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Comment by timtadh | Hacker News Reader

@chadcf and @tptacek

I was responding to @tptacek criticism of the parent not the deck. The deck is great and it mirrors the wisdom I have picked up from optimizing my own code over the years. I personally find it really frustrating not being able to easily pre-alloc lists in Python. I think that having better APIs would go a long way.

As the deck says:

"Line for line these languages are fast!"

"We need better no-copy/preallocate APIs"

"Take care in data structures"

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Forgive the naive question, but why not:

    l = [object()] * 100

Perhaps the difference is stack vs. heap?

That will create a list of 100 instances of the same object.

  object[0].x = 1
  print object[1].x
  > 1

Edit: On second read, it looks like you're asking something other than what I thought you were asking. Yes, you could create a list of 100 items and then replace its elements, but that's not idiomatic.

but it is idiomatic in C, which is the point of the slide. C was built around a performance focused idiom, which is to pre-allocate memory and then do in place writes and swaps to mutate the buffer to the state you need it to be. Python is built around an idiom of largely creating copies of objects and appending them to dynamically allocated lists. Its a much slower idiom.

Yes, the question was regarding this line in the original post: "I personally find it really frustrating not being able to easily pre-alloc lists in Python."

So my mind of course wandered in the direction of how to do that.

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