Custom serializer fields in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this custom serializer field?

Consider this Django REST Framework serializer field that converts a string to uppercase during serialization.
What will be the serialized output for the input {'name': 'alice'}?

Django

from rest_framework import serializers

class UpperCaseField(serializers.Field):
    def to_representation(self, value):
        return value.upper()

class UserSerializer(serializers.Serializer):
    name = UpperCaseField()

serializer = UserSerializer({'name': 'alice'})
print(serializer.data)

A Raises AttributeError

B {'name': 'alice'}

C {'name': 'Alice'}

D {'name': 'ALICE'}

💡 Hint

Look at what the to_representation method does to the input value.

The custom field overrides to_representation to convert the string to uppercase. So the output will have 'ALICE' as the name.

📝 Syntax

intermediate

Identify the syntax error in this custom serializer field

Which option correctly fixes the syntax error in this custom serializer field code?

Django

from rest_framework import serializers

class CustomField(serializers.Field):
    def to_representation(self, value)
        return str(value) + '!'

A Add a colon after the method definition: def to_representation(self, value):

B Change 'def' to 'function' in the method definition

C Remove 'self' parameter from the method

D Indent the return statement one level less

💡 Hint

Python method definitions require a colon at the end.

The method definition is missing a colon at the end, which causes a SyntaxError.

❓ state_output

advanced

What is the value of 'data' after serialization?

Given this serializer with a custom field that reverses strings during serialization, what is the value of serializer.data?

Django

from rest_framework import serializers

class ReverseField(serializers.Field):
    def to_representation(self, value):
        return value[::-1]

class MessageSerializer(serializers.Serializer):
    message = ReverseField()

serializer = MessageSerializer({'message': 'hello'})
data = serializer.data

A {'message': 'olleh'}

B Raises TypeError

C {'message': 'HELLO'}

D {'message': 'hello'}

💡 Hint

The to_representation method reverses the string.

The custom field reverses the string 'hello' to 'olleh' during serialization.

🔧 Debug

advanced

Which option causes a runtime error when using this custom serializer field?

This custom field expects an integer but receives a string. Which option will cause a runtime error when serializing?

Django

from rest_framework import serializers

class DoubleIntegerField(serializers.Field):
    def to_representation(self, value):
        return value * 2

class NumberSerializer(serializers.Serializer):
    number = DoubleIntegerField()

serializer = NumberSerializer({'number': input_value})
output = serializer.data

A input_value = '5'

B input_value = None

C input_value = 5

D input_value = 3.5

💡 Hint

Consider what happens when None is multiplied by 2.

Multiplying None by 2 raises a TypeError because NoneType cannot be multiplied.

🧠 Conceptual

expert

Which option correctly implements a custom serializer field that validates input as a positive integer?

You want to create a custom serializer field that only accepts positive integers during input validation. Which option correctly implements the to_internal_value method to enforce this?

A

def to_internal_value(self, data):
    value = int(data)
    if value < 0:
        raise serializers.ValidationError('Must be positive')
    return value

B

def to_internal_value(self, data):
    if data > 0:
        return data
    else:
        raise serializers.ValidationError('Must be positive')

C

def to_internal_value(self, data):
    value = int(data)
    if value <= 0:
        raise serializers.ValidationError('Must be positive')
    return value

D

def to_internal_value(self, data):
    value = float(data)
    if value <= 0:
        raise serializers.ValidationError('Must be positive')
    return value

💡 Hint

Remember to convert input to int and check for positive values including zero.

Option C converts input to int, checks if value is less than or equal to zero, and raises error if so. This correctly enforces positive integers (greater than zero).