from functools import reduce

def str2float(s):

def char2num(c):
    return {'1':1,'2':2,'3':3,'4':4,'5':5,'6':6,'7':7,'8':8,'9':9}[c]

idx = s.index('.')
s = s.replace('.','')

return reduce(lambda x,y:x*10+y,map(char2num,s))/10**(len(s)-idx)

print('str2float(\'123.456\') =',str2float('123.456'))