函数返回值为None，无法返回第一个键

def get_first_key():
    my_dict = {"key1": "value1", "key2": "value2", "key3": "value3"}
    return list(my_dict.keys())[0]

first_key = get_first_key()
print(first_key)  # 输出：key1

first_key = None

def set_first_key():
    global first_key
    my_dict = {"key1": "value1", "key2": "value2", "key3": "value3"}
    first_key = list(my_dict.keys())[0]

set_first_key()
print(first_key)  # 输出：key1

def get_first_key():
    my_dict = {}
    try:
        return list(my_dict.keys())[0]
    except IndexError:
        raise Exception("无法返回第一个键")

try:
    first_key = get_first_key()
    print(first_key)
except Exception as e:
    print(e)  # 输出：无法返回第一个键