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I've read several articles on Python generators now, and I had almost got my head around them, but then I read this: Iterators and Databases by Andy Todd, which demonstrates a simple but intuitive way of using generators to iterate through rows from a database query without having to load all of the rows in to a list in memory first. Brilliant.

Andy links to this article which includes a nice one line summary of why generators exist (generators return iterators):

The motivation behind iterators is twofold. Working with data as sequences is often the most straightforward approach, and a sequence that is processed in linear order often does not need to actually exist all at once.

More on generators:

• Charming Python: Iterators and simple generators is a good introductory tutorial
• PEP 255: Simple Generators is the PEP that formally describes the language feature