Using A List Comprehension To Look Up Variables Works With Globals() But Not Locals(). Why?

I am updating a project from python 2.7 to python 3.6. I have a list comprehension that looks up variables from locals which worked in python 2.7. It only works in python 3.6 when

Solution 1:

If we dissemble a more basic list-comprehension. we can see why this is happening:

>>> import dis
>>> dis.dis("[i for i in range(10)]")
  10 LOAD_CONST               0 (<code object <listcomp>)
              3 LOAD_CONST               1 ('<listcomp>')
              6 MAKE_FUNCTION            09 LOAD_NAME                0 (range)
             12 LOAD_CONST               2 (10)
             15 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             18 GET_ITER
             19 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             22 RETURN_VALUE

So, on line 9, we see that a list-comp actually makes a function which is called (you may be more used to this when using generators). Therefore, the function would have its own set of local variables as it is in a separate scope - thus yielding this unexpected behaviour.

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Furthermore, as pointed out by @juanpa, the "What's New in Python3" docs do explicitly state the following:

list comprehensions have different semantics: they are closer to syntactic sugar for a generator expression inside a list() constructor, and in particular the loop control variables are no longer leaked into the surrounding scope.

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Note also that there is a bug page on this: https://bugs.python.org/issue21161, however juanpa has convinced me that this isn't a bug for the reason that it is clearly documented to work this way!

Solution 2:

List comprehensions define a new scope in Python 3, which they did not do in Python 2. Simply put, A, B, and C aren't local variables in the list comprehension in Python 2.

As an aside, you probably should have been using an explicit dict {'A': 1, 'B': 2, 'C': 3} instead of separate variables all along. Now is a good time to fix that.