List comprehension works but not generator expression

Question

I am doing this problem:

Given an array of integers arr, a lucky integer is an integer which has a frequency in the array equal to its value. Return a lucky integer in the array. If there are multiple lucky integers return the largest of them. If there is no lucky integer return -1.

I am using this code, which works and passed all tests:

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        values = [n for n in arr if n if arr.count(n) == n]
        return max(values) if values else -1

However, when I try to change the list comprehension to a generator expression like so:

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        values = (n for n in arr if n if arr.count(n) == n)
        return max(values) if values else -1

I get this error:

Traceback (most recent call last):
  File "...", line 10, in <module>
    print(Solution().findLucky([2, 3, 4]))
  File "...", line 7, in findLucky
    return max(values) if values else -1
ValueError: max() arg is an empty sequence

Why does a list comprehension work whereas a generator expression fails?

I've tried converting the generator expression to a list in the comprehension, but that won't work because then the generator is exhuased.

Answer 1

Every generator is true, so you always call max(values) even if the generator is "empty". The right way to do it is to tell max what to do then:

return max(values, default=-1)