>>> a = 0xA
>>> b = 0xa
>>> a + b
20
>>> hex(a + b)
'0x14'

The int builtin has an optional base parameter, which you can use to read hex values.

with open(r'\\network\loc\build_ver.txt','r+') as f:
    value = int(f.read(), 16)
    f.seek(0)
    f.write(hex(value + 1))

You can use hex for base 16 output or str for base 10 output.

>>> val = int("9a", base=16)
>>> val
154
>>> hex(val + 1)
'0x9b'
>>> str(val + 1)
'155'

It's also worth noting that you should either validate the input or have a try block somewhere:

>>> int("g", 16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 16: 'g'

Thanks to @Peri461

basehex = input()
sechex = input()

basehexin = int(basehex, 16)
sechexin = int(sechex, 16)



sum = basehexin - sechexin



print(hex(sum))

This code will do it, by converting the hexadecimals to decimals, subtracting them, then converting(representing) them to hexadecimals again.

To represent as hexadecimal, use binascii.hexlify:

>>> import binascii
>>> binascii.hexlify('\x03\x2F\x00\x00\x02\x12\x01\x47')
'032f000002120147'
>>> ' '.join(binascii.hexlify(ch) for ch in '\x03\x2F\x00\x00\x02\x12\x01\x47')
'03 2f 00 00 02 12 01 47'

To calculate checksum, iterate each bytes:

data = '\x03\x2F\x00\x00\x02\x12\x01\x47'

checksum = 0
for ch in data:
    checksum += ord(ch)  # ord('\x03`) -> 3

or using sum and map:

checksum = sum(map(ord, data))