Python Tutorial

Python Add Dictionary Items

Adding an item is assigning to a new key. update() can add several keys.

Adding Items

A new key creates a new item.

thisdict = {"brand": "Ford", "model": "Mustang", "year": 1964}
thisdict["color"] = "red"
thisdict.update({"owner": "Luna"})
print(thisdict)

📘 Real-World Deep Dive

Adding to a dict has subtle variants — <code>__setitem__</code>, update, setdefault, merge <code>|</code>. The right choice depends on whether you want to preserve, overwrite, or collapse duplicates.

Real-Life Scenario

Build a call-frequency counter from a log line-by-line; combine a manual loop with final merge to read two days worth of files.

Real-Life Example

import csv, io
from collections import Counter

day1 = "api,/users,200\napi,/login,200\ndb,SELECT,401\napi,/users,200\n"
day2 = "api,/users,500\ndb,SELECT,401\napi,/users,200\napi,/login,200\n"
def parse(line: str):
    # route,endpoint,code
    route, ep, code = line.strip().split(",", 2)
    return (route, ep), int(code)

c: Counter = Counter()
for line in (day1 + day2).splitlines():
    if not line: continue
    key, _ = parse(line)
    c[key] += 1
print("frequencies:")
for key, n in c.most_common():
    print(f"  {key}: {n}")

Expected Output

frequencies:
  ('api', '/users'): 4
  ('api', '/login'): 3
  ('db', 'SELECT'): 2

Common mistakes

Mutating d[k] works on existing keys; use d.setdefault(k, default) only if you specifically need lazy initialisation.
d.update(other) silently overwrites; carry the old value first if you need to log changes.
d |= other requires Python 3.9+; use d.update(other) on older interpreters.

🚀 Performance & Best Practices

d[k] = v is O(1) on average; rehashing happens at load-factor boundaries.
Bulk insertion from a sequence of pairs is faster with a single update than per-key writes.
For huge dicts, use array.array for keys plus a parallel value array.

🧪 Try It Yourself

Build a coalesce(d1, d2, prefer=last) helper that accepts an override-fail mode.
Add a decorator that records every write to d for replay.
Switch the example to use collections.Counter directly.

FAQ: Python Add Dictionary Items

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