From the python docs on urllib.urlopen(), talking about the file-like object the function returns on success:

(It is not a built-in file object, however, so it can’t be used at those few places where a true built-in file object is required.)

What are those few places where a true built-in file object is required?
NB: This is purely out of curiosity... no practical problem to be solved here.
For example, f.fileno() doesn't necessarily return a true OS-level file descriptor that you could use with os.read().
python: where is a true built-in file object required?