Fix ValidationError: {"price": ["A valid number is required."]} in Django

This error means a Django REST Framework serializer rejected the input data during validation. Fix it by ensuring the request data matches the serializer field types, checking that required fields are present, and verifying that custom validate methods return the validated data instead of silently discarding it.

Reading the Stack Trace

Here's what each line means:

• File "/app/api/views.py", line 18, in create: The view calls serializer.is_valid(raise_exception=True) which raises ValidationError if any field fails validation.
• File "/venv/lib/python3.11/site-packages/rest_framework/serializers.py", line 235, in is_valid: DRF's serializer ran all field validators and found that the price field received a non-numeric value.
• rest_framework.exceptions.ValidationError: {"price": ["A valid number is required."]}: The client sent a non-numeric value (like an empty string or text) for a DecimalField.

Common Causes

// Frontend JavaScript
fetch('/api/products/', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ name: 'Widget', price: '' }),  // Empty string
});

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ['name', 'price', 'category']

    def validate_price(self, value):
        if value <= 0:
            raise serializers.ValidationError('Price must be positive.')
        # Missing: return value

class OrderSerializer(serializers.ModelSerializer):
    items = OrderItemSerializer()  # Missing many=True for list data

    class Meta:
        model = Order
        fields = ['id', 'items']

The Fix

Always return the validated value from validate_<field> methods. Add null checks for required fields. Use the validate() method for cross-field validation. DRF silently sets the field to None if validate_<field> does not return a value.

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ['name', 'price', 'category']

    def validate_price(self, value):
        if value <= 0:
            raise serializers.ValidationError('Price must be positive.')
        # Missing return!

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ['name', 'price', 'category']

    def validate_price(self, value):
        if value is None:
            raise serializers.ValidationError('Price is required.')
        if value <= 0:
            raise serializers.ValidationError('Price must be positive.')
        return value  # Always return the validated value

    def validate(self, attrs):
        """Cross-field validation."""
        if attrs.get('category') == 'free' and attrs.get('price', 0) > 0:
            raise serializers.ValidationError('Free products must have a price of 0.')
        return attrs

Testing the Fix

import pytest
from rest_framework.test import APITestCase, APIClient
from products.models import Product


class TestProductSerializer(APITestCase):
    def setUp(self):
        self.client = APIClient()

    def test_create_product_with_valid_data(self):
        response = self.client.post('/api/products/', {
            'name': 'Widget',
            'price': '19.99',
            'category': 'gadgets',
        }, format='json')
        assert response.status_code == 201

    def test_create_product_with_invalid_price(self):
        response = self.client.post('/api/products/', {
            'name': 'Widget',
            'price': 'not-a-number',
            'category': 'gadgets',
        }, format='json')
        assert response.status_code == 400
        assert 'price' in response.json()

    def test_create_product_with_negative_price(self):
        response = self.client.post('/api/products/', {
            'name': 'Widget',
            'price': '-5.00',
            'category': 'gadgets',
        }, format='json')
        assert response.status_code == 400

    def test_create_product_with_empty_price(self):
        response = self.client.post('/api/products/', {
            'name': 'Widget',
            'price': '',
            'category': 'gadgets',
        }, format='json')
        assert response.status_code == 400

Run your tests:

pytest

Pushing Through CI/CD

git checkout -b fix/serializer-validation-return,git add api/serializers.py,git commit -m "fix: return validated values from serializer validate methods",git push origin fix/serializer-validation-return

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    services:
      postgres:
        image: postgres:15
        env:
          POSTGRES_DB: test_db
          POSTGRES_USER: postgres
          POSTGRES_PASSWORD: postgres
        ports:
          - 5432:5432
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: '3.11'
          cache: 'pip'
      - run: pip install -r requirements.txt
      - run: pytest --tb=short -q

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

pip install bugstack

import bugstack

bugstack.init(api_key=os.environ["BUGSTACK_API_KEY"])

Deploying the Fix (Manual Path)

1. Run the full test suite locally to confirm serializer validation.
2. Open a pull request with the serializer fixes.
3. Wait for CI checks to pass on the PR.
4. Have a teammate review and approve the PR.
5. Merge to main and verify API validation in staging.

Frequently Asked Questions