f i e l d s . p y

Serializer fields

Each field in a Form class is responsible not only for validating data, but also for "cleaning" it — normalizing it to a consistent format.

— Django documentation

Serializer fields handle converting between primitive values and internal datatypes. They also deal with validating input values, as well as retrieving and setting the values from their parent objects.

Note: The serializer fields are declared in fields.py, but by convention you should import them using from rest_framework import serializers and refer to fields as serializers.<FieldName>.

Core arguments

Each serializer field class constructor takes at least these arguments. Some Field classes take additional, field-specific arguments, but the following should always be accepted:

read_only

Read-only fields are included in the API output, but should not be included in the input during create or update operations. Any 'read_only' fields that are incorrectly included in the serializer input will be ignored.

Set this to True to ensure that the field is used when serializing a representation, but is not used when creating or updating an instance during deserialization.

Defaults to False

write_only

Set this to True to ensure that the field may be used when updating or creating an instance, but is not included when serializing the representation.

Defaults to False

required

Normally an error will be raised if a field is not supplied during deserialization. Set to false if this field is not required to be present during deserialization.

Setting this to False also allows the object attribute or dictionary key to be omitted from output when serializing the instance. If the key is not present it will simply not be included in the output representation.

Defaults to True.

default

If set, this gives the default value that will be used for the field if no input value is supplied. If not set the default behaviour is to not populate the attribute at all.

The default is not applied during partial update operations. In the partial update case only fields that are provided in the incoming data will have a validated value returned.

May be set to a function or other callable, in which case the value will be evaluated each time it is used. When called, it will receive no arguments. If the callable has a set_context method, that will be called each time before getting the value with the field instance as only argument. This works the same way as for validators.

When serializing the instance, default will be used if the the object attribute or dictionary key is not present in the instance.

Note that setting a default value implies that the field is not required. Including both the default and required keyword arguments is invalid and will raise an error.

allow_null

Normally an error will be raised if None is passed to a serializer field. Set this keyword argument to True if None should be considered a valid value.

Note that, without an explicit default, setting this argument to True will imply a default value of null for serialization output, but does not imply a default for input deserialization.

Defaults to False

source

The name of the attribute that will be used to populate the field. May be a method that only takes a self argument, such as URLField(source='get_absolute_url'), or may use dotted notation to traverse attributes, such as EmailField(source='user.email'). When serializing fields with dotted notation, it may be necessary to provide a default value if any object is not present or is empty during attribute traversal.

The value source='*' has a special meaning, and is used to indicate that the entire object should be passed through to the field. This can be useful for creating nested representations, or for fields which require access to the complete object in order to determine the output representation.

Defaults to the name of the field.

validators

A list of validator functions which should be applied to the incoming field input, and which either raise a validation error or simply return. Validator functions should typically raise serializers.ValidationError, but Django's built-in ValidationError is also supported for compatibility with validators defined in the Django codebase or third party Django packages.

error_messages

A dictionary of error codes to error messages.

label

A short text string that may be used as the name of the field in HTML form fields or other descriptive elements.

help_text

A text string that may be used as a description of the field in HTML form fields or other descriptive elements.

initial

A value that should be used for pre-populating the value of HTML form fields. You may pass a callable to it, just as you may do with any regular Django Field:

import datetime
from rest_framework import serializers
class ExampleSerializer(serializers.Serializer):
    day = serializers.DateField(initial=datetime.date.today)

style

A dictionary of key-value pairs that can be used to control how renderers should render the field.

Two examples here are 'input_type' and 'base_template':

# Use <input type="password"> for the input.
password = serializers.CharField(
    style={'input_type': 'password'}
)

# Use a radio input instead of a select input.
color_channel = serializers.ChoiceField(
    choices=['red', 'green', 'blue'],
    style={'base_template': 'radio.html'}
)

For more details see the HTML & Forms documentation.

Boolean fields

BooleanField

A boolean representation.

When using HTML encoded form input be aware that omitting a value will always be treated as setting a field to False, even if it has a default=True option specified. This is because HTML checkbox inputs represent the unchecked state by omitting the value, so REST framework treats omission as if it is an empty checkbox input.

Note that default BooleanField instances will be generated with a required=False option (since Django models.BooleanField is always blank=True). If you want to change this behaviour explicitly declare the BooleanField on the serializer class.

Corresponds to django.db.models.fields.BooleanField.

Signature: BooleanField()

NullBooleanField

A boolean representation that also accepts None as a valid value.

Corresponds to django.db.models.fields.NullBooleanField.

