Choice Selection Fields in serializers – Django REST Framework

In Django REST Framework the very concept of Serializing is to convert DB data to a datatype that can be used by javascript. Every serializer comes with some fields (entries) which are going to be processed. For example if you have a class with name Employee and its fields as Employee_id, Employee_name, is_admin, etc. Then, you would need AutoField, CharField and BooleanField for storing and manipulating data through Django. Similarly, serializer also works with same principle and has fields that are used to create a serializer. 
This article revolves around Choice Selection Fields in Serializers in Django REST Framework. There are two major fields – Choice and MultipleChioceField.

ChoiceField

ChoiceField is basically a CharField that validates the input against a value out of a limited set of choices. This field is same as ChoiceField – Django Forms.
It has the following arguments – 

• 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…”

Syntax – 

field_name = serializers.ChoiceField(*args, **kwargs)

MultipleChoiceField

ChoiceField is basically a CharField that validates the input against a set of zero, one or many values, chosen from a limited set of choices. This field is same as MultipleChoiceField – Django Forms.
Syntax – 

field_name = serializers.MultipleChoiceField(*args, **kwargs)

How to use Choice Selection Fields in Serializers ?

To explain the usage of Choice Selection Fields, let’s use the same project setup from – How to Create a basic API using Django Rest Framework ?. 
Now that you have a file called serializers in your project, let’s create a serializer with ChoiceField and MultipleChoiceField as the fields. 

Python3

# import serializer from rest_framework
from rest_framework import serializers

class Geeks(object):
    def __init__(self, choices, multiplechoices):
        self.choices = choices
        self.multiplechoices = multiplechoices

# create a tuple
GEEKS_CHOICES =(
    ("1", "One"),
    ("2", "Two"),
    ("3", "Three"),
    ("4", "Four"),
    ("5", "Five"),
)

# create a serializer
class GeeksSerializer(serializers.Serializer):
    # initialize fields
    choices = serializers.ChoiceField(
                        choices = GEEKS_CHOICES)
    multiplechoices = serializers.MultipleChoiceField(
                        choices = GEEKS_CHOICES)

Now let us create some objects and try serializing them and check if they are actually working, Run, – 

Python manage.py shell

Now, run following python commands in the shell 

# import everything from serializers
>>> from apis.serializers import *

# create a object of type Geeks
>>> obj = Geeks("One", ["One", "Two"])

# serialize the object
>>> serializer = GeeksSerializer(obj)

# print serialized data
>>> serializer.data
{'choices': 'One', 'multiplechoices': {'Two', 'One'}}

Here is the output of all these operations on terminal – 

Validation on Choice Selection Fields

Note that prime motto of these fields is to impart validations, such as ChoiceField validates the data to a selected give choices only. Let’s check if these validations are working or not – 

# Create a dictionary and add invalid values
>>> data={}
>>> data['choices'] = "Naveen"
>>> data['multiplechoices'] = ["One", "Two"]

# dictionary created
>>> data
{'choices': 'Naveen', 'multiplechoices': ['One', 'Two']}

# deserialize the data
>>> serializer = GeeksSerializer(data=data)

# check if data is valid
>>> serializer.is_valid()
False

# check the errors
>>> serializer.errors
{'choices': [ErrorDetail(string='"Naveen" is not a valid choice.', code='invalid_choice')],
'multiplechoices': [ErrorDetail(string='"One" is not a valid choice.', code='invalid_choice')]}

Here is the output of these commands which clearly shows email and phone_number as invalid – 

Advanced concepts

Validations are part of Deserialization and not serialization. As explained earlier, serializing is process of converting already made data into another data type, so there is no requirement of these default validations out there. Deserialization requires validations as data needs to be saved to database or any more operation as specified. So if you serialize data using these fields that would work. 

Core arguments in serializer fields

Argument	Description
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
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.
required	Setting this to False also allows the object attribute or dictionary key to be omitted from output when serializing the instance.
default	If set, this gives the default value that will be used for the field if no input value is supplied.
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.
source	The name of the attribute that will be used to populate 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.
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.