Convert hex string to int in Python

Without the 0x prefix, you need to specify the base explicitly, otherwise there's no way to tell:

x = int("deadbeef", 16)

With the 0x prefix, Python can distinguish hex and decimal automatically.

>>> print(int("0xdeadbeef", 0))
3735928559
>>> print(int("10", 0))
10

(You must specify 0 as the base in order to invoke this prefix-guessing behavior; if you omit the second parameter int() will assume base-10.)

int(hexstring, 16) does the trick, and works with and without the 0x prefix:

>>> int("a", 16)
10
>>> int("0xa", 16)
10

Convert hex string to int in Python

I may have it as "0xffff" or just "ffff".

To convert a string to an int, pass the string to int along with the base you are converting from.

Both strings will suffice for conversion in this way:

>>> string_1 = "0xffff"
>>> string_2 = "ffff"
>>> int(string_1, 16)
65535
>>> int(string_2, 16)
65535

Letting `int` infer

If you pass 0 as the base, int will infer the base from the prefix in the string.

>>> int(string_1, 0)
65535

Without the hexadecimal prefix, 0x, int does not have enough information with which to guess:

>>> int(string_2, 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 0: 'ffff'

literals:

If you're typing into source code or an interpreter, Python will make the conversion for you:

>>> integer = 0xffff
>>> integer
65535

This won't work with ffff because Python will think you're trying to write a legitimate Python name instead:

>>> integer = ffff
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ffff' is not defined

Python numbers start with a numeric character, while Python names cannot start with a numeric character.

Convert hex string to int in Python