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Comment by koide | Hacker News Reader

Javascript has true closures, Python doesn't (or didn't?)

Python has closures, it just doesn't have a very convenient anonymous function syntax. I can't say I miss function () { .. } very often though.

AFAIU Python's closure have been incomplete for a long time. I think they've become true, complete closures after the addition of the nonlocal keyword on 3.0

Please correct me if I'm mistaken. This all comes from a cursory research on the subject.

To my eyes they always were true and complete, but you had to play the minor trick of using mutable data to get them. Here is an example of what I mean that has worked forever.

  def outer ():
      counter = [0]
      def inner ():
          counter[0] += 1
          return counter[0]
      return inner

I think I heard that this wasn't working around 2.0 or earlier... but I'm not curious enough to go research and I wasn't using Python until 2.4. Can you maybe confirm or deny this?

I've never heard that. I also did not use Python until 2.4.

In short, in your opinion, Python has no "complete" closures because you cannot bind a new value to the name from enclosing lexical scope from within a nested scope. I disagree.

This is not possible in Haskell either. You never hear people claiming Haskell doesn't support closures.

Which is why I disagree with claims that Python doesn't support them either. It does, of course; we could argue if Python supports "complete" closures, but we won't, because we know better than to use meaningless terms in discussion, right?

In any case it's not been the case in a while, so I stand corrected.

Although being able to write on the closed over variable is a neat feature to have, I would argue that feature does make a difference. Whether you call that complete or read-write or however you want to.

I like Python's scoping rules but declaring functions inline is a lot nicer and nicer-looking than defining functions starting with underscores.

That is a false dichotomy. There are plenty of nice ways of working with closures with the given syntax.

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