[Baypiggies] A python newbie questoin

[Ashok Chippa]

Thanks for your help.

On Fri, Feb 20, 2015 at 10:04 PM, Ashok Chippa <a.n.chippa at gmail.com> wrote:

> 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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