Signature: NullBooleanField()

String fields

CharField

A text representation. Optionally validates the text to be shorter than max_length and longer than min_length.

Corresponds to django.db.models.fields.CharField or django.db.models.fields.TextField.

Signature: CharField(max_length=None, min_length=None, allow_blank=False, trim_whitespace=True)

The allow_null option is also available for string fields, although its usage is discouraged in favor of allow_blank. It is valid to set both allow_blank=True and allow_null=True, but doing so means that there will be two differing types of empty value permissible for string representations, which can lead to data inconsistencies and subtle application bugs.

EmailField

A text representation, validates the text to be a valid e-mail address.

Corresponds to django.db.models.fields.EmailField

Signature: EmailField(max_length=None, min_length=None, allow_blank=False)

RegexField

A text representation, that validates the given value matches against a certain regular expression.

Corresponds to django.forms.fields.RegexField.

Signature: RegexField(regex, max_length=None, min_length=None, allow_blank=False)

The mandatory regex argument may either be a string, or a compiled python regular expression object.

Uses Django's django.core.validators.RegexValidator for validation.

SlugField

A RegexField that validates the input against the pattern [a-zA-Z0-9_-]+.

Corresponds to django.db.models.fields.SlugField.

Signature: SlugField(max_length=50, min_length=None, allow_blank=False)

URLField

A RegexField that validates the input against a URL matching pattern. Expects fully qualified URLs of the form http://<host>/<path>.

Corresponds to django.db.models.fields.URLField. Uses Django's django.core.validators.URLValidator for validation.

Signature: URLField(max_length=200, min_length=None, allow_blank=False)

UUIDField

A field that ensures the input is a valid UUID string. The to_internal_value method will return a uuid.UUID instance. On output the field will return a string in the canonical hyphenated format, for example:

"de305d54-75b4-431b-adb2-eb6b9e546013"

Signature: UUIDField(format='hex_verbose')

FilePathField

A field whose choices are limited to the filenames in a certain directory on the filesystem

Corresponds to django.forms.fields.FilePathField.

Signature: FilePathField(path, match=None, recursive=False, allow_files=True, allow_folders=False, required=None, **kwargs)

IPAddressField

A field that ensures the input is a valid IPv4 or IPv6 string.

Corresponds to django.forms.fields.IPAddressField and django.forms.fields.GenericIPAddressField.

Signature: IPAddressField(protocol='both', unpack_ipv4=False, **options)

Numeric fields

IntegerField

An integer representation.

Corresponds to django.db.models.fields.IntegerField, django.db.models.fields.SmallIntegerField, django.db.models.fields.PositiveIntegerField and django.db.models.fields.PositiveSmallIntegerField.

Signature: IntegerField(max_value=None, min_value=None)

FloatField

A floating point representation.

Corresponds to django.db.models.fields.FloatField.

Signature: FloatField(max_value=None, min_value=None)

DecimalField

A decimal representation, represented in Python by a Decimal instance.

Corresponds to django.db.models.fields.DecimalField.

Signature: DecimalField(max_digits, decimal_places, coerce_to_string=None, max_value=None, min_value=None)

Example usage

To validate numbers up to 999 with a resolution of 2 decimal places, you would use:

serializers.DecimalField(max_digits=5, decimal_places=2)

And to validate numbers up to anything less than one billion with a resolution of 10 decimal places:

serializers.DecimalField(max_digits=19, decimal_places=10)

This field also takes an optional argument, coerce_to_string. If set to True the representation will be output as a string. If set to False the representation will be left as a Decimal instance and the final representation will be determined by the renderer.

If unset, this will default to the same value as the COERCE_DECIMAL_TO_STRING setting, which is True unless set otherwise.

Date and time fields

DateTimeField

A date and time representation.

Corresponds to django.db.models.fields.DateTimeField.

Signature: DateTimeField(format=api_settings.DATETIME_FORMAT, input_formats=None)

DateTimeField format strings.

Format strings may either be Python strftime formats which explicitly specify the format, or the special string 'iso-8601', which indicates that ISO 8601 style datetimes should be used. (eg '2013-01-29T12:34:56.000000Z')

When a value of None is used for the format datetime objects will be returned by to_representation and the final output representation will determined by the renderer class.

auto_now and auto_now_add model fields.

When using ModelSerializer or HyperlinkedModelSerializer, note that any model fields with auto_now=True or auto_now_add=True will use serializer fields that are read_only=True by default.

If you want to override this behavior, you'll need to declare the DateTimeField explicitly on the serializer. For example:

class CommentSerializer(serializers.ModelSerializer):
    created = serializers.DateTimeField()

    class Meta:
        model = Comment

DateField

A date representation.

