Python convert hex to float

Question

How to convert the following hex string to float (single precision 32-bit) in python?

"41973333" -> 1.88999996185302734375E1

"41995C29" -> 1.91700000762939453125E1

"470FC614" -> 3.6806078125E4

Thanks

Answer 1

I'm guessing this question relates to this one and you are working with 4 bytes rather than 8 hex digits.

"\x41\x91\x33\x33" is a 4 byte string even though it looks like 16

>>> len("\x41\x91\x33\x33")
4
>>> import struct  
>>> struct.unpack(">fff","\x41\x97\x33\x33\x41\x99\x5C\x29\x47\x0F\xC6\x14")
(18.899999618530273, 19.170000076293945, 36806.078125)

If you do need to deal with the string of hexdigits rather than the actual bytes, you can use struct.pack to convert it, like this

>>> for hx in ["41973333","41995C29","470FC614"]:
...     print(struct.unpack(">f",struct.pack(">i",int(hx,16)))[0])
... 
18.8999996185
19.1700000763
36806.078125

Answer 2

I recommend using the ctypes module which basically lets you work with low level data types. In your case you could say

from ctypes import *

def convert(s):
    i = int(s, 16)                   # convert from hex to a Python int
    cp = pointer(c_int(i))           # make this into a c integer
    fp = cast(cp, POINTER(c_float))  # cast the int pointer to a float pointer
    return fp.contents.value         # dereference the pointer, get the float

print convert("41973333")    # returns 1.88999996185302734375E1

print convert("41995C29")    # returns 1.91700000762939453125E1

print convert("470FC614")    # returns 3.6806078125E4

I believe that the ctypes module makes sense here, because you're essentially asking how to perform low-level bit casting. Your question is basically, how do I tell Python to take some data and interpret that data as if those exact same bits were a different data type?

In C if you had an int and wanted to interpret its bits as a float, you'd do roughly the same thing, taking a pointer and then casting and dereferencing it:

int i = 0x41973333;
float f = *((float*)&i);

and that's exactly what the Python code using the ctypes library is doing in my example.

Answer 3

Slice up the hex strings into 2-character chunks (bytes), make each chunk into the right byte with int formatting, struct.unpack when done. I.e.:

import struct 

testcases = {
"41973333": 1.88999996185302734375E1,
"41995C29": 1.91700000762939453125E1,
"470FC614": 3.6806078125E4,
}

def hex2float(s):
    bins = ''.join(chr(int(s[x:x+2], 16)) for x in range(0, len(s), 2))
    return struct.unpack('>f', bins)[0]

for s in testcases:
  print hex2float(s), testcases[s]

emitting, as desired:

18.8999996185 18.8999996185
19.1700000763 19.1700000763
36806.078125 36806.078125

Answer 4

>>> struct.unpack('!f', '41973333'.decode('hex'))[0]
18.899999618530273
>>> struct.unpack('!f', '41995C29'.decode('hex'))[0]
19.170000076293945
>>> struct.unpack('!f', '470FC614'.decode('hex'))[0]
36806.078125

Update: see comment on how to do this in Python 3.