How to append to bytes in python 3

Question

I have some bytes.

b'\x01\x02\x03'

And an int in range 0..255.

Now I want to append the int to the bytes like this:

b'\x01\x02\x03\x05'

How to do it? There is no append method in bytes. I don't even know how to make the integer become a single byte.

>>> bytes(5)
b'\x00\x00\x00\x00\x00'

score 81 · Accepted Answer · answered Nov 18 '14 at 18:31

81

bytes is immutable. Use bytearray.

xs = bytearray(b'\x01\x02\x03')
xs.append(5)

answered Nov 18 '14 at 18:31

simonzack

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Note that `bytearray` can only store integers in range 0-256. Though in this case OP is only concerned with the same range. :-) – Ashwini Chaudhary Nov 18 '14 at 18:53
40

The `bytearray` has the `.extend` method to add more bytes at once like `xs.extend(b'\x11\x22\x33')`. – pepr Nov 18 '14 at 19:20
17

@Ashwini, (0-255) – Larry Apr 08 '19 at 05:49
14

@AshwiniChaudhary What else would be expected to fit into one byte? – Sherlock70 Feb 04 '20 at 15:28

score 27 · Answer 2 · edited Jul 14 '22 at 15:04

First of all passing an integer(say n) to bytes() simply returns an bytes string of n length with null bytes. So, that's not what you want here:

Either you can do:

>>> bytes([5]) #This will work only for range 0-256.
b'\x05'

Or:

>>> bytes(chr(5), 'ascii')
b'\x05'

As @simonzack already mentioned, bytes are immutable, so to update (or better say re-assign) its value, you need to use the += operator.

>>> s = b'\x01\x02\x03'
>>> s += bytes([5])     #or s = s + bytes([5])
>>> s
b'\x01\x02\x03\x05'

>>> s = b'\x01\x02\x03'
>>> s += bytes(chr(5), 'ascii')   ##or s = s + bytes(chr(5), 'ascii')
>>> s
b'\x01\x02\x03\x05'

Help on bytes():

>>> print(bytes.__doc__)
bytes(iterable_of_ints) -> bytes
bytes(string, encoding[, errors]) -> bytes
bytes(bytes_or_buffer) -> immutable copy of bytes_or_buffer
bytes(int) -> bytes object of size given by the parameter initialized with null bytes
bytes() -> empty bytes object

Construct an immutable array of bytes from:
  - an iterable yielding integers in range(256)
  - a text string encoded using the specified encoding
  - any object implementing the buffer API.
  - an integer

Or go for the mutable bytearray if you need a mutable object and you're only concerned with the integers in range 0-256.

Both solutions can be optimized by using `bytes((5,))` instead of the first one and `(5).to_bytes(1, "little")` for the second one. `int.to_bytes()` can be used to get longer byte sequences in the specified order from an integer of arbitrary size. — Bachsau, Dec 14 '18 at 03:15
+1 for mentioning the += operator, i dont know why .append() only works on integers and doesn't take a bytes or another bytearray object, how infuriating — mgrandi, Jan 25 '21 at 08:19

How to append to bytes in python 3

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