From TODO:

    # TODO: Add good error checking.
    # TODO: Add good exception handling.
    # TODO: Set timestamp? Make optional, default being to set it if possible?

Also, consider renaming get_contents_to_filename and get_contents_to_file. The get is a misleading verb, since a read into a file is what is happening.

See gcloud.storage.Key.get_contents_to_filename for relevant code and #146 for some context.