在Python中从int到Byte的转换不以十六进制表示

Question

I'm having some difficulty in understanding how python converts between int and byte data types and specifically why it isn't consistent with representing it as hexadecimal numbers.

Consider the following where I convert the number 13 into a 2 byte representation:

>>> (13).to_bytes(2, byteorder='big')
b'\x00\r'

Why does it use the character r in the second byte location?

In this case I would have expected it to output:

b'\x00\xD'

Doing the reverse in both cases outputs the correct answer.

>>> int.from_bytes(b'\x00\x0D', byteorder='big')
13
>>> int.from_bytes(b'\x00\r', byteorder='big')
13

And both have the correct number of bytes

>>> len(b'\x00\x0D')
2
>>> len(b'\x00\r')
2

Answer 1

There are some special escape sequences which are so common that they are not explicitly represented with their hex values:

\a <-> \x07  alert
\b <-> \x08  backspace
\t <-> \x09  tab (horizontal)
\n <-> \x0A  new line
\v <-> \x0B  vertical tab
\f <-> \x0C  formfeed
\r <-> \x0D  carriage return
\" <-> \x22  "
\' <-> \x27  '
\\ <-> \x5C  \

you find a full list of escape sequences and how they work here . Note that there is also \\0 <-> \\x00 in the C language.

Answer 2

There is a difference between bytes and a hexadecimal representation. bytes is a datatype; hexadecimal is a way of representing bit patterns on the screen.

A bytes is an immutable sequence of 8-bit values. The interpreter displays it, where possible, as characters or as string escape sequences, and where not possible, in hexadecimal. The corresponding literal is called a byte string . In other words, hexadecimal is a sort of last resort. You can construct the bytes b'ABC' using hexadecimal notation: b'\\x41\\x42\\x43' but the interpreter will still report it as b'ABC' . That is no different from the way quotes are handled:

>>> a = "ABC"
>>> a
'ABC'
>>> a = 'AB\'C'
>>> a
"AB'C"

The interpreter has a standard way of displaying its data and takes no account of the way you entered that data in the first place. This isn't a roundtrip failure, because no information is lost. You are seeing an equivalent representation rather than the same representation, is all.

If you want to see a hexadecimal representation then you should ask for it explicitly, instead of relying on the interpreter's default way of displaying a particular datatype.

>>> fmt = '\\x{0:02X}\\x{1:02X}'
>>> print(fmt.format(*((13).to_bytes(2, byteorder='big'))))
\x00\0D