How to parse packets in a python library?

I tried that and then tried pcapy. I choose pcapy because my use was similar to an example which I found googling.

http://snipplr.com/view/3579/live-packet-capture-in-python-with-pcapy/ (or see the same code copied below)

import pcapy
from impacket.ImpactDecoder import *

# list all the network devices
pcapy.findalldevs()

max_bytes = 1024
promiscuous = False
read_timeout = 100 # in milliseconds
pc = pcapy.open_live("name of network device to capture from", max_bytes, 
    promiscuous, read_timeout)

pc.setfilter('tcp')

# callback for received packets
def recv_pkts(hdr, data):
    packet = EthDecoder().decode(data)
    print packet

packet_limit = -1 # infinite
pc.loop(packet_limit, recv_pkts) # capture packets

pycapfile can be also used. Link to pip: https://pypi.python.org/pypi/pypcapfile

I recommend you to use Pyshark. this is wrapper for tshark. it also support all of tshark filter, decoder lib, ... and easy to use! This is a great package for parsing .pcap file and also livecapturing

https://pypi.python.org/pypi/pyshark

sample code (from the link):

import pyshark
cap = pyshark.FileCapture('/root/log.cap')
cap
>>> <FileCapture /root/log.cap>
print cap[0]
Packet (Length: 698)
Layer ETH:
        Destination: BLANKED
        Source: BLANKED
        Type: IP (0x0800)
Layer IP:
        Version: 4
        Header Length: 20 bytes
        Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00: Not-ECT (Not ECN-Capable Transport))
        Total Length: 684s
        Identification: 0x254f (9551)
        Flags: 0x00
        Fragment offset: 0
        Time to live: 1
        Protocol: UDP (17)
        Header checksum: 0xe148 [correct]
        Source: BLANKED
        Destination: BLANKED
  ...
dir(cap[0])
['__class__', '__contains__', '__delattr__', '__dict__', '__dir__', '__doc__', '__format__', '__getattr__', '__getattribute__', '__getitem__', '__getstate__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_packet_string', 'bssgp', 'captured_length', 'eth', 'frame_info', 'gprs-ns', 'highest_layer', 'interface_captured', 'ip', 'layers', 'length', 'number', 'pretty_print', 'sniff_time', 'sniff_timestamp', 'transport_layer', 'udp']
cap[0].layers
[<ETH Layer>, <IP Layer>, <UDP Layer>, <GPRS-NS Layer>, <BSSGP Layer>]
....

Try scapy. It is a very powerful program for packet inspection, manipulation and creation.

You can use it to build your own tools.