Corresponds to django.db.models.fields.DateField

Signature: DateField(format=api_settings.DATE_FORMAT, input_formats=None)

DateField format strings

Format strings may either be Python strftime formats which explicitly specify the format, or the special string 'iso-8601', which indicates that ISO 8601 style dates should be used. (eg '2013-01-29')

TimeField

A time representation.

Corresponds to django.db.models.fields.TimeField

Signature: TimeField(format=api_settings.TIME_FORMAT, input_formats=None)

TimeField format strings

Format strings may either be Python strftime formats which explicitly specify the format, or the special string 'iso-8601', which indicates that ISO 8601 style times should be used. (eg '12:34:56.000000')

DurationField

A Duration representation. Corresponds to django.db.models.fields.DurationField

The validated_data for these fields will contain a datetime.timedelta instance. The representation is a string following this format '[DD] [HH:[MM:]]ss[.uuuuuu]'.

Signature: DurationField(max_value=None, min_value=None)

Choice selection fields

ChoiceField

A field that can accept a value out of a limited set of choices.

Used by ModelSerializer to automatically generate fields if the corresponding model field includes a choices=… argument.

Signature: ChoiceField(choices)

Both the allow_blank and allow_null are valid options on ChoiceField, although it is highly recommended that you only use one and not both. allow_blank should be preferred for textual choices, and allow_null should be preferred for numeric or other non-textual choices.

MultipleChoiceField

A field that can accept a set of zero, one or many values, chosen from a limited set of choices. Takes a single mandatory argument. to_internal_value returns a set containing the selected values.

Signature: MultipleChoiceField(choices)

As with ChoiceField, both the allow_blank and allow_null options are valid, although it is highly recommended that you only use one and not both. allow_blank should be preferred for textual choices, and allow_null should be preferred for numeric or other non-textual choices.

File upload fields

Parsers and file uploads.

The FileField and ImageField classes are only suitable for use with MultiPartParser or FileUploadParser. Most parsers, such as e.g. JSON don't support file uploads. Django's regular FILE_UPLOAD_HANDLERS are used for handling uploaded files.

FileField

A file representation. Performs Django's standard FileField validation.

Corresponds to django.forms.fields.FileField.

Signature: FileField(max_length=None, allow_empty_file=False, use_url=UPLOADED_FILES_USE_URL)

ImageField

An image representation. Validates the uploaded file content as matching a known image format.

Corresponds to django.forms.fields.ImageField.

Signature: ImageField(max_length=None, allow_empty_file=False, use_url=UPLOADED_FILES_USE_URL)

Requires either the Pillow package or PIL package. The Pillow package is recommended, as PIL is no longer actively maintained.

Composite fields

ListField

A field class that validates a list of objects.

Signature: ListField(child=<A_FIELD_INSTANCE>, min_length=None, max_length=None)

For example, to validate a list of integers you might use something like the following:

scores = serializers.ListField(
   child=serializers.IntegerField(min_value=0, max_value=100)
)

The ListField class also supports a declarative style that allows you to write reusable list field classes.

class StringListField(serializers.ListField):
    child = serializers.CharField()

We can now reuse our custom StringListField class throughout our application, without having to provide a child argument to it.

DictField

A field class that validates a dictionary of objects. The keys in DictField are always assumed to be string values.

Signature: DictField(child=<A_FIELD_INSTANCE>)

For example, to create a field that validates a mapping of strings to strings, you would write something like this:

document = DictField(child=CharField())

You can also use the declarative style, as with ListField. For example:

class DocumentField(DictField):
    child = CharField()

HStoreField

A preconfigured DictField that is compatible with Django's postgres HStoreField.

Signature: HStoreField(child=<A_FIELD_INSTANCE>)

Note that the child field must be an instance of CharField, as the hstore extension stores values as strings.

JSONField

A field class that validates that the incoming data structure consists of valid JSON primitives. In its alternate binary mode, it will represent and validate JSON-encoded binary strings.

Signature: JSONField(binary)

Miscellaneous fields

ReadOnlyField

A field class that simply returns the value of the field without modification.

This field is used by default with ModelSerializer when including field names that relate to an attribute rather than a model field.

Signature: ReadOnlyField()

For example, if has_expired was a property on the Account model, then the following serializer would automatically generate it as a ReadOnlyField:

class AccountSerializer(serializers.ModelSerializer):
    class Meta:
        model = Account
        fields = ('id', 'account_name', 'has_expired')

HiddenField

A field class that does not take a value based on user input, but instead takes its value from a default value or callable.

