Question

I am trying to convert hex value to float using (Python 2.7) the following method:

def hex2float(x):
    y = 0
    z = x.decode('hex') 
    try:
        y = struct.unpack('!f', z)[0]
    except:
        print sys.exc_info()[1]    
    print 'z = ' + z 
    print 'y = %s' % (y) 
    print 'x = ' + x
    return

def foo28():
    x = '615885'   #8.9398e-039
    hex2float(x)

The output is as follows:

unpack requires a string argument of length 4
z = aXà
y = 0
x = 615885

I notice that I get the exception message for really small values. Is there a proper way to convert hex values to floating values for such cases.

Answer 1

You need four bytes to unpack, so prepend null bytes if necessary:

z = x.decode('hex') 
z = '\0' * (4 - len(z)) + z

Normally str.decode only outputs as much bytes as necessary to represent the value, so that’s why you only see it happen for small values.

This works perfectly:

>>> z = '615885'.decode("hex")
>>> z = '\0' * (4 - len(z)) + z
>>> struct.unpack('!f', z)
(8.939797951825212e-39,)

If you’re going to do doubles as well this solution still works, just change 4 to 8.