Why does a Python bytearray work with value >= 256

Question

The Python documentation for bytearray states:

The bytearray type is a mutable sequence of integers in the range 0 <= x < 256.

However the following code suggests values can be >= 256. I store a 9 bit binary number which has a maximum value of: 2^9-1 = 512-1 = 511

ba = bytes([0b111111111])
print '%s' % (ba)

The 9 bit binary number is printed as decimal 511:

[511]

I don't know what the intended behavior is, but I assumed the most significant bit(s) would be dropped to give an 8 bit number.

What version of python are you using? The code in your example gives a `ValueError` in 3.3.1 — Patrick Haugh, Oct 24 '16 at 14:57
@PatrickHaugh I would assume it's 2.x. Since the `print` statement — MooingRawr, Oct 24 '16 at 14:58
Huh...... it first linked to 2.7 for me my apologys (might have looked at the wrong tab....) — MooingRawr, Oct 24 '16 at 15:00

PM 2Ring · Accepted Answer · 2016-10-24T15:03:23.503

11

You aren't actually creating a bytearray or a bytes object, you're just creating a string containing '[511]', since bytes in Python 2 is just a synonym for str. In Python 3, you would get an error message:

ValueError: byte must be in range(0, 256)

The following code works in Python 2 or Python 3; note that I'm passing an 8 bit number, so it's in range.

ba = bytearray([0b11111111])
print(repr(ba))

output

bytearray(b'\xff')

edited Oct 24 '16 at 15:03

answered Oct 24 '16 at 15:01

PM 2Ring

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Also if you type `help(bytes)` in Python 2 it shows the `str` help page. – Adam Van Prooyen Oct 24 '16 at 15:04
So according to the error message `ValueError: byte must be in range(0, 256)`, the value 256 is allowed as well as value 0. This makes 257 possible values in a byte. Python always impresses me... – Rusty Gear Apr 27 '22 at 16:07
@RustyGear No, `range(0, 256)` produces 256 numbers, from 0 up to 255. Similarly, `"abcdef"[0:3]` is the string `"abc"`. Almost everything in Python uses that convention. – PM 2Ring Apr 27 '22 at 16:41

score 1 · Answer 2 · answered Sep 20 '20 at 18:40

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code:

a = 511
byte = a.to_bytes(byte length goes here, 'little')

to decode:

a = int.from_bytes(byte, 'little')

answered Sep 20 '20 at 18:40

Yash

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thanks,it's work.and byte = (-511).to_bytes(2, 'little', signed=True) can Solve signed type – thouger Feb 11 '22 at 03:37

Why does a Python bytearray work with value >= 256

2 Answers2