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How do you write a basic list comprehension, and how does it compare to an equivalent for loop? How do you add a filtering condition to a list comprehension using 'if'? What is a generator function, and how does 'yield' make it different from a regular function? How do dict comprehensions and set comprehensions differ in syntax from list comprehensions? How do you write a nested comprehension, like flattening a list of lists into a single flat list? How do you send a value INTO a generator using the .send() method, and how does this differ from just calling next()? How does 'yield from' delegate iteration to a sub-generator, and what does it return as its own expression value? How does generator.throw() let you inject an exception into a paused generator, and how would the generator handle it? How does lazy evaluation in a chained sequence of generator expressions actually process elements, one at a time end-to-end, without intermediate lists? How would you implement a memory-efficient sliding window or batching generator over a large iterable?
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What is a generator expression, and how does it differ syntactically and behaviorally from a list comprehension?

Intermediate

Answer

A generator expression uses parentheses `(expr for item in iterable)` instead of square brackets, and produces a LAZY generator object (computing values one at a time on demand) rather than an eagerly-built list — using far less memory for large or infinite sequences since it never materializes the whole sequence.
sum_of_squares = sum(n * n for n in range(1_000_000))  # generator expression: no intermediate list built

# Wasteful alternative: builds a full 1,000,000-element list just to sum it
sum_of_squares2 = sum([n * n for n in range(1_000_000)])
Real-world example Computing a sum, max, or any() result over a large sequence without ever materializing the full intermediate list in memory.

Common follow-ups: Can a generator expression be passed directly as a function's sole argument without extra parentheses, like sum(x for x in range(10))?

Memory Management & Garbage Collection
All 15 Comprehensions & Generators questions