Generator behaviour

[bla123bla]

Hello,

encountered this smart piece of breadth-first traversal:

def bfs(root):
    yield root
    for n in bfs(root):
        yield n.left
        yield n.right

but am confused about pythons behaviour here.

If you omit the first yield, the (recursive?) generator call on line 3 is
empty and this results in endless recursion, so behaves like a function
call. (One would expect it to not get executed at all).

But with the first yield, the non-empty generator gets iterated over.
Looks like the call behaves like a function in one case and a generator in
the other?