ListField 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 ListField in Serializers in Django REST Framework.

ListField

ListField is basically a list field that validates the input against a list of objects.

It has the following arguments –

  • 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.

Syntax –

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

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



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

How to use ListField in Serializers ?

To explain the usage of ListField, 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 ListField as the fields.

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# import serializer from rest_framework
from rest_framework import serializers
  
class Geeks(object):
    def __init__(self, integers):
        self.integers = integers
  
# create a serializer
class GeeksSerializer(serializers.Serializer):
    # intialize fields
    integers = serializers.ListField(
    child = serializers.IntegerField(min_value = 0, max_value = 100)
    )

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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([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

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

# print serialized data
>>> serializer.data
{'integers': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]}

Here is the output of all these operations on terminal –

ListField-in-serializers-Django-REST-Framework

Validation on ListField

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

# Create a dictionary and add invalid values
>>> data = {}
>>> data['integers'] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# dictionary created
>>> data
{'integers': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]}

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

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

# check the errors
>>> serializer.errors
{}

Here is the output of these commands which clearly shows integers valid –

listfield-in-serializers

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.



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