Byte array in Python only accepts unsigned integers

Question

I'm working on audio codecs in Python (yikes) using byte arrays to edit individual byte data from an audio file.

I have a certain encryption in mind that requires me to perform bit wise operations on single bytes stored in the byte-array.

One of those operations is the ~ operator (bitwise NOT) which essentially reverses the bit (b'0001 becomes b'1110).

The problem is when you reference a single element of a byte array, it returns an int (does Python by default consider untyped 8 bit data integers?). Integers in Python are by default signed (I don't think unsigned integers even exist in Python).

When you try to perform bit-wise NOT on a byte in the byte-array, you get the following error:

>>> array[0] = ~array[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: byte must be in range(0, 256)

This is because it expects an unsigned int between 0-255. How do I convert a signed int into an unsigned int such that the bits used to represent both values remain the same?

Cheers

I think numpy should provide unsigned ints. I would try np.array(array).astype(np.uint8) — SpghttCd
– SpghttCd, Commented May 2, 2018 at 22:31
"I have a certain encryption in mind" - are you writing your own crypto? Please don't write your own crypto. — user2357112
– user2357112, Commented May 2, 2018 at 22:39
Hahah no... I wouldn't ever try that... trying to emulate something else — jmkmay
– jmkmay, Commented May 2, 2018 at 22:40

fferri · Accepted Answer · 2018-05-02 23:19:32Z

2

Use a different operation for bit flipping.

E.g.:¹

array[i] = 255 - array[i]

or also:

array[i] = 255 ^ array[i]

will flip all (i.e.: 8) bits.

¹ the math behind this can be worked out from two's complement wikipedia page.

edited May 2, 2018 at 23:19

answered May 2, 2018 at 22:31

fferri

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6 Comments

jmkmay Over a year ago

Wouldn't it be 256 - array[0] ? Appreciate the quick response. Stuff like this makes me feel stupid hehe.

user2357112 Over a year ago

@JonathanMay: No. If array[0] is initially 0, then 256 - array[0] is 256, but the desired output is 255.

fferri Over a year ago

@JonathanMay: why don't you try it and see? for i in range(256): print('{:08b} {:08b}'.format(i, 255-i))

fferri Over a year ago

@JonathanMay: actually: all([{'0':'1','1':'0'}[c] for c in f'{i:08b}']==list('{:08b}'.format(255-i)) for i in range(256)) == True

user2357112 Over a year ago

@JonathanMay: ~0 gives -1, but this answer doesn't involve ~.

|

jmkmay · Accepted Answer · 2018-05-02 22:47:33Z

-1

The solution is actually remarkably simple after playing around with a binary calculator a little bit.

Just subtract the magnitude of the SIGNED int from 256, to get the value of the UNSIGNED int with the same binary representation.

So,

-23 signed would be 233 unsigned.

Hope this helps anyone else looking for a solution :)

EDIT: For those saying answer is 255 - array[0]. In this case I'm looking for a way to go from post NOT'd int to its unsigned counter part. So I've already performed the bitwise NOT on the integer, now I'm just getting it back to a form that can be inputted into the byte-array.

So in the end it would look something like this:

tmp = ~array[0]
array[0] = 256 + tmp

or

array[0] = 256 - abs(tmp)

This gets me the correct answer :)

edited May 2, 2018 at 22:47

answered May 2, 2018 at 22:32

jmkmay

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3 Comments

user2357112 Over a year ago

Not 256. 255. Subtracting from 256 changes the range from 0-255 to 1-256.

user2357112 Over a year ago

Or if you're already working with the output of ~, then you need to add it to 256, not subtract.

jmkmay Over a year ago

If you take a number, say -12. In binary, that's 11110100. If you then convert 11110100 into an unsigned int, you get 244. Did I go wrong anywhere there?

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