index = pd.Index(['01/01/2012', '01/01/2012', '01/01/2012', '01/02/2012',
    '01/02/2012'], name='Date')
df = pd.DataFrame({'ID': [100, 101, 102, 201, 202], 'wt': [0.5, 0.75, 1, 
    0.5, 1], 'value': [60, 80, 100, 100, 80]}, index=index)
Calculate weighted average using a pandas/dataframe