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Python 2 enumerate returns an 'enumerate' object that is more or less a light weight wrapper of the sequence that was passed in.

Generators provide a convenient syntax to implement that sort of object.

D'oh. I put in a map(), that returns a list.

Turning them into generators would have broken a lot of existing code, though, so it's reasonable to leave them until a major version change. Rather, these are the kind of small, obviously-useful changes that should have come immediately in 3.0, giving people some encouragement to switch.

Most of the breaking changes (including making map, zip, filter, etc lazy) were done in Python 3.0.

Ah, you're absolutely right. for line in open(filename) returns a generator. I added map(str.upper, xxx) there to make it more it make sense.

But the point should be obvious, without generators that would potentially consume a lot of memory.