File uploads to S3

import os
import boto3
from moto import mock_aws             # fakes AWS locally; remove it (and use real credentials) in production
 
os.environ.update(AWS_ACCESS_KEY_ID="testing", AWS_SECRET_ACCESS_KEY="testing", AWS_DEFAULT_REGION="eu-west-2")
ALLOWED = {"application/pdf", "text/markdown", "text/plain"}
MAX_BYTES = 20 * 1024 * 1024
 
@mock_aws
def demo():
    s3 = boto3.client("s3")
    s3.create_bucket(Bucket="rag-uploads", CreateBucketConfiguration={"LocationConstraint": "eu-west-2"})
 
    # 1. The API checks the request, then hands the browser a short-lived PRESIGNED URL to upload directly.
    def start_upload(user: str, filename: str, content_type: str, size: int):
        if content_type not in ALLOWED or size > MAX_BYTES:
            return {"error": "file type or size not allowed"}
        key = f"uploads/{user}/{filename}"                       # never trust the filename for paths in real code
        url = s3.generate_presigned_url("put_object", ExpiresIn=300,
                                        Params={"Bucket": "rag-uploads", "Key": key, "ContentType": content_type})
        return {"key": key, "upload_url": url.split("?")[0] + "?<signature>"}
 
    print(start_upload("alice", "policy.pdf", "application/pdf", 1_200_000))
    print(start_upload("alice", "tool.exe", "application/x-msdownload", 50_000))
 
    # 2. The browser PUTs the file to that URL (simulated here); 3. an event triggers ingestion.
    s3.put_object(Bucket="rag-uploads", Key="uploads/alice/policy.pdf", Body=b"%PDF-1.7 ...",
                  ContentType="application/pdf")
    for obj in s3.list_objects_v2(Bucket="rag-uploads", Prefix="uploads/alice/")["Contents"]:
        head = s3.head_object(Bucket="rag-uploads", Key=obj["Key"])
        print("stored:", obj["Key"], obj["Size"], "bytes,", head["ContentType"])
 
demo()

{'key': 'uploads/alice/policy.pdf', 'upload_url': 'https://rag-uploads.s3.amazonaws.com/uploads/alice/policy.pdf?<signature>'}
{'error': 'file type or size not allowed'}
stored: uploads/alice/policy.pdf 12 bytes, application/pdf