Ctrl-D is recognized not necessarily as EOF, but as "terminate current read() call".
If you have an empty line (or just pressed Ctrl-D) and press Ctrl-D, your read() terminates immediately and returns 0 read bytes. And this is a sign for EOF.
If you have data in a line and press Ctrl-D, your read() terminates with whatever there has been typed, of course without a terminating newline ('\n').
So if you have input data, you press Ctrl-D twice of a non-empty line or once on a empty one, i.e. with Enter before.
This all holds for the normal OS interface, accessible from Python via os.read().
Python file objects, and also file iterators, treat the first EOF recognized as termination for the current read() call, as they suppose there is nothing any longer. A next read() call tries again and needs another Ctrl-D in order to really return 0 bytes. The reason is that a file object read() always tries to return as many bytes as requested and tries to fill up if a OS read() returns less than requested.
As opposite to file.readline(), iter(file) uses the internal read() functions to read and thus always has this special requirement of the extra Ctrl-D.
I always use iter(file.readline, '') to read line-wise from a file.
