Generators and yield

A generator produces values one at a time, only when asked, holding nothing ahead of time.

Core idea

• Lazy evaluation: work is delayed until a value is needed; eager does it all up front (a list).
• A generator hands you one value, pauses, and does more work only when asked for the next.

Making one

• Generator function: uses yield instead of return; any yield in the body makes it a generator.
• Calling it does not run the body; it returns a generator object.
• Generator expression: inline, round parens instead of square brackets: (n*n for n in range(5)).
• Drop the extra parens when it is a function’s only arg: sum(n*n for n in range(5)).

Pause and resume

• At yield the function freezes: locals keep their values, execution resumes on the next line next time.
• next(gen) pulls exactly one value; a for loop calls it automatically.
• Exhaustion raises StopIteration; a for loop catches this signal for you.

loop --next()--> gen: yield value, freeze
loop --next()--> gen: resume, yield, freeze
loop --next()--> gen: StopIteration (done)

Cost

• List of n values: O(n) space (all held at once).
• Generator: O(1) space (only current position + locals), any number of values.
• Time is O(n) either way; only the memory footprint shrinks.

Infinite generators

• while True: yield ... never ends on its own; the caller stops with break.
• The stop condition lives in the consumer, not the generator.

When to use

• Large or infinite/unknown-length sequences processed one item at a time.
• You might stop early and skip unneeded work.
• Prefer a list when data is small, reused (a generator is single-use, exhausted after one pass), or needs random access / len().

Pitfalls

• Consumed once: after list() or a for loop, re-iterating yields nothing; recreate it.
• (...) is lazy, [...] builds a full list; easy typo, very different memory.
• len(gen) and gen[0] raise TypeError; convert with list(gen) (brings back O(n)).
• print(gen) shows the object, not values; use list() or a loop.
• Infinite generator with no break hangs: list(count_from(0)) never finishes.