OLife'S
defaultdict(list)
Standard Library Idioms

Meaning

A defaultdict with list factory automatically creates an empty list for missing keys, enabling automatic accumulation of values per key without explicit existence checks.

Primary Function

Automatically provides a new empty list for any missing key, allowing direct appends to grouped collections.

Communicative Purpose

Signals the intent to collect items into buckets keyed by some key, eliminating boilerplate existence checks.

Pattern

defaultdict(<factory>)

Core Structure

defaultdict(list)

Função primária

Automatically provides a new empty list for any missing key, allowing direct appends to grouped collections.

Propósito comunicativo

Signals the intent to collect items into buckets keyed by some key, eliminating boilerplate existence checks.

Situações de gatilho

When you need to group items by a key (e.g., words by first letter, graph edges by source node) or accumulate values per category.

Contextos

Data processing pipelines, grouping logs, building adjacency lists, aggregating results, any scenario requiring a dict of lists.

Padrão

defaultdict(<factory>)

Estrutura central

defaultdict(list)

Slots de substituição

<factory> where factory is any callable returning a default value (e.g., list, set, int, lambda: 0)

Colocados típicos

  • .get
  • .update
  • .items()
  • for key
  • vals in dd.items()
  • dd[key].append(item)

Substituições comuns

  • defaultdict(set)
  • defaultdict(int)
  • defaultdict(lambda: 0)

Erros comuns

Forgetting to import from collections; assuming the default value is shared across keys (it is not—each missing key gets a fresh instance); using a mutable literal like [] directly as the factory (which would share the same list).

Similar / contraste

dict.setdefault for per‑key defaults; regular dict with manual key checks; collections.Counter for counting occurrences.

Interferências

Using a mutable literal like [] as the factory would reuse the same list across keys; defaultdict(list) safely creates a new list per missing key.

Família do chunk

  • defaultdict
  • Counter
  • setdefault
  • grouping patterns

Nuance

The factory is invoked each time a missing key is accessed, guaranteeing a fresh empty list for each new key.

Efeito pragmático

Signals the programmer’s intent to accumulate values per key, improving readability and reducing boilerplate.

Dica de memória

defaultdict with list factory

Upgrade path

defaultdict(set) for unique items, or collections.Counter for counting frequencies

Tipo de construção: constructionTag de espaçamento: Medium-termIdioma?: Sim

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