isatty() for file-like objects: Implement or not?

[HOWARD GOLDEN]

The standard documentation for isatty() says:

        "Return True if the file is connected to a tty(-like) device, else 
False. Note: If a file-like object is not associated with a real file, 
this method should not be implemented."

In his book, "Text Processing in Python," David Mertz says: "... 
implementing it to always return 0 is probably a better approach."

My reaction is to agree with Mertz. I'd appreciate hearing from someone 
why the standard documentation is the way it is. Is there some concept of 
a file-like, non-real file that is to be distinguished using the 
non-implementation of isatty()? If this is the intention, then my 
suggestion is that this distinction is too indirectly expressed, and a 
more direct approach would be easier for non-language lawyers to 
understand. For example, there could be an attribute "unreal" (or, 
contrarily, "real").

Howard B. Golden