How can I convert a bytestring array in a hex string in Python 3.4?

The int.from_bytes method will help.

int.from_bytes(bytes, byteorder, *, signed=False) -> int

Return the integer represented by the given array of bytes.

...

If byteorder is 'little', the most significant byte is at the end of the byte array.

It will convert bytes to integer:

In [1]: int.from_bytes(b'\xb5\x1a', 'little') # 'little' for little-endian order
Out[1]: 6837

Then you can use hex

In [2]: hex(int.from_bytes(b'\xb5\x1a', 'little'))
Out[2]: '0x1ab5'

or format(..., '#x')

In [3]: format(int.from_bytes(b'\xb5\x1a', 'little'), '#x')
Out[3]: '0x1ab5'

to get hexadecimal representation.

Other solutions include base64.b16encode

In [4]: import base64

In [5]: '0x' + base64.b16encode(b'\xb5\x1a'[::-1]).decode('ascii')
Out[5]: '0x1AB5'

and binascii.hexlify:

In [24]: '0x' + binascii.hexlify(b'\xb5\x1a'[::-1]).decode('ascii')
Out[24]: '0x1ab5'

Some timings for bytestr = b'\xb5\x1a':

In [32]: %timeit hex(int.from_bytes(bytestr, 'little'))
1000000 loops, best of 3: 267 ns per loop

In [33]: %timeit format(int.from_bytes(bytestr, 'little'), '#x')
1000000 loops, best of 3: 465 ns per loop

In [34]: %timeit '0x' + base64.b16encode(bytestr[::-1]).decode('ascii')
1000000 loops, best of 3: 746 ns per loop

In [35]: %timeit '0x' + binascii.hexlify(bytestr[::-1]).decode('ascii')
1000000 loops, best of 3: 545 ns per loop

For bytestr = b'\xb5\x1a' * 100:

In [37]: %timeit hex(int.from_bytes(bytestr, 'little'))
1000000 loops, best of 3: 992 ns per loop

In [38]: %timeit format(int.from_bytes(bytestr, 'little'), '#x')
1000000 loops, best of 3: 1.2 µs per loop

In [39]: %timeit '0x' + base64.b16encode(bytestr[::-1]).decode('ascii')
1000000 loops, best of 3: 1.38 µs per loop


In [40]: %timeit '0x' + binascii.hexlify(bytestr[::-1]).decode('ascii')
1000000 loops, best of 3: 983 ns per loop

int.from_bytes is (predictably) fast for small byte strings, binascii.hexlify is fast for longer byte strings.