stack menu driven program

s=[]
def isEmpty():
       return s == []
def pus(i):
    s.append(i)
    print(s)
    return s
def po():
    return q.pop()
def size():
    return len(s)
def pr():
        return s
 
while True:
    print('Press a for push')
    print('Press b for pop')
    print('press c for the size')
    print('Press d for display')
    print('Press e for quit')
   
    op = input('What would you like to do')
    if op == 'a':
        n=int(input("enter a number to push"))
        q=pus(n)
    elif op == 'b':
        if isEmpty():
            print('Stack is empty.')
        else:
            print('Deleted value: ',po())
    elif op=='c':
        print(size())
    elif op=='d':
        print(pr())     
    elif op == 'e':
        break