I'm sure you have seen this link about a library to process PCAP files. Now the question is how to acquire in real time without storing in a file.
Probably easiest is to use a fifo
$ mkfifo /tmp/tcpdump.fifo
Now you can capture and feed data into the named fifo
$ sudo tcpdump -s0 -i eth0 -f /tmp/tcpdump.fifo tcp port 80
And in your python program you can open '/tmp/tcpdump.fifo' as the input file as per the instructions in the link.
Alternatively you can try opening '/dev/stdin' in your program and reading the data from there; you could then pipe the PCAP data straight into stdin using the shell and skipping the intermediate named fifo.
$ sudo tcpdump -s0 -i eth0 -f - tcp port 80 | ./youprogram.py