The question

"How can I convert a string to a hex value?"

is often asked, but it's not quite the right question. Better would be

"How can I convert a hex string to an integer value?"

The reason is, an integer (or char or long) value is stored in binary fashion in the computer.

"6A" = 01101010

It is only in human representation (in a character string) that a value is expressed in one notation or another

"01101010b"   binary
"0x6A"        hexadecimal
"106"         decimal
"'j'"         character

all represent the same value in different ways.

But in answer to the question, how to convert a hex string to an int

char hex[] = "6A";                          // here is the hex string
int num = (int)strtol(hex, NULL, 16);       // number base 16
printf("%c\n", num);                        // print it as a char
printf("%d\n", num);                        // print it as decimal
printf("%X\n", num);                        // print it back as hex

Output:

j
106
6A

The hex() function converts a specified integer number into a hexadecimal string representation.

Use int(x, base) with 16 as base to convert the string x to an integer. Call hex(number) with the integer as number to convert it to hexadecimal.

hex_string = "0xAA"

"0x" also required

an_integer = int(hex_string, 16)

hex_value = hex(an_integer)

print(hex_value)

Output

0xaa

Try this:

hex_str = "0xAD4"
hex_int = int(hex_str, 16)
new_int = hex_int + 0x200
print hex(new_int)

If you don't like the 0x in the beginning, replace the last line with

print hex(new_int)[2:]