Data transformation

Generate a set of unique results from an iterable

Data analysis: extracting unique identifiers from a list, Web scraping: deduplicating URLs before download, Algorithm design: building a lookup set of processed items for fast membership tests

Python scripts, data processing pipelines, algorithmic problems requiring set membership tests

{expression for item in iterable}

{... for ... in ...}

expression: any, item: identifier, iterable: iterable

• conditional filter (if)
• function calls
• other comprehensions

• list comprehension
• generator expression
• map/filter with set()

Using parentheses instead of braces (creates a generator), forgetting braces (syntax error), using unhashable expression types

List comprehension [x for x in range(5)] (produces list), generator expression (x for x in range(5)) (lazy), dict comprehension {x: x*2 for x in range(5)}

Coming from JavaScript: may expect Set constructor; in JavaScript use new Set([...]) or array filter. Coming from Java: may think of HashSet.add in loops.

• set comprehension
• dict comprehension
• list comprehension
• generator expression

Set comprehension does not preserve order; duplicates are removed; if expression yields unhashable items (e.g., list), TypeError is raised.

Efficiently builds a deduplicated collection without explicit loops or temporary lists

Curly braces with a 'for' inside make a set

Set comprehension does not preserve order; duplicates are automatically removed. The expression must produce hashable items; using unhashable types (e.g., list, dict) raises a TypeError.

{expression for item in iterable if condition}