Recursive solution for N-Queens problem in Python

nqueens.py

from math import *
import sys

chosen = {}
n = int(sys.argv[1])

def place(xpos, ypos):
    if (ypos in chosen.values()):
        return False
    opponent = 1
    while(opponent < xpos):
        if abs(chosen[opponent]-ypos) == abs(opponent-xpos):
            return False
        opponent+=1
    return True

def clear_all_future_positions(xpos):
    for i in range(xpos,n+1):
       chosen[i]=None

def NQueens(xpos):
 #  print 'NQueens(',xpos,') entering'
    for ypos in range(1, n + 1):
        clear_all_future_positions(xpos)
        if place(xpos, ypos):
            chosen[xpos] = ypos
   #        print 'chosen=',chosen
            if (xpos==n):
                for opponent in chosen:
                    print chosen[opponent]
                print '------------------'
            else:
                NQueens(xpos+1)
 #  print 'NQueens(',xpos,') returns'


NQueens(1)

#code error line 14, in
n = int(sys.argv[1])

IndexError: list index out of range

#code error line 14, in
n = int(sys.argv[1])

IndexError: list index out of range

Try running with python3 nqueens.py [value of n that you want]

#code error line 14, in
n = int(sys.argv[1])

IndexError: list index out of range