Pitrou,

I think you are right on "On the other hand, it will appear quite bizarre to people who have another, dedicated key for auto-completion." If I assign another key for auto-completion, the behavior becomes really strange.  It happens in ipython too.

But I guess most people just use tab as the default key for auto-completion, and the readline module document say:"
get_begidx( ) 
    Get the beginning index of the readline tab-completion scope. 
get_endidx( ) 
    Get the ending index of the readline tab-completion scope. 
set_completer_delims( string) 
    Set the readline word delimiters for tab-completion. 
get_completer_delims( ) 
    Get the readline word delimiters for tab-completion. 
"
And now, I think the problem becomes:convenience for the majority or protection for the minority. If we decide to protect the minority, I think I can submit another patch for the readline module document, to make the document constant, so, the document looks like:
get_endidx( ) 
    Get the ending index of the readline auto-completion scope. 

PS: Is there a way to find out the current key-binding on auto-completion? If we can find this, I think we can "if else" this behavior to satisfy everybody.