help with relative imports

[John Salerno]

Robert Kern wrote:

> What is ambiguous about A.B.D, A.E, and A.F.G? But if you like:

I guess maybe I was looking at it backwards. From the way it was worded, 
I thought the only information we had to use was the structure A.B.C, 
and then given a statement like:

from . import D

we just had to figure out for ourselves that this results in A.B.D, 
instead of, for example, A.C.D, or any other possibility.

But I'm still a little confused about the use of the single or double 
period. In this case:

from . import D                 # Imports A.B.D
from .. import E                # Imports A.E

why do you need a single period in the first example, and a double in 
the second, if they both are importing from A? If E is directly under A, 
couldn't you just use a single period? And since D is nested twice 
beneath A (i.e., in A, then in B), wouldn't you need two periods there 
instead?