How to join list of integers into one integer python

I suggest you to switch the values to bytes type in order to concatenate them, them switch back to int type, as follows:

myList = [0x1A, 0xF3, 0x74, 0xA3]
# Conversion to 'bytes'
a = bytes()
for b in myList:
    a += b.to_bytes(1,'big')
# Conversion back to 'int'
a = int.from_bytes(a, 'big')
print(hex(a))  # 0x1af374a3

And you can replace the classic for loop with a generator comprehension passed as parameter to join() method, in order to concatenate the bytes items. It is still readable and a bit shorter, as follows:

myList = [0x1A, 0xF3, 0x74, 0xA3]
a = b''.join(b.to_bytes(1,'big') for b in myList) # Conversion to 'bytes'
a = int.from_bytes(a, 'big')                      # Conversion back to 'int'
print(hex(a))  # 0x1af374a3

Note that if an integer in the input list exceeds 255, then you logically get the error OverflowError: int too big to convert due to b.to_bytes(1,'big'). Of course it can be improved by managing the exception if this case can happen.

I finally may also suggest you the solution using the multiplication by powers of 256, just to show you that it can be achieved in only one-line:

myList = [0x1A, 0xF3, 0x74, 0xA3]
a = sum(nb*(256**i) for i,nb in enumerate(reversed(myList)))
print(hex(a))  # 0x1af374a3

l = [0x1A, 0xF3, 0x74, 0xA3]

merged = ''.join([str(hex(el))[2:] for el in l])

If you need the hex you can get it as:

hex(int(merged, 16)) --> 0x1af374a3

There is nothing extremely inefficient nor ugly in the following code, although it may seem so at a first glance. If you need even more speed you will have to use bitwise operators to jam all ints together.

l = [0x1A, 0xF3, 0x74, 0xA3]
i = int("".join(hex(x)[2:] for x in l), 16)
# And to get the hex representation as requested:
print(hex(i).upper())

There is another way to exploit poor strings in this by using the string formatting like this:

i = int(len(l)*"%x" % tuple(l), 16)
print(hex(i))

mylist =[0x1A, 0xF3, 0x74, 0xA3]

most_significant = str(hex(max(mylist)))
print(most_significant + ''.join([str(el).replace('0x', '') for el in mylist if el != max(mylist)]))

0xf326116163

You can convert each to decimal then back to hex but remove the 0x, the join them together and then add 0x at the beginning. format() is good at doing such conversions.

mylist = [0x1A, 0xF3, 0x74, 0xA3]
result = '0x' + ''.join([format(int(n), 'x')for n in [format(i , 'd') for i in mylist]])

Output:

'0x1af374a3'

I think there is a clean and "pythonic" way to solve this, using the Python bytearray type:

foolist = [0x1A, 0xF3, 0x74, 0xA3]
# convert to bytearray and then to a single integer in one line:
x = int.from_bytes(bytearray(foolist), "big")
print(hex(x))
# '0x1af374a3'

In case the most significant byte in the list is at the end of the list, use the parameter "little" instead of "big" in the int.from_bytes function.

In case your list contains an integer bigger than 255, ValueError exception will be raised:

>>> foolist1 = [0xFF, 255]                                                                                 

>>> bytearray(foolist1)                                                                                    
bytearray(b'\xff\xff')


>>> foolist2 = [0xFF, 256]                                                                                 

>>> bytearray(foolist2)                                                                                    
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: byte must be in range(0, 256)

byte must be in range(0, 256)