Issue 13557: exec of list comprehension fails on NameError (original) (raw)
Created on 2011-12-08 20:25 by sdeibel, last changed 2022-04-11 14:57 by admin. This issue is now closed.
Messages (9)
Author: Stephan R.A. Deibel (sdeibel)
Date: 2011-12-08 20:25
Calling exec() on code that includes a list comprehension that references a defined local variable x fails incorrectly on "NameError: global name 'x' not defined".
Author: Amaury Forgeot d'Arc (amaury.forgeotdarc) * 
Date: 2011-12-09 14:10
This is expected and documented: http://docs.python.org/py3k/reference/executionmodel.html#interaction-with-dynamic-features "Free variables are not resolved in the nearest enclosing namespace, but in the global namespace.", a free variable being a variable "used in a code block but not defined there". And yes, a list comprehension defines a code block.
Try using exec(code, locals()). It's a good habit anyway to always pass a namespace to exec().
Author: Stephan R.A. Deibel (sdeibel)
Date: 2011-12-09 14:20
Ah, thanks, there it is... I thought this must be dealt with somewhere but couldn't find it. Maybe should add something to the 'exec' statement docs http://docs.python.org/py3k/library/functions.html#exec to reference this (from a usability-of-docs standpoint).
BTW, my code already sends the namespaces to exec (from current stack frame; it's part of a debugger) and probably would break other cases to alter this.
Well, anyway, sorry for the invalid bug report...
Author: Éric Araujo (eric.araujo) *
Date: 2011-12-10 16:32
Would you like to make a doc patch?
Author: Stephan R.A. Deibel (sdeibel)
Date: 2011-12-12 21:24
Here's a patch to the docs that notes the issue and refers the reader to the relevant execution model docs page. I have also attempted to clarify the "interaction with dynamic features" section of the execution model page.
Author: Terry J. Reedy (terry.reedy) *
Date: 2012-02-25 00:18
Issues like this, about exec, have come up multiple times. I just closed #14049 as a duplicate of this, and listed there some other issues. So I think that the doc for exec (and execfile in 2.7) could be better still. I would like to see something like the following added, whether instead of or in addition to the current proposal -- perhaps after "If provided, locals can be any mapping object." (quote from the 3.2 exec entry)
"At module level, globals and locals are the same dictionary. If one passes two separate objects as globals and locals, the code will be executed as if it were embedded in a class definition."
To me, this is friendlier and less intimidating than 'free variable' and the Execution model doc chapter. It summarizes the essential problem with such exec calls.
To illustrate, the following gives the same NameError. and for the same reason, as exec(code,{},{}), where code is the quoted unindented version with the class line deleted.
class x: x = 1 def incx(): return x+1 print(incx())
Author: Éric Araujo (eric.araujo) *
Date: 2012-02-25 05:30
Stefan: This fell off my radar, sorry I haven’t reviewed your patch yet.
Terry: +1
Author: Florent Xicluna (flox) *
Date: 2012-07-07 15:25
Issue #11796 marked as duplicate of this one.
However the issue described in #11796 does not involve exec/execfile. It is about scopes for list comprehension like this one.
Another doc patch should probably be written to cover the case described in #11796 too.
Author: Roundup Robot (python-dev) 
Date: 2012-07-08 21:53
New changeset ab22ffa6fb2e by Terry Jan Reedy in branch '2.7': Issue #13557: Clarify effect of giving two different namespaces to exec or http://hg.python.org/cpython/rev/ab22ffa6fb2e
New changeset ea670d71a36d by Terry Jan Reedy in branch '3.2': Issue #13557: Clarify effect of giving two different namespaces to exec or http://hg.python.org/cpython/rev/ea670d71a36d
New changeset b47ae7a9e685 by Terry Jan Reedy in branch 'default': Merge 3.2 closes issue 13557 http://hg.python.org/cpython/rev/b47ae7a9e685
History
Date
User
Action
Args
2022-04-11 14:57:24
admin
set
github: 57766
2012-11-25 18:03:32
mark.dickinson
link
2012-07-08 21:53:49
python-dev
set
status: open -> closed
nosy: + python-dev
messages: +
resolution: fixed
stage: needs patch -> resolved
2012-07-07 15:25:42
flox
set
messages: +
2012-07-07 15:21:17
flox
set
nosy: + rhettinger, michael.foord, Trundle, jonathan.hartley, flox, daniel.urban, westley.martinez, mjs0, josmiley
2012-07-07 15:20:51
flox
link
2012-02-25 05:30:18
eric.araujo
set
messages: +
2012-02-25 00🔞06
terry.reedy
set
nosy: + terry.reedy
messages: +
versions: + Python 2.7, Python 3.2
2012-02-25 00:15:24
terry.reedy
link
2011-12-12 21:24:52
sdeibel
set
files: + exec-eval-clarification.diff
keywords: + patch
messages: +
versions: - Python 2.7, Python 3.2
2011-12-10 16:32:41
eric.araujo
set
assignee: docs@python
components: + Documentation
versions: + Python 2.7, Python 3.3
keywords: + easy
nosy: + docs@python, eric.araujo
messages: +
resolution: not a bug -> (no value)
stage: needs patch
2011-12-09 14:20:53
sdeibel
set
status: pending -> open
messages: +
2011-12-09 14:10:58
amaury.forgeotdarc
set
status: open -> pending
nosy: + amaury.forgeotdarc
messages: +
resolution: not a bug
2011-12-08 20:25:54
sdeibel
create