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 I switch to using globals.
Below is a toy example to illustrate the issue.
The relevant code is:
(A,B,C) = (1,2,3)
myvars = ['A','B','C']
If I execute the following code:
[locals().get(var) for var in myvars]
the return value in python 3.6 is:
[None, None, None]
However, the return value in python 2.7 is:
[1, 2, 3]
If I execute the following code using globals:
[globals().get(var) for var in myvars]
then I get the same result in python 2.7 and 3.6:
[1, 2, 3]
Can anyone explain why the code using locals() no longer works in python 3.6?
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)]")
1 0 LOAD_CONST 0 (<code object <listcomp>)
3 LOAD_CONST 1 ('<listcomp>')
6 MAKE_FUNCTION 0
9 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.
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.
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!
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