The patch in review includes a PathBase.as_uri() method that raises UnsupportedOperation. I expect that some subclasses of PathBase will override that method, e.g. to return s3:// or ftp:// URIs.
Q: Should we add a symmetrical PathBase.from_uri() classmethod? This would provide an explicit means to contruct a path object (and its backend) from a URI - for example, an FTPPath.from_uri() method could parse the host/port/user/passwd from the URI, construct an ftplib.FTP object, and then wrap it in an FTPPath object.
For Path.from_uri() (local paths), I have a local branch that handles RFC 8089 file: URIs, including the weird ones with 4 or 5 leading slashes, such as those produced by urllib.request.pathname2url().