app.py

# Signed URL generation demo
# Author: Fil Zembowicz <fil@formsort.com>

usage = "AWS_REGION=... AWS_BUCKET=... AWS_UPLOAD_FOLDER=... flask run"

import boto3
import os
import uuid
from flask import Flask, request, abort, jsonify

app = Flask(__name__)

AWS_REGION = os.getenv("AWS_REGION")
AWS_BUCKET = os.getenv("AWS_BUCKET")
AWS_UPLOAD_FOLDER = os.getenv("AWS_UPLOAD_FOLDER")
if not AWS_REGION or not AWS_BUCKET or not AWS_UPLOAD_FOLDER:
    raise (AssertionError(usage))
if AWS_UPLOAD_FOLDER.startswith("/"):
    raise AssertionError("AWS_UPLOAD_FOLDER should not have a leading slash")
MIN_SIZE_BYTES = 1
MAX_SIZE_BYTES = 10_000_000


@app.route("/presigned-post")
def presigned_post():

    # You can pass other args from the formsort answer set.
    # When creating the signedUrl, just include them like https://example.com/get-signed-url?something={{some_answer}}
    # Filename will automatically be added onto that.

    filename = request.args.get("filename")
    if not filename:
        abort(400)

    filename, ext = os.path.splitext(filename)

    # Mime types are also verified on the frontend per configuration set within Formsort,
    # but you should also do it on the backend.
    allowed_upload_mime_types = {".jpg": "image/jpg"}
    if ext not in allowed_upload_mime_types:
        abort(400)
    mime_type = allowed_upload_mime_types[ext]

    filename = f"{uuid.uuid4()}{ext}"
    location = f"{AWS_UPLOAD_FOLDER}/{filename}"

    headers = {
        "acl": "public-read",
        "Content-Type": mime_type,
        "Cache-Control": "max-age=31536000,public",
    }

    conditions = [
        {"acl": "public-read"},
        {"Content-Type": mime_type},
        {"Cache-Control": "max-age=31536000,public"},
        # File sizes are also frontend-verified, but you should also verify on the backend.
        ["content-length-range", MIN_SIZE_BYTES, MAX_SIZE_BYTES],
        {"key": location},
    ]

    s3 = boto3.client("s3", region_name=AWS_REGION)
    post = s3.generate_presigned_post(
        Bucket=AWS_BUCKET, Key=location, Conditions=conditions, Fields=headers
    )

    # You must return these three keys since they are required to actually upload the content.
    # If you specify an answerValue in the response, that will be saved in the Formsort answer set.
    # Otherwise, the full url (url + key) will be saved. This way, if your bucket allows GET, you can view
    # the image immediately after.
    response = jsonify({"url": post["url"], "headers": headers, "key": location})

    # This is just for the demo. In practice you will want to restrict to *.yourdomain.com and *.formsort.com
    response.headers.add("Access-Control-Allow-Origin", "*")
    return response


if __name__ == "__main__":
    app.run(debug=True)