Dicts, defaultdict and Counter

import json
customers = json.load(open("customers.json"))
by_id = {c["customer_id"]: c for c in customers}

print(by_id[2]["name"])
print(by_id.get(99))            # None, no KeyError
print(by_id.get(99, {}).get("name", "unknown"))

sana = by_id[6]
print(sana["address"]["city"])  # key exists, is None
city = (sana.get("address") or {}).get("city") or "Unknown"
print(city)

Diya
None
unknown
None
Unknown

import csv
from collections import Counter, defaultdict

rows = list(csv.DictReader(open("order_items.csv")))

items_per_order = defaultdict(list)       # group
for r in rows:
    items_per_order[r["order_id"]].append(r["product_id"])
print(dict(list(items_per_order.items())[:3]))

counts = Counter(r["product_id"] for r in rows)   # count
print(counts.most_common(3))

merged = {"retries": 3, "env": "dev"} | {"env": "prod"}
print(merged)                             # right side wins

{'101': ['1', '5'], '102': ['3', '6'], '103': ['2']}
[('1', 5), ('5', 4), ('3', 3)]
{'retries': 3, 'env': 'prod'}

prices = {"mouse": 799, "hub": 1499, "pens": 149}
for name, p in prices.items():
    if p < 500:
        del prices[name]

RuntimeError: dictionary changed size during iteration

prices = {"mouse": 799, "hub": 1499, "pens": 149}
prices = {k: v for k, v in prices.items()
          if v >= 500}
print(prices)

{'mouse': 799, 'hub': 1499}