Comment by ekiru | Hacker News Reader

Comment by ekiru | Hacker News Reader

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I don't think that's what's happening. In your example with the generator expression, you're calling each lambda as you iterate through the generator, which due to the lazy evaluation of the generator means that the value of the single i variable shared across all each lambda is still only the latest value reached.

If you instead fully evaluate the generator expression before calling any of the functions (for example, by passing it to the list constructor), you get the same behavior as the list comprehension case:

    >>> [f() for f in list(lambda: i for i in range(0, 10))]
    [9, 9, 9, 9, 9, 9, 9, 9, 9, 9]

You're right, but that means that sequence comprehension also leaks the variable, so it's even worse than I thought.

Side note: I think that commenters above didn't quite understand what I meant by "leaking", because there's more than one scope boundary here. Roughly speaking, any comprehension or loop can be desugared into something that looks like a C-style for-loop:

   /* scope 1 */
   for (/* scope 2 */) {
      /* scope 3 */
   }

Scope 1 is outside relative to the loop. Scope 2 is specific to the loop but shared by all its iterations. Scope 3 is specific to one loop iteration. The "leaking" I referred to above is from scope 3 to scope 2. I think other commenters took it to mean leaking from scope 2 to scope 1 - i.e. the ability to use the variable outside of the comprehension; that is, indeed, something that changed between Python 2 and 3.

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