32位浮点单精度hex 字符串转 十进制 小数

新增粉尘仪检测设备调试中需要对 hex 32位浮点数 转10 进制小数（保留三位小数）

def ieee_transfer(hex_str):
    """
    :param hex_str: hex string like '4145851F'
    :return: float type
    """
    ret = ''
    for i in range(0, len(hex_str), 2):
        a = hex_str[i: i + 2]
        intItem = int(a, 16)
        binary_str = bin(intItem)[2:len(bin(intItem))]
        binary_str = "%(binary)08d" % {'binary': int(binary_str)}
        ret = ret + binary_str
    sign_head = int(ret[0])
    exponent = int(ret[1:9], 2)
    real_num = ret[9:len(ret) - 1]
    m = float()
    for i in range(1, len(real_num) - 1, 1):
        if real_num[i - 1] == "1":
            m = m + 0.5 ** i
    val = (-1) ** sign_head * 2 ** (exponent - 127) * (1 + m)
    return round(val, 3)