Name is not defined in a list comprehension with multiple loops
Solution 1:
You have the order of your loops mixed up; they are considered nested from left to right, so for r in a[g] is the outer loop and executed first. Swap out the loops:
print [r['n'] for g in good for r in a[g]]
Now g is defined for the next loop, for r in a[g], and the expression no longer raises an exception:
>>> a={
... 1: [{'n': 1}, {'n': 2}],
... 2: [{'n': 3}, {'n': 4}],
... 3: [{'n': 5}],
... }
>>> good = [1,2]
>>> [r['n'] for g in good for r in a[g]]
[1, 2, 3, 4]