[Baypiggies] A python newbie questoin

[Ashok Chippa]

Thought I fixed it :( But seeing this new one:

root at ashok-vb:/home/achippa/ppl# vi ethernet.py
root at ashok-vb:/home/achippa/ppl# !py
python test.py
Traceback (most recent call last):
  File "test.py", line 23, in <module>
    test()
  File "test.py", line 14, in test
    eth.gen(buf)
  File "/home/achippa/ppl/ethernet.py", line 123, in gen
    0x0800)
TypeError: 'str' object does not support item assignment
root at ashok-vb:/home/achippa/ppl# !vi


ethernet.py:
    ...
    def gen(self, buf):
        buf[len(buf):] = struct.pack('!6s6sH', '\x01\x02\x03\x04\x05\x06',
\           <<<<<<======== line 123

'\x07\x08\x09\x0a\x0b\x0c',  0x0800)
...
struct.pack() returns a string, right?!!!! Why does it complain about the
assignment?



On Fri, Feb 20, 2015 at 4:09 PM, Ashok Chippa <a.n.chippa at gmail.com> wrote:

> Thank you Alex, Jeff and Nam. Just after sending the question, I suspected
> it may be the type instead of length... Thank you for your answers, I think
> I have enough to fix it now... :-)
>
> Thanks a lot,
>
> On Fri, Feb 20, 2015 at 3:54 PM, Alex Martelli <aleax at google.com> wrote:
>
>> struct.pack returns *a string* and struct.unpack accepts *a string*.  buf
>> is *a list*, not a string.  struct.unpack[buf[-1]] (or buf[0] as buf only
>> has one item -- the string in question).  No idea of how it can work in the
>> case you say it works:-).
>>
>> Alex
>>
>> On Fri, Feb 20, 2015 at 3:49 PM, Ashok Chippa <a.n.chippa at gmail.com>
>> wrote:
>>
>>> A newbie python question:
>>>
>>> THIS WORKS:
>>>
>>> ethernet.py:
>>>
>>> ...
>>> def test():
>>>     buf = []
>>>     #eth_gen(buf)
>>>     buf = struct.pack('!6s6sH', '\x01\x02\x03\x04\x05\x06', \
>>>                                 '\x07\x08\x09\x0a\x0b\x0c', 0x0800)
>>>     eth = Eth()
>>>     eth.unpack(buf)
>>>     eth.show()
>>>
>>> root at ashok-vb:/home/achippa/ppl# python ethernet.py
>>> smac = 07:08:09:0a:0b:0c dmac = 01:02:03:04:05:06 type = 0800
>>>
>>>
>>> THIS DOES NOT WORK:
>>>
>>> # Build an ethernet header into the specified buffer.
>>> #
>>> def eth_gen(buf):
>>>     n = len(buf)
>>>     buf[n:] = struct.pack('!6s6sH', '\x01\x02\x03\x04\x05\x06', \
>>>                                     '\x07\x08\x09\x0a\x0b\x0c', 0x0800)
>>> def test():
>>>     buf = []
>>>     eth_gen(buf)
>>>     eth = Eth()
>>>     eth.unpack(buf)
>>>     eth.show()
>>>     #
>>>
>>> root at ashok-vb:/home/achippa/ppl# !py
>>> python ethernet.py
>>> Traceback (most recent call last):
>>>   File "ethernet.py", line 181, in <module>
>>>     test()
>>>   File "ethernet.py", line 173, in test
>>>     eth.unpack(buf)
>>>   File "ethernet.py", line 89, in unpack
>>>     dpkt.Packet.unpack(self, buf)
>>>   File "/usr/local/lib/python2.7/dist-packages/dpkt/dpkt.py", line 127,
>>> in unpack
>>>     struct.unpack(self.__hdr_fmt__, buf[:self.__hdr_len__])):
>>> struct.error: unpack requires a string argument of length 14
>>>
>>> I have verified (by printing) that calcsize(self.__hdr_fmt__) and
>>> self.__hdr_len__ are both 14. The buf has only 14 bytes of ethernet header.
>>> Putting the struct.pack(..) in a function (eth_gen()) causes the error...
>>>
>>> Any idea what may be happening... Thanks a lot
>>>
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>>
>>
>
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