A class I wrote as an utility when dealing with byte data. It can create bytes out of nibbles, words out of bytes and break words into a byte lists. All inputs are validated and the exceptions are raised.

hexTools.py

class HexTools():
    """
    Performs simple operations on byte or word data
    """
    MAX_VALUE_FOR_BYTE = 0xFF
    MAX_VALUE_FOR_WORD = 0xFFFF
    MAX_VALUE_FOR_NIBBLE = 0x0F

    @staticmethod
    def getHighNibbleFromByte(byte=0):
        if byte > HexTools.MAX_VALUE_FOR_BYTE:
            raise ValueError("value must be an integer less than 256")
        if not isinstance(byte, int):
            raise ValueError("value must be an integer less than 256")
        byte = (byte >> 4) & 0x0F
        #print hex(byte)
        return byte

    @staticmethod
    def getLowNibbleFromByte(byte=0):
        if byte > HexTools.MAX_VALUE_FOR_BYTE:
            raise ValueError("value must be an integer less than 256")
        if not isinstance(byte, int):
            raise ValueError("value must be an integer less than 256")
        byte = (byte) & 0x0F
        #print hex(byte)
        return byte

    @staticmethod
    def getWordFromBytes(msb=0x00, lsb=0x00):
        """
        raises TypeError
        """
        word = msb << 8
        word += lsb
        return word

    @staticmethod
    def getByteFromNibbles(ms, ls):
        """
        raises TypeError
        """
        if ms > HexTools.MAX_VALUE_FOR_NIBBLE or ls > HexTools.MAX_VALUE_FOR_NIBBLE:
            raise TypeError
        byte = (ms << 4) & 0xF0
        byte |= ls
        return byte

    @staticmethod
    def getByteListFromWord(word):
        """
        raises TypeError
        """
        if word > HexTools.MAX_VALUE_FOR_WORD:
            raise TypeError
        highByte = (word >> 8) & 0x00FF
        lowByte = (word) & 0x00FF
        byteList = []
        byteList.append(highByte)
        byteList.append(lowByte)
        return byteList