How to convert a string representing a binary fraction to a number in Python

The following is a shorter way to express the same algorithm:

def parse_bin(s):
    return int(s[1:], 2) / 2.**(len(s) - 1)

It assumes that the string starts with the dot. If you want something more general, the following will handle both the integer and the fractional parts:

def parse_bin(s):
    t = s.split('.')
    return int(t[0], 2) + int(t[1], 2) / 2.**len(t[1])

For example:

In [56]: parse_bin('10.11')
Out[56]: 2.75

It is reasonable to suppress the point instead of splitting on it, as follows. This bin2float function (unlike parse_bin in previous answer) correctly deals with inputs without points (except for returning an integer instead of a float in that case).

For example, the invocations bin2float('101101'), bin2float('.11101'), andbin2float('101101.11101')` return 45, 0.90625, 45.90625 respectively.

def bin2float (b):
    s, f = b.find('.')+1, int(b.replace('.',''), 2)
    return f/2.**(len(b)-s) if s else f