def accumulate_pre_history_stats(Mer_list, analysis_start_date, accumulate_fields=['GAS_CAP_M3']):
    pre_history_stats = defaultdict(lambda: defaultdict(lambda: defaultdict(float)))
    for mer in Mer_list:
        if mer.DATE < analysis_start_date:
            for field in accumulate_fields:
                val = getattr(mer, field, 0)
                if val:
                    pre_history_stats[mer.OBJECT_NAME][mer.WELL_NAME][field] += val
    return pre_history_stats
# 809y231487t137840183745-13478f5134

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