Django Storages S3 skill

Django Storages S3

Senior Django specialist for production-grade file storage on AWS S3 via django-storages and boto3 — public and private media, static files, presigned URLs, and CloudFront.

When to Use This Skill

• Serving static and/or media files from AWS S3 instead of the local filesystem
• Configuring the Django 4.2+ STORAGES dict or legacy DEFAULT_FILE_STORAGE
• Separating public (CDN-served) and private (presigned) file backends
• Generating presigned download or direct browser-to-S3 upload URLs
• Fronting S3 with CloudFront and writing a least-privilege IAM policy
• Migrating local FileField/ImageField storage to S3 without code changes
• Testing storage code without hitting S3

Core Workflow

1. Install & register — pip install django-storages[s3] boto3; add "storages" to INSTALLED_APPS
2. Configure credentials — Load from env vars or rely on an attached IAM role; never hardcode
3. Wire the STORAGES dict — Set default (media) and staticfiles backends with separate location prefixes
4. Add named backends — Split public vs. private buckets/ACLs as additional STORAGES entries when needed
5. Verify & test — Run collectstatic, confirm uploads land in S3, and mock S3 in tests with InMemoryStorage or moto

Reference Guide

Load detailed guidance based on context:

Topic	Reference	Load When
Settings & STORAGES	references/configuration.md	Core settings, 4.2+ vs legacy, CloudFront
Custom backends	references/custom-backends.md	Public vs. private buckets, per-field storage
Presigned URLs	references/presigned-urls.md	Download links, direct browser uploads
Testing & IAM	references/testing-storages.md	Mocking S3, IAM policy, common pitfalls

Minimal Working Example

The snippet below demonstrates the core MUST DO constraints: env-loaded credentials, STORAGES dict, separate media/static locations, and default_acl=None on the media backend.

# settings.py
import os

AWS_STORAGE_BUCKET_NAME = os.environ["AWS_STORAGE_BUCKET_NAME"]
AWS_S3_REGION_NAME = os.environ.get("AWS_S3_REGION_NAME", "us-east-1")
AWS_S3_CUSTOM_DOMAIN = f"{AWS_STORAGE_BUCKET_NAME}.s3.{AWS_S3_REGION_NAME}.amazonaws.com"
# On EC2/ECS/Lambda, omit keys entirely — boto3 uses the attached IAM role.

STORAGES = {
    "default": {  # media uploads
        "BACKEND": "storages.backends.s3boto3.S3Boto3Storage",
        "OPTIONS": {
            "bucket_name": AWS_STORAGE_BUCKET_NAME,
            "location": "media",
            "default_acl": None,        # rely on bucket policy, not per-object ACLs
            "file_overwrite": False,
            "querystring_auth": False,  # public objects → clean URLs
        },
    },
    "staticfiles": {
        "BACKEND": "storages.backends.s3boto3.S3StaticStorage",
        "OPTIONS": {
            "bucket_name": AWS_STORAGE_BUCKET_NAME,
            "location": "static",
        },
    },
}

MEDIA_URL = f"https://{AWS_S3_CUSTOM_DOMAIN}/media/"
STATIC_URL = f"https://{AWS_S3_CUSTOM_DOMAIN}/static/"

# models.py — uploads go straight to S3 on save()
from django.db import models

class Document(models.Model):
    file = models.FileField(upload_to="docs/")  # uses STORAGES["default"]

Auditing an Existing Configuration

When reviewing a project that already uses S3 (not greenfield), walk this checklist — each item is a constraint below rephrased as "find X, confirm Y":

1. Credentials — grep -rn "AWS_SECRET_ACCESS_KEY\|aws_secret" settings/ → confirm values come from os.environ/django-environ or an IAM role, never literals committed to the repo.
2. ACLs — grep -rn "default_acl\|AWS_DEFAULT_ACL" . → on buckets created after April 2023, every value must be None. Any "public-read"/"private" will raise AccessControlListNotSupported; public access belongs in a bucket policy.
3. Storage backend — confirm Django 4.2+ uses the STORAGES dict, not DEFAULT_FILE_STORAGE/STATICFILES_STORAGE (removed in Django 5.1, so silently ignored on 5.1/5.2/6.0); confirm the static class is S3StaticStorage, not a fabricated name.
4. Locations — confirm default (media) and staticfiles have distinct location prefixes or buckets so collectstatic never collides with uploads.
5. Region — confirm region_name (or the global AWS_S3_REGION_NAME) matches the bucket's real region and that AWS_S3_CUSTOM_DOMAIN includes the region segment for non-us-east-1 buckets.
6. Presigning — for private backends, confirm querystring_auth=True and custom_domain=None; confirm presigned .url() results aren't cached past AWS_QUERYSTRING_EXPIRE.
7. Overwrite cleanup — where file_overwrite=False, confirm replaced files are explicitly deleted (otherwise superseded objects leak).
8. IAM — confirm the policy grants only Get/Put/Delete/ListBucket on the bucket ARN, not broader S3 access.

Constraints

MUST DO

• Load AWS credentials from environment variables or an attached IAM role
• Set default_acl=None so bucket policies (not object ACLs) control access
• Give static and media files separate location prefixes or separate buckets
• Use the STORAGES dict on Django 4.2+ (same config through 5.2 LTS and 6.0); DEFAULT_FILE_STORAGE/STATICFILES_STORAGE were removed in 5.1, so reserve them for < 4.2 only
• Set custom_domain=None on any backend that issues presigned URLs
• Mock S3 (InMemoryStorage or moto) in tests instead of hitting real buckets

MUST NOT DO

• Hardcode AWS_SECRET_ACCESS_KEY in settings.py or commit it
• Mix querystring_auth=True with a custom_domain (presigning breaks)
• Mix static and media files under the same prefix
• Grant the IAM user broader than Get/Put/Delete/ListBucket on the bucket ARN
• Rely on per-object ACLs on buckets created after April 2023 (ACLs disabled by default)

Knowledge Reference

django-storages, S3Boto3Storage, S3StaticStorage, boto3, STORAGES dict, presigned URLs, generate_presigned_post, CloudFront, IAM policy, InMemoryStorage, moto

Related Skills

• django-expert — core Django models, DRF, and ORM that produce the files this skill persists to S3
• fullstack-guardian — secure end-to-end upload flows and access control around stored files
• devops-engineer — provisioning the S3 buckets, IAM roles, and CloudFront distributions this skill targets

Documentation