Moving to Functional

[Gribouillis]

I would try the following

from itertools import groupby
def report():
    data = Load()
    report = []
    for date, irec in groupby(data, key=lambda r: RecToDate(r)):
        irec = list(irec)
        counts = [0] * 10
        for rec in irec:
            for j in range(10):
                counts[j] += RecRan(rec, j)
            report.append(str(rec))
        report.append(Summary(len(irec), counts))
    return ''.join(report)