Is it possible to write a firewall in python? Say it would block all traffic?
"Yes" - that's usually the answer to "is it possible...?" questions.
How difficult and specific implementations are something else entirely. I suppose technically in a don't do this sort of way, if you were hell-bent on making a quick firewall in Python, you could use the socket libraries and open connections to and from yourself on every port. I have no clue how effective that would be, though it seems like it wouldn't be. Of course, if you're simply interested in rolling your own, and doing this as a learning experience, then cool, you have a long road ahead of you and plenty of education.
OTOH, if you're actually worried about network security there are tons of other products out there that you can use, from iptables on *nix, to ZoneAlarm on windows. Plenty of them are both free and secure so there's really no reason to roll your own except on an "I want to learn" basis.
Is it possible to write a firewall in python?