利用map和reduce编写一个str2float函数，把字符串'123.456'转换成浮点数123.456：

 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-

 from functools import reduce

 CHAR_TO_INT = {
     '': 0,
     '': 1,
     '': 2,
     '': 3,
     '': 4,
     '': 5,
     '': 6,
     '': 7,
     '': 8,
     '': 9
 }

 def str2int(s):
     ints = map(lambda ch: CHAR_TO_INT[ch], s)
     return reduce(lambda x, y: x * 10 + y, ints)

 print(str2int(''))
 print(str2int(''))
 print(str2int(''))

 CHAR_TO_FLOAT = {
     '': 0,
     '': 1,
     '': 2,
     '': 3,
     '': 4,
     '': 5,
     '': 6,
     '': 7,
     '': 8,
     '': 9,
     '.': -1
 }

 def str2float(s):
     nums = map(lambda ch: CHAR_TO_FLOAT[ch], s)
     point = 0
     def to_float(f, n):
         nonlocal point
         if n == -1:
             point = 1
             return f
         if point == 0:
             return f * 10 + n
         else:
             point = point * 10
             return f + n / point
     return reduce(to_float, nums, 0.0)

 print(str2float(''))
 print(str2float('123.456'))
 print(str2float('123.45600'))
 print(str2float('0.1234'))
 print(str2float('.1234'))
 print(str2float('120.0034'))