Python 3.9 is now generally available.

google.appengine.api.blobstore.create_upload_url_async

Asynchronously creates the upload URL for a POST form.

success_path The path within the application to call when a POST call is successful and the upload is complete.
max_bytes_per_blob The maximum size in bytes that any one blob in the upload can be, or None for no maximum size.
max_bytes_total The maximum size in bytes that the aggregate sizes of all of the blobs in the upload can be, or None for no maximum size.
rpc Optional UserRPC object.
gs_bucket_name The Google Cloud Storage bucket name to which the blobs should be uploaded. The application's service account must have the correct permissions to write to this bucket. The bucket name can be of the format bucket/path/, in which case the included path will be prepended to the uploaded object name.

A UserRPC whose result will be the upload URL.

TypeError If max_bytes_per_blob or max_bytes_total are not integral types.
ValueError If max_bytes_per_blob or max_bytes_total are not positive values.