Remove a char in string and then use dictionary to do key match

I want write a function to convert string to float by using map and reduce. (no int() here). Here is my code so far:

from functools import reduce

def str2float(s):
    def char2number(s):
        s = s.replace('.', '')
        return {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}[s] #here i try to get pure numbers 

    def find_digt(s):
        return (len(s) - s.find('.') - 1) if ',' in s else 0

    return reduce(lambda x, y: (10 * x + y), (map(char2number, s))) / pow(10, find_digt(s))


print(str2float('1456.124'))

So after this, I get an error:

return {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}[sn]  # [s] is the [key]
KeyError: ''

Which means that the '' is not in the dict. I did some test like:

s = '1234.456'
s = s.replace(',', '')
print('' in s) #True

So the question now is, once

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

the '.' replace by '', it did not clear the '.' in the string.

I'm wondering what is going on here. what is the right way to clear a char in a string since it is immutable.