How to convert "str" to "int"

[colt]

Could it be an error similar to this one ?

>>> rom_bytes = b'hello world'
>>> import struct
>>> struct.unpack("I", rom_bytes[4])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: a bytes-like object is required, not 'int'
>>> struct.unpack("I", rom_bytes[:4])
(1819043176,)

I don't understand why would be, since is a different error message.

Anyway, I don't know why, but now the code is working on python 3.2, since it converts the values to int. However in 2.6 where the type is 'str', first I receive

self.data_bytes = rom_bytes [HEADER_SIZE:HEADER_SIZE + (16 + KB_SIZE *(self.num_prg_blocks))]
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Then adding a int before with:

self.data_bytes = rom_bytes [HEADER_SIZE:HEADER_SIZE + (16 + KB_SIZE * int (self.num_prg_blocks))]

I receive:

ValueError: invalid literal for int() with base 10: '\x02'

finally doing this:

self.temp = self.num_prg_blocks
		struct.unpack ("I", self.temp)
        self.data_bytes = rom_bytes [HEADER_SIZE:HEADER_SIZE + (16 + KB_SIZE * self.temp)] 

makes me receive on python 2.6

struct.error: unpack requires a string argument of length 4

and on python 3.2

TypeError: 'int' does not support the buffer interface

What shall be my next step?