当我运行程序时,"无"一直出现

Question

每当我运行我创建的计算器程序时,它工作正常,但文本"无"一直出现,我不知道为什么.这是代码:

def add():
    print 'choose 2 numbers to add'
    a=input('add this')
    b=input('to this')
    print a+b
    return menu()
def sub():
    print 'choose 2 numbers to subract'
    a=input('subract this')
    b=input('from this')
    print b-a
    return menu()
def menu():
    print "hello, Welcome"
    print "these are your options"
    print "1. add"
    print "2. sub"
print menu()
loop=2
def sys():
    while loop==2:
        a=input("please choose")
        if a==1:
            print add()
        elif a==2:
            print sub()
        else:
            return menu(),sys()
print sys()

这是输出:

hello, Welcome
these are your options
1. add
2. sub
None    <-------------------------(this is what I'm talking about)
please choose

---

如果它帮助这里的任何人都是我完成的计算器的代码(当我通过它时看起来很乱,但是当你复制和粘贴它时它会起作用)

def add():
    print 'choose 2 numbers to add'
    a=input('add this')
    b=input('to this')
    print a+b
def sub():
    print 'choose 2 numbers to subract'
    a=input('subract this')
    b=input('from this')
    print b-a
def mul():
    print 'choose 2 numbers to multiply'
    a=input("multiply this")
    b=input("by this")
    print b*a
def div():
    print 'choose what numbers your want to divide'
    a=input('divide this')
    b=input('by this')
    print a/b
def exp():
    print 'choose your number you want to exponentiate'
    a=input('multiply this')
    b=input('by the power of this')
    print a**b
def menu():
    print "hello, Welcome"
    print "these are your options"
    print "1. add"
    print "2. sub"
    print "3. mul"
    print "4. div"
    print "5. expo"
    print "0. to end"
menu()
def sys():
    while True:
        a=input("please choose")
        if a==1:
             add()
             menu()
        elif a==2:
             sub()
             menu()
        elif a==3:
             mul()
             menu()
        elif a==4:
             div()
             menu()
        elif a==5:
             exp()
             menu()
        elif a==0:
            break 
        else:
            return menu(),sys()
sys()

Answer 1

那是因为函数menu()没有返回任何东西,默认情况下python函数返回None

>>> def func():pass
>>> print func()  #use `print` only if you want to print the returned value
None

只需使用:

menu() #no need of print as you're already printing inside the function body.

从和中sys()删除后的新版本.而不是在每个函数内部使用,只需在函数结尾处调用函数.return menu()add()sub()return menu()menu()while loop

def sys():
    while True:
        a = input("please choose")
        if a == 1:
            add()    # call add(), no need of print as you're printing inside add() itself
        elif a==2: 
            sub()  
        menu()       # call menu() at the end of the loop

while loop==2实际上loop==2首先评估表达式,如果是,True那么while loop继续其他的立即中断.在你的情况下,因为你没有改变loop变量的值,所以你可以简单地使用while True.

>>> loop = 2
>>> loop == 2
True

相关:关于"同时真实"的基本问题