如何为堆栈实现一些函数？

Question

我正在尝试为堆栈实现这些函数：push（S）、isEmpty（S）、top（S）、nextToTop（S）。它说语法错误。如何测试

S= [1, 5, 13, 21]
def push(S):
    return S

def isEmpty(S):
    if S = []:
        print ("Empty String")
    else:
        print ("String is not empty")
def Top(S):
     if S = []:
         print ("Empty String")
    else:
        return S[0]

def nextToTop(S):
    if S = []:
        print ("String is not empty")
    else:
        return S[1]

print(push(S))
print(isEmpty(S))
print(Top(S))
print(nextToTop(S))

Answer 1

S = []是赋值运算符，其中as==是conditional equal to运算符。因此，检查空列表的if条件应该类似于if S == []

同样的行为也可以通过简单的操作来实现：

if S:   # check if S is not empty
    print "list is not empty"  # S is list, not string
else:
    print "list is empty"