializer = serializers.get_serializer("xml")
xml_serializer = XMLSerializer()
xml_serializer.serialize(queryset)
data = xml_serializer.getvalue()
This is useful if you want to serialize data directly to a file-like object (which includes an HttpResponse):
out = open("file.xml", "w")
xml_serializer.serialize(SomeModel.objects.all(), stream=out)
If you only want a subset of fields to be serialized, you can specify a fields argument to the serializer:
from django.core import serializers
data = serializers.serialize('xml', SomeModel.objects.all(), fields=('name','size'))
In this example, only the name and size attributes of each model will be serialized.
Note
Depending on your model, you may find that it is not possible to deserialize a model that only serializes a subset of its fields. If a serialized object doesn't specify all the fields that are required by a model, the deserializer will not be able to save deserialized instances.
If you have a model that is defined using an abstract base class, you don't have to do anything special to serialize that model. Just call the serializer on the object (or objects) that you want to serialize, and the output will be a complete representation of the serialized object.
However, if you have a model that uses multi-table inheritance, you also need to serialize all of the base classes for the model. This is because only the fields that are locally defined on the model will be serialized. For example, consider the following models:
class Place(models.Model):
name = models.CharField(max_length=50)
class Restaurant(Place):
serves_hot_dogs = models.BooleanField()
If you only serialize the Restaurant model:
data = serializers.serialize('xml', Restaurant.objects.all())
the fields on the serialized output will only contain the serves_hot_dogs attribute. The name attribute of the base class will be ignored.
In order to fully serialize your Restaurant instances, you will need to serialize the Place models as well:
all_objects = list(Restaurant.objects.all()) + list(Place.objects.all())
data = serializers.serialize('xml', all_objects)
Deserializing data is also a fairly simple operation:
for obj in serializers.deserialize("xml", data):
do_something_with(obj)
As you can see, the deserialize function takes the same format argument as serialize, a string or stream of data, and returns an iterator.
However, here it gets slightly complicated. The objects returned by the deserialize iterator aren't simple Django objects. Instead, they are special DeserializedObject instances that wrap a created -- but unsaved -- object and any associated relationship data.
Calling DeserializedObject.save() saves the object to the database.
This ensures that deserializing is a non-destructive operation even if the data in your serialized representation doesn't match what's currently in the database. Usually, working with these DeserializedObject instances looks something like:
for deserialized_object in serializers.deserialize("xml", data):
if object_should_be_saved(deserialized_object):
deserialized_object.save()
In other words, the usual use is to examine the deserialized objects to make sure that they are "appropriate" for saving before doing so. Of course, if you trust your data source you could just save the object and move on.
The Django object itself can be inspected as deserialized_object.object.
Django supports a number of serialization formats, some of which require you to install third-party Python modules:
| Identifier | Information |
|---|---|
| xml | Serializes to and from a simple XML dialect. |
| json | Serializes to and from JSON (using a version of simplejson bundled with Django). |
| python | Translates to and from "simple" Python objects (lists, dicts, strings, etc.). Not really all that useful on its own, but used as a base for other serializers. |
| yaml | Serializes to YAML (YAML Ain't a Markup Language). This serializer is only available if PyYAML is installed. |
If you're using UTF-8 (or any other non-ASCII encoding) data with the JSON serializer, you must pass ensure_ascii=False as a parameter to the serialize() call. Otherwise, the output won't be encoded correctly.
For example:
json_serializer = serializers.get_serializer("json")()
json_serializer.serialize(queryset, ensure_ascii=False, stream=response)
The Django source code includes the simplejson module. However, if you're using Python 2.6 (which includes a builtin version of the module), Django will use the builtin json module automatically. If you have a system installed version that includes the C-based speedup extension, or your system version is more recent than the version shipped with Django (currently, 2.0.7), the system version will be used instead of the version included with Django.
Be aware that if you're serializing using that module directly, not all Django output can be passed unmodified to simplejson. In particular, lazy translation objects need a special encoder written for them. Something like this will work:
from django.utils.functional import Promise
from django.utils.encoding import force_unicode
class LazyEncoder(simplejson.JSONEncoder):
def default(self, obj):
if isinstance(obj, Promise):
return force_unicode(obj)
return super(LazyEncoder, self).default(obj)
Sep 20, 2009