Python 脚本返回意外的“无"函数执行后 [英] Python Script returns unintended "None" after execution of a function

问题描述

背景:Python 初学者；搜索了这个问题，但我找到的答案更多是关于什么"的比为什么";

我打算做什么:创建一个函数，该函数从用户输入测试分数并根据等级量表/曲线输出字母等级；代码如下:

score = input("请输入考试成绩:")分数 = 整数(分数)def letter_grade(分数):如果 90 <= 分数 <= 100:打印(A")elif 80 <= 分数 <= 89:打印(B")elif 70 <= 分数 <= 79:打印(C")elif 60 <= 分数 <= 69:打印(D")elif分数<60:打印(F")打印(字母等级(分数))

这在执行时返回:

请输入考试成绩:45F没有任何

None 不是故意的.我发现如果我使用 letter_grade(score) 而不是 print (letter_grade(score)) ， None 不再出现.>

我能找到的最接近的答案说的是python 中的函数返回 None ，除非明确指示否则".但我确实在最后一行调用了一个函数，所以我在这里有点困惑.

所以我想我的问题是:是什么导致了 None 的消失?我确信这是非常基本的东西，但我找不到任何解释幕后"的答案.机制.因此，如果有人可以对此有所了解，我将不胜感激.谢谢！

解决方案

在python中，函数的默认返回值是None.

<预><代码>>>>def func():pass>>>print func() #print 或 print() 打印返回值没有任何>>>func() #remove 打印，返回值不打印.>>>

所以，只需使用:

letter_grade(score) #去除打印

另一种选择是用 return 替换所有打印:

def letter_grade(score):如果 90 <= 分数 <= 100:返回A"elif 80 <= 分数 <= 89:返回B"elif 70 <= 分数 <= 79:返回C"elif 60 <= 分数 <= 69:返回D"elif分数<60:返回F"别的:#如果不满足所有其他条件则返回返回无效标记"

现在使用print():

<预><代码>>>>打印(字母等级(91))一种>>>打印(字母等级(45))F>>>打印(字母等级(75))C>>>打印 letter_grade(1000)无效标记

Background: total beginner to Python; searched about this question but the answer I found was more about "what" than "why";

What I intended to do: Creating a function that takes test score input from the user and output letter grades according to a grade scale/curve; Here is the code:

score = input("Please enter test score: ")
score = int(score)

def letter_grade(score):
    if 90 <= score <= 100:
        print ("A")
    elif 80 <= score <= 89:
        print ("B")
    elif 70 <= score <= 79:
        print("C")
    elif 60 <= score <= 69:
        print("D")
    elif score < 60:
        print("F")

print (letter_grade(score))

This, when executed, returns:

Please enter test score: 45
F
None

The None is not intended. And I found that if I use letter_grade(score) instead of print (letter_grade(score)) , the None no longer appears.

The closest answer I was able to find said something like "Functions in python return None unless explicitly instructed to do otherwise". But I did call a function at the last line, so I'm a bit confused here.

So I guess my question would be: what caused the disappearance of None? I am sure this is pretty basic stuff, but I wasn't able to find any answer that explains the "behind-the-stage" mechanism. So I'm grateful if someone could throw some light on this. Thank you!

解决方案

In python the default return value of a function is None.

>>> def func():pass
>>> print func()     #print or print() prints the return Value
None
>>> func()           #remove print and the returned value is not printed. 
>>>

So, just use:

letter_grade(score) #remove the print

Another alternative is to replace all prints with return:

def letter_grade(score):
    if 90 <= score <= 100:
        return "A"
    elif 80 <= score <= 89:
        return "B"
    elif 70 <= score <= 79:
        return  "C"
    elif 60 <= score <= 69:
        return "D"
    elif score < 60:
        return "F"
    else:
        #This is returned if all other conditions aren't satisfied
        return "Invalid Marks"

Now use print():

>>> print(letter_grade(91))
A
>>> print(letter_grade(45))
F
>>> print(letter_grade(75))
C
>>> print letter_grade(1000)
Invalid Marks

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