Join two hex integers in Python

Question

I am only new to Python, and currently reading 2 bytes from a digital compass using I2C on the Raspberry Pi. The MSB and LSB values are being stored in an array e.g.
a = [0x07, 0xFF]

I would like to join these two bytes into one variable such as
b == 0x07FF

How would I go about doing this?
I thought it would be as easy as multiplying the MSB by 256 and adding it to the LSB but I keep getting "IndexError: list index out of range"
Any help on this would be appreciated :)

My code is:

import smbus
import time
bus = smbus.SMBus(1)
addr = 0x1E
bus.write_byte_data(addr, 0x00, 0x70)
bus.write_byte_data(addr, 0x01, 0xA0)
bus.write_byte_data(addr, 0x02, 0x00)
time.sleep(0.006)
for i in range(0,10):
 x = bus.read_i2c_block_data(addr,0x03,2)
 y = bus.read_i2c_block_data(addr,0x07,2)
 z = bus.read_i2c_block_data(addr,0x05,2)
 xval = 256*x[2]+x[1]
 print x, y, z
 print xval
 time.sleep(1)
print 'exiting...'

The error I get is:

Traceback (most recent call last):
 File "compass2.py", line 18, in <module>
 xval = 256*x[2]+x[1]
IndexError: list index out of range

• eyllanesc: Python indexes start at 0. Change xval = 256*x[2]+x[1] to xval = 256*x[1]+x[0]

Answer 1

As noted in a comment, in Python, indexing begins at 0, not at 1. In your code, start at x[0], not x[1].

To merge two integers from 0 to 255:

Using bitwise

Credit: this answer

MAXINT = 255 # Must be a power-of-two minus one
NUM_BITS = MAXINT.bit_length()
def merge(a, b):
 c = (a << NUM_BITS) | b
 return c
def split(c):
 a = (c >> NUM_BITS) & MAXINT
 b = c & MAXINT
 return a, b
# Test:
EXPECTED_MAX_NUM_BITS = NUM_BITS * 2
for a in range(MAXINT + 1):
 for b in range(MAXINT + 1):
 c = merge(a, b)
 assert c.bit_length() <= EXPECTED_MAX_NUM_BITS
 assert (a, b) == split(c)

Using arithmetic

This technique works but is not preferred.

MAXINT_PLUS1 = 256
def merge(a, b):
 c = MAXINT_PLUS1 * a + b
 return c
def split(c):
 a, b = divmod(c, MAXINT_PLUS1)
 return a, b
# Test:
EXPECTED_MAX_NUM_BITS = (MAXINT_PLUS1 - 1).bit_length() * 2
for a in range(MAXINT_PLUS1):
 for b in range(MAXINT_PLUS1):
 c = merge(a, b)
 assert c.bit_length() <= EXPECTED_MAX_NUM_BITS
 assert (a, b) == split(c)