Meaning
The chunk builds a list by iterating over an iterable and optionally filtering items based on a condition. It evaluates the expression for each selected item and collects the results into a new list. This is useful when you need a transformed collection without writing an explicit loop.
Primary Function
List comprehension
Communicative Purpose
Enables concise creation of filtered lists from iterables.
Pattern
[expression for item in iterable if condition]
Core Structure
[... for ... in ... if ...]
Função primária
List comprehension
Propósito comunicativo
Enables concise creation of filtered lists from iterables.
Situações de gatilho
Data processing: extracting even numbers from a range of integers; Algorithm preparation: generating a list of squared values for elements that meet a threshold; Configuration parsing: collecting valid entries from a list of strings based on a pattern.
Contextos
Used in data processing, replacing explicit loops, generating numeric sequences, preprocessing data for algorithms.
Padrão
[expression for item in iterable if condition]
Estrutura central
[... for ... in ... if ...]
Slots de substituição
expression: any expression involving the item variable; item: variable representing each element; iterable: any iterable object; condition: boolean expression involving the item.
Colocados típicos
- range
- len
- zip
- map
- filter
- lambda
Substituições comuns
- list(filter(lambda x: x%2==0
- range(20)))
- [x for x in range(20) if x%2==0] (equivalent)
- for-loop with append.
Erros comuns
Using '=' instead of '==' in condition; forgetting the 'if' keyword; misplacing brackets; confusing list comprehension with generator expression (using parentheses); omitting the iterable.
Similar / contraste
Similar: generator expression (x for x in range(20) if x%2==0), set comprehension {x for x in range(20) if x%2==0}, dictionary comprehension {x: x*2 for x in range(20) if x%2==0}. Contrast: for loop with append, filter() with lambda.
Interferências
Coming from languages with generator-only comprehensions: may forget brackets → Python list comprehension requires brackets.
Família do chunk
- list comprehension idioms
Nuance
Produces a list immediately (eager evaluation); preserves order of items; allows duplicate values if expression yields same result.
Efeito pragmático
Conveys a declarative, concise intent to filter and transform data, improving readability compared to explicit loops.
Dica de memória
Like a net that catches only even numbers from a stream of integers.
Nota
This specific example yields even numbers from 0 to 18 inclusive; adjusting the range upper bound changes the maximum value.
Upgrade path
Consider using a generator expression for lazy evaluation with large iterables, or NumPy vectorized operations for numeric arrays.
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