List comprehensions for the win:
all_b_values = [t.b for t in list_of_named_tuples]
all_a_values = [t.a for t in list_of_named_tuples]
or
all_b_2s = [t for t in list_of_named_tuples if t.b == 2]
There is nothing namedtuple
-specific about these list comprehensions; they work for any sequence of objects with common attributes.