Forming an n-stock portfolio

The following program generates a return matrix with three stocks plus S&P500:

import statsimport numpy as np
import pandas as pd
tickers=['IBM','dell','wmt']
path1='http://chart.yahoo.com/table.csv?s=^GSPC'
final=pd.read_csv(path1,usecols=[0,6],index_col=0)
final.columns=['^GSPC']
path2='http://chart.yahoo.com/table.csv?s=ttt'
for ticker in tickers:
    print ticker
    x = pd.read_csv(path2.replace('ttt',ticker),usecols=[0,6],index_col=0)
    x.columns=[ticker]
    final=pd.merge(final,x,left_index=True,right_index=True)

To show the first and last few lines, we use the .head() and .tail() functions as follows:

>>>final.head()
              ^GSPC     IBM   dell    wmt
Date                                     
2013-10-18  1744.50  172.85  13.83  75.71
2013-10-17  1733.15  173.90  13.85  75.78
2013-10-16  1721.54  185.73  13.85  75.60
2013-10-15  1698.06  183.67  13.83  74.37
2013...