问题描述:
Write a python function to find the largest negative number from the given list.

测试用例:
['assert largest_neg([1,2,3,-4,-6]) == -6', 'assert largest_neg([1,2,3,-8,-9]) == -9', 'assert largest_neg([1,2,3,4,-1]) == -1']

生成的代码:
def largest_neg(numbers):
    max_neg = None
    for num in numbers:
        if num < 0:
            if max_neg is None or num > max_neg:
                max_neg = num
    return max_neg