make an s3 policy from a bucket name, secret key, and filename

make_s3_policy.py

def make_s3_policy(aws_bucket, aws_secret_key, filename, expiration_delta):
  import datetime
  import base64
  import json
  import hmac
  from hashlib import sha1
  expiration = (datetime.datetime.utcnow() + \
    datetime.timedelta(seconds=expiration_delta)).strftime("%Y-%m-%dT%H:%M:%SZ")
  policy = {
    "expiration": expiration,
      "conditions": [
        {"key": filename},
        {"bucket": aws_bucket},
        {"acl": "public-read"},
        {"Content-type": "image/png"},
        ["content-length-range", 0, 5000000]
      ]
  }
  policy = base64.b64encode(json.dumps(policy))
  signed = base64.b64encode(
    hmac.new(str(aws_secret_key), str(policy), sha1).digest())
  return policy, signed

if __name__ == '__main__':
  print make_s3_policy("foo", "barbazquux", "xyzzy.png", 300)