Dictionary manipulation

Combine or update the contents of one dictionary with another.

Configuration handling: merging default settings with user-provided options; Data aggregation: accumulating counts from multiple sources; Caching: updating a cache with new entries

Common in configuration handling, data processing pipelines, and any code that aggregates dictionary-based data.

target_dict.update(source_dict)

... .update(...)

target_dict: dict, source_dict: dict|iterable of (key, value) pairs

• dict literal
• **kwargs
• collections.ChainMap
• dict comprehension

• Using the union operator (|) (Python 3.9+): target_dict | source_dict → creates a new dict
• leaves originals unchanged
• requires Python 3.9+
• Using dictionary unpacking: {**target_dict
• **source_dict} → creates a new dict
• works in older Python versions but less readable.

Assuming update returns a new dictionary (misconception: thinking it returns merged dict) → consequence: assigning the result to a variable yields None, causing downstream errors; Forgetting that update modifies the original in‑place (misconception: thinking it works on a copy) → consequence: unintended mutation of source data; Using update with a non‑iterable argument (syntax error: passing an int) → consequence: TypeError at runtime; Passing keyword arguments that clash with positional keys (misconception: thinking keywords are ignored) → consequence: unexpected key overwrites.

dict.copy() (shallow copy without merging); dict.setdefault() (only adds missing keys).

Coming from languages where merge creates a new object (e.g., JavaScript Object.assign): may expect update to return a merged dict → dict.update modifies the original in‑place and returns None.

• dict.copy
• dict.setdefault
• dict.merge (Python 3.9+)
• dict union operator

When NOT to use: when you need to keep the original dictionary unchanged (use a copy or the union operator). Performance/resource implications: runs in O(n) time where n is the number of items from the source, with minimal overhead as it modifies in‑place. Non‑obvious boundary conditions: accepts any iterable of key‑value pairs (including generators); keyword arguments are treated as additional pairs and override positional keys; duplicate keys within the same call follow left‑to‑right order with the later value winning.

Provides a concise, in-place way to merge configurations, reducing boilerplate code.

Think 'update my dict with their dict'.

Remember that dict.update returns None; to keep the original dictionary unchanged, copy it first or use the union operator (|) available in Python 3.9+.

Using the dict union operator (|) for immutable merging: merged = dict1 | dict2