Errno 22 opening file for appending Python 2.7

Question

I am parsing data into a dictionary and then saving it to file. After the file reaches a size, I start getting the error:

Traceback (most recent call last):
  File "storepcap.py", line 123, in storeData
    with open(dataFileName, 'a+')  as dataFile:
IOError: [Errno 22] invalid mode ('a+') or filename: 'alldata.dat'

The file does grow in size as data is being appended so I know that opening it is very likely not the issue. It probably has something to do with me unpickling data, adding more data to the pickled object, and then resaving it. (I'd very much welcome more efficient solutions).

4,294,969,812 bytes

import dpkt
from socket import inet_ntoa
import traceback
import datetime
import multiprocessing
import sys
try:
    import cPickle as pickle
except:
    import pickle

myAddress = '10.0.0.3'       

def inet_to_str(inet):
    return socket.inet_ntop(socket.AF_INET, inet)

badFrames = 0
data = ""
maxProcesses = 100
pool = []
jobsProcessed = 0

def storeData(timestamp, buf):
    global badFrames


    dataFileName = 'alldata.dat'
    with open(dataFileName, 'rb')  as dataFile:
        try:
            packetData = pickle.load(dataFile)
        except:
            #print 'Doing data from scratch'
            packetData = {}    

    try:
        eth = dpkt.ethernet.Ethernet(buf)

        if not isinstance(eth.data, dpkt.ip.IP):
            return


        ip = eth.data
        info = {}
        try:
            info = {'source':inet_ntoa(ip.src),
                    'destination':inet_ntoa(ip.dst),
                    'len':ip.len,
                    'id':ip.id,
                    'off':ip.off,
                    'ttl':ip.ttl,
                    'p':ip.p,
                    'sum':ip.sum,
                    'payload':repr(ip.data),
                    'options':ip.opts,
                    'type':eth.type}  

            if isinstance(ip.data, dpkt.udp.UDP):
                info['packet'] = 'UDP'
                info['sport'] = ip.data.sport
                info['dport'] = ip.data.dport
                info['ulen'] = ip.data.ulen

            elif isinstance(ip.data, dpkt.tcp.TCP):
                info['packet'] = 'TCP'
                info['sport'] = ip.data.sport
                info['dport'] = ip.data.dport
                info['sequence'] = ip.data.seq
                info['acknowledgement'] = ip.data.ack
                info['flags'] = ip.data.flags
                info['window'] = ip.data.win            

            elif isinstance(ip.data, dpkt.igmp.IGMP):
                info['packet'] = 'IGMP'
                info['group'] = ip.data.group

            elif isinstance(ip.data, dpkt.icmp.ICMP):
                info['packet'] = 'ICMP'
                info['code'] = ip.data.code
                info['payload'] = repr(ip.data.data.data)
                info['sport'] = ip.data.data.data.data.sport
                info['dport'] = ip.data.data.data.data.dport
                info['sum'] = ip.data.data.data.data.sum
                try:
                    info['packet'] = 'ICMP+TCP'
                    info['sequence'] = ip.data.data.data.data.seq
                    info['acknowledgement'] = ip.data.data.data.data.ack
                    info['flags'] = ip.data.data.data.data.flags
                    info['window'] = ip.data.data.data.data.win    
                    info['sum'] = ip.data.data.data.data.sum 
                    info['options'] = ip.data.data.data.data.opts 
                except:
                    try:
                        info['packet'] = 'ICMP+UDP'
                        info['len'] = ip.data.data.data.data.ulen
                    except:
                        print "-------------------- ping tunnel issue"
                        print {ip}
                        print "--------------------"
                    pass

            elif isinstance(ip.data, dpkt.sctp.SCTP):
                info['packet']= 'SCTP'
                info['sport'] = ip.data.sport
                info['dport'] = ip.data.dport
                for i in xrange(len(ip.data.data)):
                    info['chunk %s len'%i] = ip.data.data[i].len
                    info['chunk %s payload'%i] = ip.data.data[i].data
            else:
                info['packet'] = 'UNKNOWN'
        except:
            traceback.print_exc()
            print {ip}

        if myAddress != info['source'] and myAddress != info['destination']:
            return

        try:
            packetData[str(datetime.datetime.utcfromtimestamp(timestamp))] = info
        except:
            badFrames += 1
            return

        with open(dataFileName, 'a+')  as dataFile:
            pickle.dump(packetData, dataFile)

    except:
        traceback.print_exc()
        return



if __name__ == '__main__':
    with open("alldata2.pcap", 'rb') as data:
        pcap = dpkt.pcap.Reader(data)

        for timestamp, buf in pcap:

            storeData(timestamp, buf)

    print "%s bad frames"%badFrames