username_checker.py

import re, sys

try:
	f = open("/etc/passwd",'r')
except:
	print "\nCan't open the file!\n"
	sys.exit(0)

#enter your username here
username = "rash"
k=0

for lines in f.readlines():
	match = re.search(username, lines)
	if match:   
		k=1                   
		print match.group() + " exists."
		break 

if(k==0):
	print username + " does not exist!"