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Python Change Dictionary Items

Change a value by referring to its key. update() can change several items at once.

← Access ItemsAdd Items →

Change Values

Assign to a key that already exists.

thisdict = {"brand": "Ford", "model": "Mustang", "year": 1964}
thisdict["year"] = 2018
thisdict.update({"year": 2020})
print(thisdict)

📘 Real-World Deep Dive

Dict mutation has more variants than list mutation (<code>d[k] = v</code>, <code>d.update</code>, <code>d.setdefault</code>, comprehension). Picking the right one keeps logs intact and avoids stale references.

Real-Life Scenario

Update pricing rules: merge a small change-set into a base pricing map, then verify which keys changed.

Real-Life Example

from copy import deepcopy

BASE = {"starter": 0, "pro": 19, "enterprise": 99}
PATCH = {"pro": 25, "team": 39, "enterprise": 89}

patched = deepcopy(BASE)
patched.update(PATCH)            # later wins; new keys appear

changed_keys = sorted(set(BASE) ^ set(patched))
print("base   :", BASE)
print("patched:", patched)
print("toggled:", changed_keys)

# Conditional change with setdefault
def ensure_path(d, k, path):
    d.setdefault(k, {}).setdefault("path", path)

od = {}
ensure_path(od, "user", "/users")
ensure_path(od, "user", "/users")
ensure_path(od, "post", "/posts")
print("od:", od)

Expected Output

base   : {'starter': 0, 'pro': 19, 'enterprise': 99}
patched: {'starter': 0, 'pro': 25, 'enterprise': 89, 'team': 39}
toggled: ['enterprise', 'team']
od: {'user': {'path': '/users'}, 'post': {'path': '/posts'}}

Common mistakes

  • Mutating a dict copy still mutates the original if values are mutable objects.
  • setdefault(...) evaluates the value expression each call — side-effects in that expression are unexpected.
  • Tuple keys (including (namespace, key)) work but built-in tuples can't include lists.

🚀 Performance & Best Practices

  • d.update(other) is one C-level loop — much faster than per-key assignment.
  • Use d |= other for in-place merge since Python 3.9.
  • Avoid d.pop in tight loops; it rehashes.

🧪 Try It Yourself

  1. Write a change_set(old, new) that returns {"add": [...], "remove": [...], "update": [...]}.
  2. Replace the merge step with d |= PATCH and benchmark.
  3. Implement a deep_update that recursively merges nested dicts.

FAQ: Python Change Dictionary Items

Common questions about this page.

What is Python Change Dictionary Items?

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