TypeError: ‘builtin_function_or_method’ object is not subscriptable的一种错误情况

初学python，今天在做练习时为一个Error苦恼了很久
最终找到了解决方法

错误代码块如下

def get_new_nums():
	numbers = input('Well, I don\'t know your favorite numbers yet, '+
		'can you tell me?')
	numbers = numbers.split(',')	
	print(int(numbers[0]))
	numbers = [int(numbers[i]) for i in range(len[numbers])]
	with open('favorite_nums.json','w') as fav_nums:
		json.dump(numbers,fav_nums)

运行后返回：TypeError: ‘builtin_function_or_method’ object is not subscriptable

原因：我把倒数第三行列表解析里面的len(numbers)打成len[numbers]了,小括号打成了中括号
函数不能当字典用，因此返回 ‘builtin_function_or_method’ object is not subscriptable