Single precision big endian float values to Python's float (double precision, big endian)

I need to receive hex encoded single precision big endian float values coming from an Arduino over a serial line (RS-232). How do convert them to Python's float which are big endians with double precision?

The Arduino send something like "8192323E" and in Python I would like to have 0.174387. I found "Convert hex to开发者_开发问答 float" but it seems that all of them don't work for single precision floats.

From the linked page, this looks promising:

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

But it still doesn't work with my single precision floats.

In Java (Processing) I've been able to do that:

float decodeFloat(String inString) {
  byte [] inData = new byte[4];

  inString = inString.substring(2, 10); // discard the leading "f:"
  inData[0] = (byte) unhex(inString.substring(0, 2));
  inData[1] = (byte) unhex(inString.substring(2, 4));
  inData[2] = (byte) unhex(inString.substring(4, 6));
  inData[3] = (byte) unhex(inString.substring(6, 8));

  int intbits = (inData[3] << 24) | ((inData[2] & 0xff) << 16) | ((inData[1] & 0xff) << 8) | (inData[0] & 0xff);
  //unhex(inString.substring(0, 8));
  return Float.intBitsToFloat(intbits);
}

For your reference, this is the C code running on the Arduino implementing the hex encoding.

void serialFloatPrint(float f) {
  byte * b = (byte *) &f;
  Serial.print("f:");
  for(int i=0; i<4; i++) {

    byte b1 = (b[i] >> 4) & 0x0f;
    byte b2 = (b[i] & 0x0f);

    char c1 = (b1 < 10) ? ('0' + b1) : 'A' + b1 - 10;
    char c2 = (b2 < 10) ? ('0' + b2) : 'A' + b2 - 10;

    Serial.print(c1);
    Serial.print(c2);
  }
}

Building on Ignacio Vazquez-Abrams's answer,

import binascii
import struct

text='8192323E'
print(struct.unpack('<f',binascii.unhexlify(text))[0])
# 0.17438699305057526

>>> struct.unpack('<f', '\x81\x92\x32\x3e')
(0.17438699305057526,)

Indeed, you need to invert the byte order. Look:

>>> convert('8192323E')
-5.370402375965945e-38
>>> convert('3E329281')
0.17438699305057526

It looks like the data sequence in the hex string values has the lower-order bytes first (little-endian?). At least that's the way your Java code -- which you say works -- treats it:

  inData[0] = (byte) unhex(inString.substring(0, 2));  // gets _first_ two bytes
  inData[1] = (byte) unhex(inString.substring(2, 4));  // and the next two
  inData[2] = (byte) unhex(inString.substring(4, 6));  // etc...
  inData[3] = (byte) unhex(inString.substring(6, 8));

  // this shifts the bytes at the end of the hex string more the the leading ones
  int intbits = (inData[3] << 24) | ((inData[2] & 0xff) << 16) | 
                ((inData[1] & 0xff) << 8) | (inData[0] & 0xff);

Python's int() function expects the higher-order bytes to appear first when passed a (hex or decimal) string. So you should be able fix your convert() function by adding something like:

def convert(s):
    s = ''.join(s[i:i+2] for i in range(8,-2,-2)) # put low byte pairs first
    i = int(s, 16)                   # convert from hex to a Python int
      ...

near the beginning to fix it.

Update

Or you could fix the serialFloatPrint() function running on the Arduino to do the hex encoding properly for a little-endian architecture.