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import hashlib, base64, hmac, json, settings

def shopify_webhook(f):
  """
  A decorator thats checks and validates a Shopify Webhook request.
  """

  def _hmac_is_valid(body, secret, hmac_to_verify):
    hash            = hmac.new(body, secret, hashlib.sha256)
    hmac_calculated = base64.b64encode(hash.digest())
    return hmac_calculated == hmac_to_verify

  @wraps(f)
  def wrapper(request, *args, **kwargs):
    # Try to get required headers and decode the body of the request.
    try:
      webhook_topic = request.META['HTTP_X_SHOPIFY_TOPIC']
      webhook_hmac  = request.META['HTTP_X_SHOPIFY_HMAC_SHA256']
      webhook_data  = json.loads(request.body)
    except:
      return HttpResponseBadRequest()

    # Verify the HMAC.
    if not _hmac_is_valid(request.body, settings.SHOPIFY_API_SECRET, webhook_hmac):
      return HttpResponseForbidden()

    # Otherwise, set properties on the request object and return.
    request.webhook_topic = webhook_topic
    request.webhook_data  = webhook_data
    return f(request, args, kwargs)

  return wrapper