[Python-Dev] Byte literals (was Re: [Python-checkins] Changing string constants to byte arrays ( r55119 - in python/branches/py3k-struni/Lib: codecs.py test/test_codecs.py ))

[Josiah Carlson]

"Fred L. Drake, Jr." <fdrake at acm.org> wrote:
> 
> On Saturday 05 May 2007, Aahz wrote:
>  > I'm with MAL and Fred on making literals immutable -- that's safe and
>  > lots of newbies will need to use byte literals early in their Python
>  > experience if they pick up Python to operate on network data.
> 
> Yes; there are lots of places where bytes literals will be used the way str 
> literals are today.  buffer(b'...') might be good enough, but it seems more 
> than a little idiomatic, and doesn't seem particularly readable.
> 
> I'm not suggesting that /all/ literals result in constants, but bytes literals 
> seem like a case where what's wanted is the value.  If b'...' results in a 
> new object on every reference, that's a lot of overhead for a network 
> protocol implementation, where the data is just going to be written to a 
> socket or concatenated with other data.  An immutable bytes type would be 
> very useful as a dictionary key as well, and more space-efficient than 
> tuple(b'...').

I was saying the exact same thing last summer.  See my discussion with
Martin about parsing/unmarshaling.  What I expect will happen with bytes
as dictionary keys is that people will end up subclassing dictionaries 
(with varying amounts of success and correctness) to do something like
the following...

    class bytesKeys(dict):
        ...
        def __setitem__(self, key, value):
            if isinstance(key, bytes):
                key = key.decode('latin-1')
            else:
                raise KeyError("only bytes can be used as keys")
            dict.__setitem__(self, key, value)
        ...

Is it optimal?  No.  Would it be nice to have immtable bytes?  Yes.  Do
I think it will really be a problem in parsing/unmarshaling?  I don't
know, but the fact that there now exists a reasonable literal syntax b'...'
rather than the previous bytes([1, 2, 3, ...]) means that we are coming
much closer to having what really is about the best way to handle this;
Python 2.x str.


 - Josiah