关于函数type error的问题

有一个字典a
a = {0: (-37.4168610028693, 145.005372256037),
1: (-37.7032933463858, 144.572524145218),
2: (-37.7292612709136, 144.650631483984),
3: (-37.7777637471456, 144.772303608804),
4: (-37.5792063872414, 144.72816450831)}

我想写一个函数来计算某个点到a字典中所有点的haversine距离并且返回最短距离和字典的key，我的代码如下：

def get_min_dis2(a:dict,b:tuple):
    min_dist = 10000
    for k in a.keys():
        lat1, lon1 = a[k]
        lat2, lon2 = b
        dlat = math.radians(lat2-lat1)
        dlon = math.radians(lon2-lon1)
        a = (math.sin(dlat / 2) * math.sin(dlat / 2) + math.cos(math.radians(lat1)) * math.cos(math.radians(lat2))*
            math.sin(dlon / 2) * math.sin(dlon / 2))
        c = 2 * math.atan2(math.sqrt(a), math.sqrt(1-a))
        distance = c * 6378
        if distance < min_dist:
            min_dist = distance
            key = k
    return (key,b,min_dist)

get_min_dis2(a,(-37.8438,145.0303))
但是我运行时会一直报错TypeError: 'float' object is not subscriptable
请问要如何解决呢

你改变a的类型了

import math

a = {0: (-37.4168610028693, 145.005372256037),
     1: (-37.7032933463858, 144.572524145218),
     2: (-37.7292612709136, 144.650631483984),
     3: (-37.7777637471456, 144.772303608804),
     4: (-37.5792063872414, 144.72816450831)}


def get_min_dis2(data: dict, b: tuple):
    min_dist = float('inf')
    for k in data.keys():
        lat1, lon1 = data[k]
        lat2, lon2 = b
        dlat = math.radians(lat2 - lat1)
        dlon = math.radians(lon2 - lon1)
        a = (math.sin(dlat / 2) * math.sin(dlat / 2) + math.cos(math.radians(lat1)) * math.cos(math.radians(lat2)) *
             math.sin(dlon / 2) * math.sin(dlon / 2))
        c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a))
        distance = c * 6378
        if distance < min_dist:
            min_dist = distance
            key = k
    return (key, b, min_dist)


print(get_min_dis2(a, (1, 1)))