import datetime

promedio = [[datetime.datetime(2013, 4, 10, 12, 0), 23.4],[datetime.datetime(2013, 4, 10, 12, 0), 25.81], [datetime.datetime(2013, 4, 10, 12, 0), 25.08],
[datetime.datetime(2013, 4, 10, 13, 0), 18.49], [datetime.datetime(2013, 4, 10, 14, 0), 20.74], [datetime.datetime(2013, 4, 10, 14, 0), 25.86] ]

ultima_fecha=None
group = []
res = []
suma=0
cantidad=0

for d, v in promedio:
    if not ultima_fecha is None and ultima_fecha != d:
        res.append((ultima_fecha, suma/cantidad))
        suma=0
        cantidad=0
    ultima_fecha=d
    suma+= v
    cantidad+=1

if cantidad>0:
    res.append((ultima_fecha, suma/cantidad))

print res