what is the best way to authenticate to GCP via an HTPP request from an IAP developed with flask and deployed in APP engine?

Solution
def explicit():
    from google.cloud import storage

    # Explicitly use service account credentials by specifying the private key
    # file.
    # In your case:
    authentification = request.args.get('login')

    # storage_client = storage.Client.from_service_account_json(
    #    'service_account.json')
    storage_client = storage.Client.from_service_account_json(
        authentification)

    # Make an authenticated API request
    buckets = list(storage_client.list_buckets())
    print(buckets)
def implicit():
    from google.cloud import storage

    # If you don't specify credentials when constructing the client, the
    # client library will look for credentials in the environment.
    storage_client = storage.Client()

    # Make an authenticated API request
    buckets = list(storage_client.list_buckets())
    print(buckets)