Signature: HiddenField()

For example, to include a field that always provides the current time as part of the serializer validated data, you would use the following:

modified = serializers.HiddenField(default=timezone.now)

The HiddenField class is usually only needed if you have some validation that needs to run based on some pre-provided field values, but you do not want to expose all of those fields to the end user.

For further examples on HiddenField see the validators documentation.

ModelField

A generic field that can be tied to any arbitrary model field. The ModelField class delegates the task of serialization/deserialization to its associated model field. This field can be used to create serializer fields for custom model fields, without having to create a new custom serializer field.

This field is used by ModelSerializer to correspond to custom model field classes.

Signature: ModelField(model_field=<Django ModelField instance>)

The ModelField class is generally intended for internal use, but can be used by your API if needed. In order to properly instantiate a ModelField, it must be passed a field that is attached to an instantiated model. For example: ModelField(model_field=MyModel()._meta.get_field('custom_field'))

SerializerMethodField

This is a read-only field. It gets its value by calling a method on the serializer class it is attached to. It can be used to add any sort of data to the serialized representation of your object.

Signature: SerializerMethodField(method_name=None)

The serializer method referred to by the method_name argument should accept a single argument (in addition to self), which is the object being serialized. It should return whatever you want to be included in the serialized representation of the object. For example:

from django.contrib.auth.models import User
from django.utils.timezone import now
from rest_framework import serializers

class UserSerializer(serializers.ModelSerializer):
    days_since_joined = serializers.SerializerMethodField()

    class Meta:
        model = User

    def get_days_since_joined(self, obj):
        return (now() - obj.date_joined).days

Custom fields

If you want to create a custom field, you'll need to subclass Field and then override either one or both of the .to_representation() and .to_internal_value() methods. These two methods are used to convert between the initial datatype, and a primitive, serializable datatype. Primitive datatypes will typically be any of a number, string, boolean, date/time/datetime or None. They may also be any list or dictionary like object that only contains other primitive objects. Other types might be supported, depending on the renderer that you are using.

The .to_representation() method is called to convert the initial datatype into a primitive, serializable datatype.

The to_internal_value() method is called to restore a primitive datatype into its internal python representation. This method should raise a serializers.ValidationError if the data is invalid.

Note that the WritableField class that was present in version 2.x no longer exists. You should subclass Field and override to_internal_value() if the field supports data input.

Examples

A Basic Custom Field

Let's look at an example of serializing a class that represents an RGB color value:

class Color(object):
    """
    A color represented in the RGB colorspace.
    """
    def __init__(self, red, green, blue):
        assert(red >= 0 and green >= 0 and blue >= 0)
        assert(red < 256 and green < 256 and blue < 256)
        self.red, self.green, self.blue = red, green, blue

class ColorField(serializers.Field):
    """
    Color objects are serialized into 'rgb(#, #, #)' notation.
    """
    def to_representation(self, value):
        return "rgb(%d, %d, %d)" % (value.red, value.green, value.blue)

    def to_internal_value(self, data):
        data = data.strip('rgb(').rstrip(')')
        red, green, blue = [int(col) for col in data.split(',')]
        return Color(red, green, blue)

By default field values are treated as mapping to an attribute on the object. If you need to customize how the field value is accessed and set you need to override .get_attribute() and/or .get_value().

As an example, let's create a field that can be used to represent the class name of the object being serialized:

class ClassNameField(serializers.Field):
    def get_attribute(self, instance):
        # We pass the object instance onto `to_representation`,
        # not just the field attribute.
        return instance

    def to_representation(self, value):
        """
        Serialize the value's class name.
        """
        return value.__class__.__name__

Raising validation errors

Our ColorField class above currently does not perform any data validation. To indicate invalid data, we should raise a serializers.ValidationError, like so:

def to_internal_value(self, data):
    if not isinstance(data, six.text_type):
        msg = 'Incorrect type. Expected a string, but got %s'
        raise ValidationError(msg % type(data).__name__)

    if not re.match(r'^rgb\([0-9]+,[0-9]+,[0-9]+\)$', data):
        raise ValidationError('Incorrect format. Expected `rgb(#,#,#)`.')

    data = data.strip('rgb(').rstrip(')')
    red, green, blue = [int(col) for col in data.split(',')]

    if any([col > 255 or col < 0 for col in (red, green, blue)]):
        raise ValidationError('Value out of range. Must be between 0 and 255.')

    return Color(red, green, blue)

