Python Pyparsing Optional field

Question

I am currently using pyparsing module to create my own interpreter. My current code is

import pyparsing as pp

# To parse a packet with srcip,dstip,srcmac,dstmac
identifier = pp.Word(pp.alphas,pp.alphanums+'_')
ipfield = pp.Word(pp.nums,max=3)
ipAddr = pp.Combine(ipfield+"."+ipfield+"."+ipfield+"."+ipfield)
hexint = pp.Word(pp.hexnums,exact=2)
macAddr = pp.Combine(hexint+(":"+hexint)*5)
ip = pp.Combine(identifier+"="+ipAddr)
mac = pp.Combine(identifier+"="+macAddr)
pkt = ip & ip & mac & mac
arg = "createpacket<" + pp.Optional(pkt) + ">"
arg.parseString("createpacket<srcip=192.168.1.3dstip=192.168.1.4srcmac=00:FF:FF:FF:FF:00>")

While I run the last line of the code to parse an example string I get an error as follows:

File "<stdin>", line 1, in <module>
File "/usr/lib/python2.7/dist-packages/pyparsing.py", line 1041, in parseString
  raise exc
pyparsing.ParseException: Expected ">" (at char 13), (line:1, col:14)

Can anyone explain the reason for this error?

I **strongly** recommend that you read http://stackoverflow.com/editing-help then have another go at this... — jonrsharpe, Jun 02 '15 at 20:57
`pkt` contains 2 `mac`, while the string you were parsing only contains one mac address. — nymk, Jun 02 '15 at 21:17
@nymk is exactly right - since pkt expects 2 ips and 2 macs (in any order, given that they are joined by '&' operators), and your given example has only a single mac, then the grammar assumes that the optional pkt is not present, and tries to match the next item, which is the closing '>'. Not finding the '>', pyparsing then raises the exception. — PaulMcG, Jun 02 '15 at 21:52

Python Pyparsing Optional field

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