from functools import reduce

def str2num(s):
#考虑输入不全是数字的情况，应用try..except..
try:
    return eval(s)  #eval()：将字符串string对象转化为有效的表达式
                      #参与求值运算返回计算结果        
except Exception as e: 
    print('Error:This is a invalid expression of number!')

def calc(exp):
ss = exp.split('+')
ns = map(str2num, ss)
try:
    return reduce(lambda acc, x: acc + x, ns)
except Exception as e:
    return 'invalid result'

def main():
r = calc('100 + 200 + 345')
print('100 + 200 + 345 =', r)
r = calc('99 + 88 + 7.6')
print('99 + 88 + 7.6 =', r)
r = calc('99 + 88 + 2.a')
print('99 + 88 + 2.a =', r)
r = calc('99 + a8.3 + 2.f')
print('99 + a8.3 + 2.f =', r)

main()