>>> def func():pass
>>> print func() #print or print() prints the return Value
None
>>> func() #remove print and the returned value is not printed.
>>>
因此，只需使用：
letter_grade(score) #remove the print
另一种选择是将所有打印内容替换为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"
现在使用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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