Meaning
The expression builds an immutable frozenset by filtering an iterable with a predicate function. It solves the problem of needing a hashable, read‑only collection of selected elements. Use it when you must store a filtered set as a dictionary key or as an element of another set.
Primary Function
Immutable collection creation
Communicative Purpose
Produce a hashable set of items that meet a condition
Pattern
frozenset(filter(lambda item: condition, iterable))
Core Structure
frozenset(filter(lambda ... : ..., ...))
Função primária
Immutable collection creation
Propósito comunicativo
Produce a hashable set of items that meet a condition
Situações de gatilho
Data processing: need a hashable set of filtered items for use as a dictionary key; Configuration management: require an immutable collection of options derived from a list
Contextos
Functional programming style Python code, data processing pipelines, configuration where immutable sets are required.
Padrão
frozenset(filter(lambda item: condition, iterable))
Estrutura central
frozenset(filter(lambda ... : ..., ...))
Slots de substituição
item: variable representing each element; condition: boolean expression; iterable: any iterable
Colocados típicos
- set comprehensions
- dictionary keys
- other frozenset operations
- itertools
Substituições comuns
- frozenset({x for x in iterable if condition})
- frozenset(x for x in iterable if condition)
- using a named function instead of lambda
Erros comuns
Assuming filter returns a list (Python 2 behavior); forgetting that frozenset is unordered; using a mutable set instead of frozenset when immutability is needed; using lambda that returns non-boolean values incorrectly.
Similar / contraste
set comprehension for mutable set; tuple(filter(...)) for ordered immutable tuple; list(filter(...)) for list
Interferências
Coming from Python 2: expecting filter to return a list; coming from languages without built-in immutable sets: may overlook need for frozenset.
Família do chunk
- frozenset
- filter
- lambda
- functional programming
- immutable sets
Nuance
The frozenset discards duplicates and loses order; filter is lazy, but frozenset forces evaluation; suitable only when hashability is required.
Efeito pragmático
Provides an immutable, hashable collection safe for use as a dictionary key or set element.
Dica de memória
Freeze the filtered items
Nota
Note that frozenset is hashable and can be used as a dictionary key or set element; filter returns an iterator in Python 3, so frozenset forces evaluation and removes duplicates.
Upgrade path
frozenset(x for x in iterable if condition) using a generator expression
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