Iterators and generators

Streaming instead of holding

A generator produces values one at a time as they are asked for. Reading a multi-gigabyte log becomes possible on a laptop, because only the current line is ever in memory.

Writing one is a matter of yielding instead of appending to a list and returning it. Generators also compose: piping one into another builds a processing chain that still streams end to end.

The catch is that a generator is exhausted after one pass. Iterating it twice silently gives you nothing the second time, which is a genuinely confusing bug the first time you meet it.