A little script for blocking or unblocking sites editing your hosts file. It can't add lines, but it comments or uncomments it to unblock or block sites.

blocky.py

from colorama import init, Fore
from os import system, name

init()

route = "/etc/hosts"
f = open(route)
lines = f.readlines()
f.close()

def cls():
	system(["clear","cls"][name == "nt"])

def save():
	f = open(route, "w")
	for line in lines:
		f.write(line)
	f.close()

def list():
	cls()
	i = 0
	for line in lines:
		if (line[0] == "#"):
			print Fore.GREEN,
		else:
			print Fore.RED,
		print str(i) + ". " + line,
		i = i + 1 
	print Fore.RESET

def block(n):
	line = lines[n]
	if lines[n][0] == "#":
		lines[n] = lines[n][2:]

def unblock(n):
	if lines[n][0] != "#":
		lines[n] = "# " + lines[n]

def blockall():
	i = 0
	for line in lines:
		if line[0] == "#":
			lines[i] = line[2:]
		i = i + 1 

def unblockall():
	i = 0
	for line in lines:
		if line[0] != "#":
			lines[i] = "# " + line
		i = i + 1 

list()
while True:
	option = raw_input("\nOption: ").lower().split()
	if option[0] == "bye" or option[0] == "exit":
		save()
		exit()
	elif option[0][0] == "s":
		save()
		list()
	elif option[0][0] == "b":
		if (option[1][0] == "a"):
			blockall()
		else:
			block(int(option[1]))
		list()
	elif option[0][0] == "u":
		if (option[1][0] == "a"):
			unblockall()
		else:
			unblock(int(option[1]))
		list()
	else:
		print "I don't know what you want."