django-rest-framework/docs/api-guide/fields.md

source: fields.py

---

Note: This is the documentation for the version 3.0 of REST framework. Documentation for version 2.4 is also available.

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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.

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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>.

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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

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.

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.

Defaults to False

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 behavior is to not populate the attribute at all.

May be set to a function or other callable, in which case the value will be evaluated each time it is used.

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.

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('get_absolute_url'), or may use dotted notation to traverse attributes, such as EmailField(source='user.email').

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.

style

A dictionary of key-value pairs that can be used to control how renderers should render the field. The API for this should still be considered experimental, and will be formalized with the 3.1 release.

Two options are currently used in HTML form generation, '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'}
}

Note: The style argument replaces the old-style version 2.x widget keyword argument. Because REST framework 3 now uses templated HTML form generation, the widget option that was used to support Django built-in widgets can no longer be supported. Version 3.1 is planned to include public API support for customizing HTML form generation.

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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.

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()

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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)

• max_length - Validates that the input contains no more than this number of characters.
• min_length - Validates that the input contains no fewer than this number of characters.
• allow_blank - If set to True then the empty string should be considered a valid value. If set to False then the empty string is considered invalid and will raise a validation error. Defaults to False.
• trim_whitespace - If set to True then leading and trailing whitespace is trimmed. Defaults to 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"

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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)

• max_value Validate that the number provided is no greater than this value.
• min_value Validate that the number provided is no less than this value.

FloatField

A floating point representation.

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

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

• max_value Validate that the number provided is no greater than this value.
• min_value Validate that the number provided is no less than this value.

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)

• max_digits The maximum number of digits allowed in the number. Note that this number must be greater than or equal to decimal_places.
• decimal_places The number of decimal places to store with the number.
• coerce_to_string Set to True if string values should be returned for the representation, or False if Decimal objects should be returned. Defaults to the same value as the COERCE_DECIMAL_TO_STRING settings key, which will be True unless overridden. If Decimal objects are returned by the serializer, then the final output format will be determined by the renderer.
• max_value Validate that the number provided is no greater than this value.
• min_value Validate that the number provided is no less than this value.

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.

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Date and time fields

DateTimeField

A date and time representation.

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

Signature: DateTimeField(format=None, input_formats=None)

• format - A string representing the output format. If not specified, this defaults to the same value as the DATETIME_FORMAT settings key, which will be 'iso-8601' unless set. Setting to a format string indicates that to_representation return values should be coerced to string output. Format strings are described below. Setting this value to None indicates that Python datetime objects should be returned by to_representation. In this case the datetime encoding will be determined by the renderer.
• input_formats - A list of strings representing the input formats which may be used to parse the date. If not specified, the DATETIME_INPUT_FORMATS setting will be used, which defaults to ['iso-8601'].

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.

In the case of JSON this means the default datetime representation uses the ECMA 262 date time string specification. This is a subset of ISO 8601 which uses millisecond precision, and includes the 'Z' suffix for the UTC timezone, for example: 2013-01-29T12:34:56.123Z.

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=None, input_formats=None)

• format - A string representing the output format. If not specified, this defaults to the same value as the DATE_FORMAT settings key, which will be 'iso-8601' unless set. Setting to a format string indicates that to_representation return values should be coerced to string output. Format strings are described below. Setting this value to None indicates that Python date objects should be returned by to_representation. In this case the date encoding will be determined by the renderer.
• input_formats - A list of strings representing the input formats which may be used to parse the date. If not specified, the DATE_INPUT_FORMATS setting will be used, which defaults to ['iso-8601'].

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=None, input_formats=None)

• format - A string representing the output format. If not specified, this defaults to the same value as the TIME_FORMAT settings key, which will be 'iso-8601' unless set. Setting to a format string indicates that to_representation return values should be coerced to string output. Format strings are described below. Setting this value to None indicates that Python time objects should be returned by to_representation. In this case the time encoding will be determined by the renderer.
• input_formats - A list of strings representing the input formats which may be used to parse the date. If not specified, the TIME_INPUT_FORMATS setting will be used, which defaults to ['iso-8601'].

#### 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')

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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)

• choices - A list of valid values, or a list of (key, display_name) tuples.
• allow_blank - If set to True then the empty string should be considered a valid value. If set to False then the empty string is considered invalid and will raise a validation error. Defaults to False.

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)

• choices - A list of valid values, or a list of (key, display_name) tuples.
• allow_blank - If set to True then the empty string should be considered a valid value. If set to False then the empty string is considered invalid and will raise a validation error. Defaults to False.

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.

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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)

• max_length - Designates the maximum length for the file name.
• allow_empty_file - Designates if empty files are allowed.
• use_url - If set to True then URL string values will be used for the output representation. If set to False then filename string values will be used for the output representation. Defaults to the value of the UPLOADED_FILES_USE_URL settings key, which is True unless set otherwise.

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)

• max_length - Designates the maximum length for the file name.
• allow_empty_file - Designates if empty files are allowed.
• use_url - If set to True then URL string values will be used for the output representation. If set to False then filename string values will be used for the output representation. Defaults to the value of the UPLOADED_FILES_USE_URL settings key, which is True unless set otherwise.

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

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Composite fields

ListField

A field class that validates a list of objects.

Signature: ListField(child)

• child - A field instance that should be used for validating the objects in the list. If this argument is not provided then objects in the list will not be validated.

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)

• child - A field instance that should be used for validating the values in the dictionary. If this argument is not provided then values in the mapping will not be validated.

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()

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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, is 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 = serializer.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)

• method_name - The name of the method on the serializer to be called. If not included this defaults to get_<field_name>.

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

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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 serializer.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

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, obj):
        return "rgb(%d, %d, %d)" % (obj.red, obj.green, obj.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 represent the class name of the object being serialized:

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

    def to_representation(self, obj):
        """
        Serialize the object's class name.
        """
        return obj.__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):
        msg = 'Incorrect type. Expected a string, but got %s'
        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 you error messages more cleanly separated from your code, and should be preferred.

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.

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.