Hex en ligne en texte Decoder

In Obj-C I used to convert an unsigned integer n to a hex string with

 NSString *st = [NSString stringWithFormat:@"%2X", n];

I tried for a long time to translate this into Swift language, but unsuccessfully.

There is the hex string A = 'FB01A4....' with length more than 83000 symbols.

Operation A.decode('hex') give wrong result.

How to decode correct?

Here is example. When I try to use second A from example (that is commented), the print result looks more correct than first.

I am trying to use str.encode() but I get

>>> "hello".encode(hex)Traceback (most recent call last):  File "<stdin>", line 1, in <module>TypeError: must be string, not builtin_function_or_method

I have tried a bunch of variations and they seem to all work in Python 2.5.2, so what do I need to do to get them to work in Python 3.1?

So, I've found various methods to convert a byteArray into a hexadecimal string... but one puzzles me :s

String s = "";for(byte b: byteArray) {    s = s.concat(String.format("%02x", b));}

Here, they format each byte in a byte array to hex using the format option %02x but why specify a width of 2 and why pad with zeros?

I have few hex numbers that the last digit mean the length of the string after. for example
2E3F00004Sara
41590000AFavhilmnfr
Except from:
515900016Mickael Barrie Linwood
Which is 22 not 16..
any idea how to get it from the number? Thank you