def merge(a: Mapping; b: Mapping; ) -> dict

Type System & Annotations

Meaning

Defines a function that takes two Mapping objects and returns a new dictionary containing every key‑value pair from both inputs. It eliminates the need to write explicit loops or call update repeatedly, reducing boiler‑plate and accidental mutation of the original mappings. Use it whenever you need to combine configuration dictionaries, counters, or any mapping‑like data structures while keeping the inputs unchanged.

Primary Function

Data transformation

Communicative Purpose

Combine two dictionaries (or Mapping objects) into one, preserving type hints.

Pattern

def function_name(first_mapping: Mapping, second_mapping: Mapping) -> return_type:

Core Structure

def ...(..., ...) -> ...:

Função primária

Data transformation

Propósito comunicativo

Combine two dictionaries (or Mapping objects) into one, preserving type hints.

Situações de gatilho

Web application configuration: merging default settings with environment‑specific overrides; Data analysis: combining count dictionaries from separate data shards; CLI tool: aggregating parsed arguments with default option values

Contextos

General‑purpose Python codebases, data‑processing scripts, configuration management modules.

Padrão

def function_name(first_mapping: Mapping, second_mapping: Mapping) -> return_type:

Estrutura central

def ...(..., ...) -> ...:

Slots de substituição

function_name: identifier, first_mapping: Mapping, second_mapping: Mapping, return_type: type

Colocados típicos

return
dict
update
items
** unpacking

Substituições comuns

return {**a
**b}
return dict(a
**b)
return dict(a)
result.update(b)

Erros comuns

Assuming the merge is in‑place and mutates the first mapping → original data is unexpectedly altered; Overlooking that values are shallow‑copied, so mutable objects remain shared → later modifications affect both dictionaries; Expecting duplicate keys to be summed automatically → later key overwrites earlier value, leading to loss of data

Similar / contraste

collections.ChainMap (provides a view without copying) vs. dict.update (mutates in place). ChainMap keeps originals separate, update modifies the first dict.

Interferências

Coming from JavaScript: using Object.assign directly may give unexpected prototype properties; Python's dict merging does not copy methods.

Família do chunk

dictionary merging
dict unpacking
ChainMap
update pattern

Nuance

Do not use this simple merge when you need custom conflict resolution such as summing values or preserving both versions of a key; The operation creates a brand‑new dict, incurring O(n) time and memory overhead proportional to the combined size of the inputs; Nested mutable objects are not deep‑copied, so changes to them affect the merged result and the original mappings

Efeito pragmático

Eliminates manual loops, reduces boiler‑plate, and makes the intent of merging explicit.

Dica de memória

Merge two mappings with a single‑line function.

Nota

The function does not modify the input mappings; it creates a new dictionary.

Upgrade path

Use collections.ChainMap for a read‑only view or the PEP 584 "dict union" operator: `a | b` (Python 3.9+).

Frequência: HighFormulaicidade: Semi-fixedTipo de construção: function definitionPrioridade de aquisição: Active recallPrioridade de output: BothTag de espaçamento: Short-term

Examples

def merge(a: Mapping[str, int], b: Mapping[str, int]) -> dict[str, int]: """Return a new dict containing all items from *a* and *b*. Keys in *b* overwrite those in *a* when duplicated. """ return {**a, **b} # Demo first = {'apple': 1, 'banana': 2} second = {'banana': 3, 'cherry': 4} print(merge(first, second)) # {'apple': 1, 'banana': 3, 'cherry': 4}

def merge_defaults(defaults: Mapping[str, int], overrides: Mapping[str, int]) -> dict[str, int]: """Combine a default config with user overrides. Overrides take precedence. """ return {**defaults, **overrides} # Demo default_cfg = {'timeout': 30, 'retries': 3, 'verbose': 0} user_cfg = {'retries': 5, 'verbose': 1} print(merge_defaults(default_cfg, user_cfg)) # {'timeout': 30, 'retries': 5, 'verbose': 1}

def merge_counts(a: Mapping[str, int], b: Mapping[str, int]) -> dict[str, int]: """Merge two count dictionaries, summing values for duplicate keys. """ result: dict[str, int] = {} for k, v in a.items(): result[k] = v for k, v in b.items(): result[k] = result.get(k, 0) + v return result # Demo c1 = {'error': 2, 'warning': 5} c2 = {'error': 3, 'info': 7} print(merge_counts(c1, c2)) # {'error': 5, 'warning': 5, 'info': 7}