python 2.7

Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
Type "copyright", "credits" or "license" for more information.

IPython 5.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: from test import f, g, s

In [2]: %timeit s()
The slowest run took 27.60 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.41 µs per loop

In [3]: %timeit f()
The slowest run took 18.01 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 2.34 µs per loop

In [4]: %timeit g()
10000 loops, best of 3: 14.5 µs per loop

Python 3.5

Python 3.5.2 (default, Nov 17 2016, 17:05:23) 
Type "copyright", "credits" or "license" for more information.

IPython 5.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: from test import f, g, s

In [2]: %timeit s()
The slowest run took 6.12 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 984 ns per loop

In [3]: %timeit f()
The slowest run took 11.51 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.73 µs per loop

In [4]: %timeit g()
100000 loops, best of 3: 9.36 µs per loop

test.py

from hashlib import sha256
from binascii import hexlify, unhexlify

a = 'a'

def s():
    global a
    a = sha256(a.encode()).hexdigest()
    return a

def f():
    return hexlify(unhexlify(s())[::-1])

def g():
        """ Switches the endianness of a hex string (in pairs of hex chars) """
        s = s()
        pairList = [s[i:i+2].encode() for i in range(0, len(s), 2)]
        return b''.join(pairList[::-1]).decode()