How to interpret python bytes class when more than one byte in one \x

Question

In Python, I am struggling to understand the following bytes:

a = b'\xff33'
b = b'\x00333'
c = b'\x00gff'

According to Pycharm debugger, a has 3 bytes, b c have 4 bytes. I don't quite get why.

Why b'\xff33' != b'\xff\x33' and b'\x00333' != b'\x00\x03\x33', and same logic for c

And a b c 's hex() conversion show:

a.hex() == 'ff3333'
b.hex() == '00333333'
c.hex() == '00676666'

I can't make sense of the results. Especially for c.hex(). It seems g == 67 and f == 66... But then why a.hex() is ff not 6666.. I feel that my head is exploding.

Can you help me make sense of these?

Thanks

J

\x is an escape sequence, but the other characters just represent the bytes corresponding to their ascii value. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Sep 28, 2020 at 21:55
So consider a, the the escape sequence \xff represents the byte with value 255, i.e. "ff" in hexadecimal. However, 3 and 3 each represent a byte, since they aren't escaped with \x, and the value corresponds to the ascii value of the character '3', i.e. ord('3') == 51. Thus, you have bytes 255 51 51, which you can confirm by doing list(a) — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Sep 28, 2020 at 22:01
Ah so \x only escape the first byte, and the rest of each char is an ascii value? That makes a lit of sense now. — J_yang
– J_yang, Commented Sep 28, 2020 at 22:03
I have a mmap read from a file that read out like this \x99\x99\x99\xff33\xb3\xff\xcd , which seems rather inconsistent to me. Why python decide to escape some character but display ascii for others... Why not just show everything per byte with \x escape. — J_yang
– J_yang, Commented Sep 28, 2020 at 22:06
Yes, you have to escape the bytes that are outside of the ascii range, of course, you can still use escape sequences for the whole thing, in other words, b'\xff33' == b'\xff\x33\x33' — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Sep 28, 2020 at 22:06

Wups · Accepted Answer · 2020-09-28 22:03:04Z

1

Python's bytes uses a mix of escaped unprintable bytes and normal printable bytes. a consists of the unprintable and escaped byte \xff and 2 printable "3".
That would be \xff\x33\x33 if one would escape all bytes, which is the same as the .hex() result.

answered Sep 28, 2020 at 22:03

Wups

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