[Python-ideas] Binary f-strings

[Sven R. Kunze]

On 02.10.2015 16:26, Eric V. Smith wrote:
> bf'{foo}'
>
> Might succeed or fail, depending on what foo returns for __format__. If
> foo is 'bar', it succeeds. If it's '\u1234', it fails.

I know a lot of functions that fail when passing the wrong kind of 
arguments. What's so wrong with it?

> But some of the other arguments are making me think bf'' is a bad idea,
> so now I'm leaning towards not implementing it.

I see. Do you think of an alternative solution?


I was digging deeper into the matter of binary/byte strings formatting 
in order to sympathise why {} is not usable for binary protocols. Let's 
look at this practical example 
https://blog.tox.chat/2015/09/fuzzing-the-new-groupchats/ I hope the tox 
protocol fully qualifies as a wireframe protocol. What he's trying to do 
it is to fuzzing his new groupchat implementation by creating 
more-or-less random packages and feed them into tox core to see if it 
breaks.

He conveniently uses this type of syntax to describe the structure of 
the first header:

Header 1:  [  Packet ID  (1 b)  |  Chat ID hash  (4 b)  | Sender PK (32 
b)  |  nonce (24 b)  ]


Interested in writing a fuzzer, I would find the following really 
helpful as it mirrors the description within his blog post:

header1 = bf'{packet_id}{chat_id}{sender_pk}{nonce}'

# which should be the same as

header1 = b'%b%b%b%b' % (packet_id, chat_id, sender_pk, nonce)

I wouldn't mind specifying the encoding for all non-byte-string arguments.
Why? Because I would be working with bytes anyway, so no formatting (as 
in format()) would be necessary in the first place. However, I like the 
syntax for specifying the structure of (byte) strings.


Does this makes sense?


Best,
Sven