為什么下面三個函數中的最后兩個會出現異常而第一個不會？

def func(lst=[]):
    def inner_func():
        lst.append(10)
        return lst
    return inner_func()


def func2(num=10):
    def inner_func():
        num += 1
        return num
    return inner_func()


def func3(lst=[]):
    def inner_func():
        lst += [10]
        return lst
    return inner_func()


if __name__ == '__main__':
    print('func:', func())

    try:
        print('func2:', func2())
    except UnboundLocalError as e:
        print('func2:', e)

    try:
        print('func3:', func3())
    except UnboundLocalError as e:
        print('func3:', e)

執(zhí)行結果：

func: [10]
func2: local variable 'num' referenced before assignment
func3: local variable 'lst' referenced before assignment