>>> a=15
>>> b=25
>>> eval(a+b)               #൱ȥַʽ˵˫
40
>>> eval(a+b)                 #ͲַͣΪʾ
Traceback (most recent call last)
   File <pyshell#4>line1in<module>
       eval(a+b)
TypeErroreval() arg 1 must be a string,bytes or code object