How do I not make a list?

Diez B. Roggisch deets at
Thu Nov 29 09:36:50 CET 2007

Just Another Victim of the Ambient Morality schrieb:
>     It may sound like a strange question but that's probably only because I 
> don't know the proper terminology.  I have an iterable object, like a list, 
> and I want to perform a transform on it (do an operation on each of the 
> elements) and then pass it onto something else that expects and iterable. 
> I'm pretty sure this something else doesn't need a list, either, and just 
> wants to iterate over elements.
>     Now, I could just make a list, using a list comprehension, performing my 
> operation on each element, and then pass that list on, knowing that it is 
> iterable.  However, I was wondering if there was a way I can do virtually 
> this without having to actually allocate the memory for a list.  Creating a 
> stock iterator or generator or whatever it's called, with a passed in 
> operation?

You want a generator expression. Or a generator.

res = (apply_something(e) for e in my_iterable)


def g(mit):
    for e in mit:
        yield apply_something(e)

Both only get evaluated step by step during the iteration, reducing 
memory consumption.


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