The .fail() method is a shortcut for raising ValidationError that takes a message string from the error_messages dictionary. For example:

default_error_messages = {
    'incorrect_type': 'Incorrect type. Expected a string, but got {input_type}',
    'incorrect_format': 'Incorrect format. Expected `rgb(#,#,#)`.',
    'out_of_range': 'Value out of range. Must be between 0 and 255.'
}

def to_internal_value(self, data):
    if not isinstance(data, six.text_type):
        self.fail('incorrect_type', input_type=type(data).__name__)

    if not re.match(r'^rgb\([0-9]+,[0-9]+,[0-9]+\)$', data):
        self.fail('incorrect_format')

    data = data.strip('rgb(').rstrip(')')
    red, green, blue = [int(col) for col in data.split(',')]

    if any([col > 255 or col < 0 for col in (red, green, blue)]):
        self.fail('out_of_range')

    return Color(red, green, blue)

This style keeps your error messages cleaner and more separated from your code, and should be preferred.

Using source='*'

Here we'll take an example of a flat DataPoint model with x_coordinate and y_coordinate attributes.

class DataPoint(models.Model):
    label = models.CharField(max_length=50)
    x_coordinate = models.SmallIntegerField()
    y_coordinate = models.SmallIntegerField()

Using a custom field and source='*' we can provide a nested representation of the coordinate pair:

class CoordinateField(serializers.Field):

    def to_representation(self, value):
        ret = {
            "x": value.x_coordinate,
            "y": value.y_coordinate
        }
        return ret

    def to_internal_value(self, data):
        ret = {
            "x_coordinate": data["x"],
            "y_coordinate": data["y"],
        }
        return ret


class DataPointSerializer(serializers.ModelSerializer):
    coordinates = CoordinateField(source='*')

    class Meta:
        model = DataPoint
        fields = ['label', 'coordinates']

Note that this example doesn't handle validation. Partly for that reason, in a real project, the coordinate nesting might be better handled with a nested serialiser using source='*', with two IntegerField instances, each with their own source pointing to the relevant field.

The key points from the example, though, are:

For completeness lets do the same thing again but with the nested serialiser approach suggested above:

class NestedCoordinateSerializer(serializers.Serializer):
    x = serializers.IntegerField(source='x_coordinate')
    y = serializers.IntegerField(source='y_coordinate')


class DataPointSerializer(serializers.ModelSerializer):
    coordinates = NestedCoordinateSerializer(source='*')

    class Meta:
        model = DataPoint
        fields = ['label', 'coordinates']

Here the mapping between the target and source attribute pairs (x and x_coordinate, y and y_coordinate) is handled in the IntegerField declarations. It's our NestedCoordinateSerializer that takes source='*'.

Our new DataPointSerializer exhibits the same behaviour as the custom field approach.

Serialising:

>>> out_serializer = DataPointSerializer(instance)
>>> out_serializer.data
ReturnDict([('label', 'testing'),
            ('coordinates', OrderedDict([('x', 1), ('y', 2)]))])

Deserialising:

>>> in_serializer = DataPointSerializer(data=data)
>>> in_serializer.is_valid()
True
>>> in_serializer.validated_data
OrderedDict([('label', 'still testing'),
             ('x_coordinate', 3),
             ('y_coordinate', 4)])

But we also get the built-in validation for free:

>>> invalid_data = {
...     "label": "still testing",
...     "coordinates": {
...         "x": 'a',
...         "y": 'b',
...     }
... }
>>> invalid_serializer = DataPointSerializer(data=invalid_data)
>>> invalid_serializer.is_valid()
False
>>> invalid_serializer.errors
ReturnDict([('coordinates',
             {'x': ['A valid integer is required.'],
              'y': ['A valid integer is required.']})])

For this reason, the nested serialiser approach would be the first to try. You would use the custom field approach when the nested serialiser becomes infeasible or overly complex.

Third party packages

The following third party packages are also available.

DRF Compound Fields

The drf-compound-fields package provides "compound" serializer fields, such as lists of simple values, which can be described by other fields rather than serializers with the many=True option. Also provided are fields for typed dictionaries and values that can be either a specific type or a list of items of that type.

DRF Extra Fields

The drf-extra-fields package provides extra serializer fields for REST framework, including Base64ImageField and PointField classes.

djangorestframework-recursive

the djangorestframework-recursive package provides a RecursiveField for serializing and deserializing recursive structures

django-rest-framework-gis

The django-rest-framework-gis package provides geographic addons for django rest framework like a GeometryField field and a GeoJSON serializer.

django-rest-framework-hstore

The django-rest-framework-hstore package provides an HStoreField to support django-hstore DictionaryField model field.