Merge Two Dicts - Data Handling in Pure Python

Combine a defaults dict and an overrides dict so that override values win on shared keys. The replay shows merged filling with all defaults first, then the second loop overwriting port and debug with the override values.

By hand

The Pythonic way

{**defaults, **overrides} unpacks both dicts into a single literal in one expression. Later entries win on duplicate keys, so placing overrides second gives it priority — the same rule as the two-loop version.

naive.py

defaults = {'host': 'localhost', 'port': 80, 'debug': False}
overrides = {'port': 443, 'debug': True}
merged = {}
for k, v in defaults.items():
    merged[k] = v
for k, v in overrides.items():
    merged[k] = v
print('RESULT:', {k: merged[k] for k in sorted(merged)})

library.py

defaults = {'host': 'localhost', 'port': 80, 'debug': False}
overrides = {'port': 443, 'debug': True}
merged = {**defaults, **overrides}
print('RESULT:', {k: merged[k] for k in sorted(merged)})

RESULT: {'debug': True, 'host': 'localhost', 'port': 443}

Implementation notes

Python 3.9+ adds the union operator: defaults | overrides produces the same result as {**defaults, **overrides} and reads more clearly as a merge.
Argument order matters: {**overrides, **defaults} would give defaults priority instead; the rightmost dict wins on any duplicate key.
Neither approach mutates the input dicts. If in-place update is acceptable, defaults.update(overrides) modifies defaults directly without creating a new dict.