On 1/06/2006 10:50 AM, Giovanni Bajo wrote:
> Hello,
>
> given the ongoing work on struct (which I thought was a dead module), I was
> wondering if it would be possible to add an API to register custom parsing
> codes for struct. Whenever I use it for non-trivial tasks, I always happen to
> write small wrapper functions to adjust the values returned by struct.
>
> An example API would be the following:
>
> ============================================
> def mystring_len():
> return 20
>
> def mystring_pack(s):
> if len(s) > 20:
> raise ValueError, "a mystring can be at max 20 chars"
> s = (s + "\0"*20)[:20]
Have you considered s.ljust(20, "\0") ?
> s = struct.pack("20s", s)
> return s
I am an idiot, so please be gentle with me: I don't understand why you
are using struct.pack at all:
|>>> import struct
|>>> x = ("abcde" + "\0" * 20)[:20]
|>>> x
'abcde\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
|>>> len(x)
20
|>>> y = struct.pack("20s", x)
|>>> y == x
True
|>>>
Looks like a big fat no-op to me; you've done all the heavy lifting
yourself.
>
> def mystring_unpack(s):
> assert len(s) == 20
> s = struct.unpack("20s", s)[0]
Errrm, g'day, it's that pesky idiot again:
|>>> z = struct.unpack("20s", y)[0]
|>>> z
'abcde\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
|>>> z == y == x
True
> idx = s.find("\0")
> if idx >= 0:
> s = s[:idx]
> return s
Have you considered this:
|>>> z.rstrip("\0")
'abcde'
|>>> ("\0" * 20).rstrip("\0")
''
|>>> ("x" * 20).rstrip("\0")
'xxxxxxxxxxxxxxxxxxxx'
>
> struct.register("S", mystring_pack, mystring_unpack, mystring_len)
>
> # then later
> foo = struct.unpack("iilS", data)
> ============================================
>
> This is only an example, any similar API which might fit better struct
> internals would do as well.
>
> As shown, the custom packer/unpacker can call the original pack/unpack as a
> basis for their work. I guess an issue with this could be the endianess
> problem: it would make sense if, when called recursively, struct.pack/unpack
> used by the default the endianess specified by the external format string.
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