Windows file paths are limited to 256 characters, and one of Windows's prescribed methods to address this is to prepend "\\?\" before a Windows absolute path (see: https://docs.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation)

urllib.request.pathname2url raises an error on such paths as this function calls nturl2path.py's pathname2url function which explicitly checks that the number of characters before the ":" in a Windows path is precisely one, which is, of course, not the case if you are using an extended-length path (e.g. "\\?\C:\Python39").

As a result, urllib cannot handle pathname2url conversion for some valid Windows paths.