main.py

from collections import defaultdict
from pprint import pprint

data = [{'order_id': [17], 'product_id': 763},
        {'order_id': [19], 'product_id': 822},
        {'order_id': [19], 'product_id': 822},
        {'order_id': [20], 'product_id': 994},
        {'order_id': [20], 'product_id': 783},
        {'order_id': [20], 'product_id': 783},
        {'order_id': [21], 'product_id': 857},
        {'order_id': [21], 'product_id': 822},
        {'order_id': [26], 'product_id': 857}]

class Order(object):
    def __init__(self, order):
        self.order_id = order['order_id']
        self.product_id = order['product_id']

    def __hash__(self):
        return hash((",".join([str(x) for x in self.order_id]), self.product_id))

    def __eq__(self, other):
        return self.order_id == other.order_id and self.product_id == other.product_id

if __name__ == "__main__":
    purchase = defaultdict(lambda: {'product_id': 0, 'count': 0, 'order_counter': []})
    orders = set([Order(o) for o in data])
    for x in orders:
        purchase[x.product_id]['product_id'] = x.product_id
        purchase[x.product_id]['count'] += 1
        purchase[x.product_id]['order_counter'].append(['order_id', x.order_id[0]])
    result = [purchase[p] for p in purchase]
    pprint(result)