Question

I am playing around with the bytearray type in python 2.7.3 and noticed this weird behaviour:

Passing an integer argument to its constructor creates a bytearray of size equal to the integer filled with zero bytes:

>>> s = bytearray(15)
>>> s
bytearray(b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')

However, if I attempt to create a bytearray with bytes set manually by me, the constructor seems to ignore/skip some of the bytes I create:

>>> s = bytearray(b'\x34\x78\x98\xFF\xFF')
>>> s
bytearray(b'4x\x98\xff\xff')

Why is this happening? Is this a bug? Or am I missing something? The python documentation is not very enlightening.

Answer 1

Nothing strange is happening, all the bytes are still there. Relax! 🙂

The confusion stems from the fact that the representation is slightly optimized: bytes whose values represent printable characters are printed as those characters. This makes them take just a single character in the output rather than four.

It’s doing just what it says:

• \x34 is the character 4, which is why the buffer starts with a ‘4‘
• \x78 is the character x, which is why there’s an ‘x‘ after the ‘4‘
• \x98\xff\xff are all kept as-is.

Note that len(s) prints 5.

Epic graphical representation:

    +---+---+------+------+------+
s = | 4 | x | \x98 | \xff | \xff |
    +---+---+------+------+------+