fields.py

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. If you're using Model Serializer default value will be False if you have specified blank=True or default or null=True at your field in your Model.

`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.

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 requires_context = True attribute, then the serializer field will be passed as an argument.

For example:

class CurrentUserDefault:
    """
    May be applied as a `default=...` value on a serializer field.
    Returns the current user.
    """
    requires_context = True

    def __call__(self, serializer_field):
        return serializer_field.context['request'].user

When serializing the instance, default will be used if 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. Beware of possible n+1 problems when using source attribute if you are accessing a relational orm model. For example:

class CommentSerializer(serializers.Serializer):
    email = serializers.EmailField(source="user.email")

This case would require user object to be fetched from database when it is not prefetched. If that is not wanted, be sure to be using prefetch_related and select_related methods appropriately. For more information about the methods refer to django documentation.

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 Django 2.1 removed the blank kwarg from models.BooleanField. Prior to Django 2.1 models.BooleanField fields were always blank=True. Thus since Django 2.1 default serializers.BooleanField instances will be generated without the required kwarg (i.e. equivalent to required=True) whereas with previous versions of Django, default BooleanField instances will be generated with a required=False option. If you want to control this behavior manually, explicitly declare the BooleanField on the serializer class, or use the extra_kwargs option to set the required flag.

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

Signature: BooleanField()

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"

Signature: UUIDField(format='hex_verbose')

format: Determines the representation format of the uuid value
- 'hex_verbose' - The canonical hex representation, including hyphens: "5ce0e9a5-5ffa-654b-cee0-1238041fb31a"
- 'hex' - The compact hex representation of the UUID, not including hyphens: "5ce0e9a55ffa654bcee01238041fb31a"
- 'int' - A 128 bit integer representation of the UUID: "123456789012312313134124512351145145114"
- 'urn' - RFC 4122 URN representation of the UUID: "urn:uuid:5ce0e9a5-5ffa-654b-cee0-1238041fb31a" Changing the format parameters only affects representation values. All formats are accepted by to_internal_value

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)

path - The absolute filesystem path to a directory from which this FilePathField should get its choice.
match - A regular expression, as a string, that FilePathField will use to filter filenames.
recursive - Specifies whether all subdirectories of path should be included. Default is False.
allow_files - Specifies whether files in the specified location should be included. Default is True. Either this or allow_folders must be True.
allow_folders - Specifies whether folders in the specified location should be included. Default is False. Either this or allow_files must be True.

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)

protocol Limits valid inputs to the specified protocol. Accepted values are 'both' (default), 'IPv4' or 'IPv6'. Matching is case-insensitive.
unpack_ipv4 Unpacks IPv4 mapped addresses like ::ffff:192.0.2.1. If this option is enabled that address would be unpacked to 192.0.2.1. Default is disabled. Can only be used when protocol is set to 'both'.

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. It must be either None or an integer 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. Note that setting localize will force the value to True.
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.
localize Set to True to enable localization of input and output based on the current locale. This will also force coerce_to_string to True. Defaults to False. Note that data formatting is enabled if you have set USE_L10N=True in your settings file.
rounding Sets the rounding mode used when quantizing to the configured precision. Valid values are decimal module rounding modes. Defaults to None.
normalize_output Will normalize the decimal value when serialized. This will strip all trailing zeroes and change the value's precision to the minimum required precision to be able to represent the value without losing data. Defaults to False.

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)

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, default_timezone=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'].
default_timezone - A tzinfo subclass (zoneinfo or pytz) representing the timezone. If not specified and the USE_TZ setting is enabled, this defaults to the current timezone. If USE_TZ is disabled, then datetime objects will be naive.

`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 be 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)

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=api_settings.TIME_FORMAT, 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')

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)

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

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.
html_cutoff - If set this will be the maximum number of choices that will be displayed by a HTML select drop down. Can be used to ensure that automatically generated ChoiceFields with very large possible selections do not prevent a template from rendering. Defaults to None.
html_cutoff_text - If set this will display a textual indicator if the maximum number of items have been cutoff in an HTML select drop down. Defaults to "More than {count} items…"

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.
html_cutoff - If set this will be the maximum number of choices that will be displayed by a HTML select drop down. Can be used to ensure that automatically generated ChoiceFields with very large possible selections do not prevent a template from rendering. Defaults to None.
html_cutoff_text - If set this will display a textual indicator if the maximum number of items have been cutoff in an HTML select drop down. Defaults to "More than {count} items…"

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)

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.

Composite fields

ListField

A field class that validates a list of objects.

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

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.
allow_empty - Designates if empty lists are allowed.
min_length - Validates that the list contains no fewer than this number of elements.
max_length - Validates that the list contains no more than this number of elements.

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>, allow_empty=True)

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.
allow_empty - Designates if empty dictionaries are allowed.

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>, allow_empty=True)

child - A field instance that is used for validating the values in the dictionary. The default child field accepts both empty strings and null values.
allow_empty - Designates if empty dictionaries are allowed.

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, encoder)

binary - If set to True then the field will output and validate a JSON encoded string, rather than a primitive data structure. Defaults to False.
encoder - Use this JSON encoder to serialize input object. Defaults to None.

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.

Note: HiddenField() does not appear in partial=True serializer (when making PATCH request). This behavior might change in future, follow updates on github discussion.

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
        fields = '__all__'

    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.

Examples

A Basic Custom Field

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

class Color:
    """
    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, str):
        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, str):
        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 serializer using source='*', with two IntegerField instances, each with their own source pointing to the relevant field.

The key points from the example, though, are:

to_representation is passed the entire DataPoint object and must map from that to the desired output.

>>> instance = DataPoint(label='Example', x_coordinate=1, y_coordinate=2)
>>> out_serializer = DataPointSerializer(instance)
>>> out_serializer.data
ReturnDict([('label', 'Example'), ('coordinates', {'x': 1, 'y': 2})])

Unless our field is to be read-only, to_internal_value must map back to a dict suitable for updating our target object. With source='*', the return from to_internal_value will update the root validated data dictionary, rather than a single key.

>>> data = {
...     "label": "Second Example",
...     "coordinates": {
...         "x": 3,
...         "y": 4,
...     }
... }
>>> in_serializer = DataPointSerializer(data=data)
>>> in_serializer.is_valid()
True
>>> in_serializer.validated_data
OrderedDict([('label', 'Second Example'),
             ('y_coordinate', 4),
             ('x_coordinate', 3)])

For completeness lets do the same thing again but with the nested serializer 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 behavior as the custom field approach.

Serializing:

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

Deserializing:

>>> 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 serializer approach would be the first to try. You would use the custom field approach when the nested serializer 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.