Serializer validation in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this serializer validation code?

Consider this Django REST Framework serializer:

from rest_framework import serializers

class ProductSerializer(serializers.Serializer):
    name = serializers.CharField(max_length=100)
    price = serializers.DecimalField(max_digits=5, decimal_places=2)

serializer = ProductSerializer(data={'name': 'Pen', 'price': '12.345'})
valid = serializer.is_valid()
errors = serializer.errors

What will be the value of valid and errors after running this code?

Django

from rest_framework import serializers

class ProductSerializer(serializers.Serializer):
    name = serializers.CharField(max_length=100)
    price = serializers.DecimalField(max_digits=5, decimal_places=2)

serializer = ProductSerializer(data={'name': 'Pen', 'price': '12.345'})
valid = serializer.is_valid()
errors = serializer.errors

A valid is False; errors contain {'price': ['Ensure that there are no more than 2 decimal places.']}

B valid is True; errors is an empty dict {}

C valid is False; errors contain {'name': ['This field is required.']}

D valid is True; errors contain {'price': ['Invalid decimal value.']}

💡 Hint

Check how DecimalField validates decimal places.

The DecimalField with decimal_places=2 expects at most 2 decimal digits. The input '12.345' has 3 decimal digits, so validation fails with an error about decimal places.

❓ state_output

intermediate

What is the value of validated_data after calling is_valid()?

Given this serializer and data:

from rest_framework import serializers

class UserSerializer(serializers.Serializer):
    username = serializers.CharField()
    age = serializers.IntegerField(min_value=18)

serializer = UserSerializer(data={'username': 'alice', 'age': 17})
serializer.is_valid()
validated = serializer.validated_data

What will be the value of validated?

Django

from rest_framework import serializers

class UserSerializer(serializers.Serializer):
    username = serializers.CharField()
    age = serializers.IntegerField(min_value=18)

serializer = UserSerializer(data={'username': 'alice', 'age': 17})
serializer.is_valid()
validated = serializer.validated_data

A Raises a ValidationError because validated_data is not set when validation fails

B {'username': 'alice', 'age': 17}

C {} (empty dict)

D None

💡 Hint

Think about what happens when is_valid() returns False.

If is_valid() returns False, validated_data is not populated and accessing it raises a ValidationError.

📝 Syntax

advanced

Which option correctly implements a custom field-level validation method?

In Django REST Framework serializers, how do you correctly write a custom validation method for the field email?

Django

from rest_framework import serializers

class ContactSerializer(serializers.Serializer):
    email = serializers.EmailField()

    # Custom validation method here

A

def validate_email(self):
    if not self.email.endswith('@example.com'):
        raise serializers.ValidationError('Email must be from example.com')
    return self.email

B

def validate(self, data):
    if not data['email'].endswith('@example.com'):
        raise serializers.ValidationError('Email must be from example.com')
    return data

C

def validate_email(self, value):
    if not value.endswith('@example.com'):
        raise serializers.ValidationError('Email must be from example.com')
    return value

D

def validate_email(value):
    if not value.endswith('@example.com'):
        raise serializers.ValidationError('Email must be from example.com')
    return value

💡 Hint

Field-level validation methods must be named validate_ and accept self and value.

The correct signature for field-level validation is def validate_fieldname(self, value):. Option C follows this pattern. Option C is for object-level validation. Options C and D have incorrect signatures.

🔧 Debug

advanced

Why does this serializer raise a KeyError during validation?

Look at this serializer code:

from rest_framework import serializers

class OrderSerializer(serializers.Serializer):
    product_id = serializers.IntegerField(read_only=True)
    quantity = serializers.IntegerField()

    def validate(self, data):
        if data['quantity'] > 10 and data['product_id'] == 0:
            raise serializers.ValidationError('Invalid order')
        return data

serializer = OrderSerializer(data={'quantity': 15})
serializer.is_valid()

Why does calling is_valid() raise a KeyError?

Django

from rest_framework import serializers

class OrderSerializer(serializers.Serializer):
    product_id = serializers.IntegerField(read_only=True)
    quantity = serializers.IntegerField()

    def validate(self, data):
        if data['quantity'] > 10 and data['product_id'] == 0:
            raise serializers.ValidationError('Invalid order')
        return data

serializer = OrderSerializer(data={'quantity': 15})
serializer.is_valid()

A Because 'quantity' is missing in input data, causing KeyError

B Because 'product_id' is missing in input data, accessing data['product_id'] causes KeyError

C Because validate method must return None, not data

D Because serializers.IntegerField() does not accept missing fields

💡 Hint

Check which fields are required and what keys exist in data inside validate().

The input data lacks 'product_id', but the validate method tries to access data['product_id'] without checking if it exists, causing a KeyError.

🧠 Conceptual

expert

Which option best describes the order of validation methods called in a serializer?

In Django REST Framework, when you call serializer.is_valid(), in what order are the following validation methods called?

• Field-level validation methods (e.g., validate_)
• Object-level validation method (validate(self, data))
• Built-in field validators

A 1, 3, 2

B 2, 1, 3

C 3, 1, 2

D 1, 2, 3

💡 Hint

Think about how DRF validates each field before running custom validations.

First, built-in field validators run to check basic constraints. Then, field-level validate_ methods run for custom field validation. Finally, the object-level validate(self, data) method runs for cross-field validation.