[Python-Dev] Python3 compiled listcomp can't see local var - bug or feature?

[Xavier de Gaye]

On 06/06/2018 03:51 PM, Nick Coghlan wrote:
 > On 6 June 2018 at 15:31, Rob Cliffe via Python-Dev <python-dev at python.org <mailto:python-dev at python.org>> wrote:
 > ...
 >     *In other words, it looks as if in Python 3.6.5, the compiled list comprehension**
 >     **can "see" a pre-existing global variable but not a local one.*
 >
 > Yes, this is expected behaviour - the two-namespace form of exec (which is what you get implicitly when you use it inside a function body) is similar to a class body, and hence nested functions
 > (including the implicit ones created for comprehensions) can't see the top level local variables.


In issue 13557 [1] Amaury gives the following explanation by quoting the documentation [2] "Free variables are not resolved in the nearest enclosing namespace, but in the global namespace" and hints 
at the same solution that is proposed by Nick.

FWIW in issue 21161 [3] folks have been bitten by this when trying to run a list comprehension in pdb.

[1] https://bugs.python.org/issue13557
[2] http://docs.python.org/py3k/reference/executionmodel.html#interaction-with-dynamic-features
[3] https://bugs.python.org/issue21161

Xavier