Connect4 winner detection

detector.py

import sys
import json

def search_line(row):
	string = ''.join(row)
	if string.find('OOOO') != -1:
		return 'O'
	if string.find('XXXX') != -1:
		return 'X'
	else:
		return None
	
def detect(board):
	for row in board:
		winner = search_line(row)
		if winner:
			return winner
	
	for col in zip(*board[::1]):
		winner = search_line(col)
		if winner:
			return winner
	
	for dia in diagonals(board):
		winner = search_line(dia)
		if winner:
			return winner
	
	return None

def diagonals(board):
	"Return lists of diagonals."
	height = len(board)
	width = len(board[0])
	
	# NW <-> SE diagonals
	diags = []
	for i, row in enumerate(board):
		diags.append(row[i:])
	
	# SW <-> NE diagonals
	for i, row in enumerate(board):
		diags.append(["."] * i + row)
	
	return zip(*diags[::1])

if __name__ == '__main__':
	board = json.load(sys.stdin)
	winner = detect(board)
	
	if winner:
		print 'Winner: {0}'.format(winner)
	else:
		print 'No winner'

[
  [".", ".", ".", ".", ".", ".", "."],
  [".", ".", ".", ".", ".", ".", "."],
  [".", ".", "O", ".", ".", ".", "."],
  [".", ".", "X", "O", "X", "X", "."],
  [".", ".", "X", "X", "O", "O", "X"],
  [".", ".", "O", "X", "X", "O", "X"]
]