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Comment by anonymoushn | Hacker News Reader

Global by default and block level scope seems like the right choice. In a popular dynamic language that uses local by default and function level scope, the language had to include two keywords for accessing variables other than locals, and the following becomes a bug when it would not be a bug in any sane language:

  x = calculate_n_dogs()
  # later
  ok = all(x.status_code == 200 for x in my_other_responses)
  # later
  let_the_dogs_out(x)
  # oh no, why isn't x n_dogs anymore

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This is clearly about Python, so I will note that this is not actually the case: The generator expression introduces a new scope, and so it does not reassign the value of x. Python does not require the "global" and "nonlocal" keywords in order to access variables, but to reassign names brought in from an outer scope.

Python 2 did have the issue where list comprehensions (though still not generator expressions) didn't introduce a new scope, and so you could indeed get issues quite similar to your example above. This was corrected in Python 3.

All that said, I will agree that Python's scoping rules are pants-on-head crazy. In very nearly every other programming language under the sun, you declare variables at the point where they exist. In Python, you declare a variable when it exists somewhere else (and you then want to reassign it). This does mean that you don't need to put a "local" or "var" or what-have-you in front of each new variable you declare (which is basically the reason Python did it this way), but the semantics of the thing can take some getting used to.

I appreciate the correction and I'm glad to know this is fixed in python 3!

I've run into similar issues where the variable was used in a for loop declaration rather than a list comprehension, but these are a bit easier to spot. Also, maybe people who don't use lua constantly won't be in the habit of thinking they can safely shadow locals with their looping variables.

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