yet another list comprehension question

[Ross]

I'm trying to set up a simple filter using a list comprehension. If I
have a list of tuples, a = [(1,2), (3,4), (5,None), (6,7), (8, None)]
and I wanted to filter out all tuples containing None, I would like to
get the new list b = [(1,2), (3,4),(6,7)].

I tried b = [i for i in a if t for t in i is not None]   but I get the
error that "t is not defined". What am I doing wrong?