[issue36426] exec() issue when used inside function

[Nick Coghlan]

Nick Coghlan  added the comment:

This is not a bug - to enable function level optimisations, the compiler must 
be able to see all local variable names at compile time.

In Python 2.x the exec statement implementation included special code to allow 
even function local variables to be rebound, but that special-casing was 
removed as part of the Python 3 change to convert exec from a statement to a 
builtin function (as per 
https://www.python.org/dev/peps/pep-3100/#core-language )

This means that to use exec() and reliably see the changes it makes to a 
namespace, you have to supply a reliably read/write dictionary of your own. 

Object instance dictionaries work well for this purpose, as you can then access 
the results as attributes on the instance:

```
>>> class Namespace:
... pass
... 
>>> def f():
... ns = Namespace()
... exec("x = 1; y = 2", vars(ns))
... print(ns.x)
... print(ns.y)
... 
>>> f()
1
2
```

--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue36426] exec() issue when used inside function

[Domen Jurkovič]

Domen Jurkovič  added the comment:

Are there any workarounds - how can we use exec() and on-the-fly created 
variables?
Also, why 'locals()['bar']' is seen by the interpretter, but not ony 'bar'?

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[issue36426] exec() issue when used inside function

[Terry J. Reedy]

Terry J. Reedy  added the comment:

This is not a bug; the behavior is covered in the docs.  I am surprised that 
this works in 2.7, as it has the same warning as 3.7:
"
locals()
Update and return a dictionary representing the current local symbol table. 
Free variables are returned by locals() when it is called in function blocks, 
but not in class blocks.
Note The contents of this dictionary should not be modified; changes may not 
affect the values of local and free variables used by the interpreter."

--
nosy: +terry.reedy
type: behavior -> enhancement

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[issue36426] exec() issue when used inside function

[Steve Dower]

Steve Dower  added the comment:

This is currently by design, which means 3.8 is likely the only viable place it 
can change. It's also not Windows specific so I removed that component (people 
may remove themselves from nosy).

But +Nick, since I know he has some interest in making locals() behave more 
consistently. Currently it's basically a read-only proxy, as locals are 
optimized within functions which is why you can't see updates via the duct.

--
components:  -Windows
nosy: +ncoghlan
versions: +Python 3.8, Python 3.9 -Python 2.7, Python 3.5, Python 3.6, Python 
3.7

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[issue36426] exec() issue when used inside function

[Emmanuel Arias]

Emmanuel Arias  added the comment:

But I confirmed the behavior reported


uhmm weird

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[issue36426] exec() issue when used inside function

[Emmanuel Arias]

Emmanuel Arias  added the comment:

I test on 3.5 and 3.8 running not in an func and I don't have the problem:

Python 3.8.0a2+ (heads/bpo-36287:ba8f342623, Mar 25 2019, 21:57:16) 
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 'bar'
>>> b = 42
>>> c  = a + ' = c'
>>> c
'bar = c'
>>> exec(c)
>>> 'bar' in locals()
True
>>> print(locals()['bar'])
bar = c
>>> print(bar)
bar = c
>>> print("'bar' OK:", bar)
'bar' OK: bar = c

--
nosy: +eamanu

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[issue36426] exec() issue when used inside function

[Domen Jurkovič]

Domen Jurkovič  added the comment:

Seems like I don't know how to write a code here. 
Anyway, issue created on stack overflow can be found on:
https://stackoverflow.com/questions/55239875/python-exec-function-broken-in-versions-above-2-7-error-name-not-defined/5524?noredirect=1#comment97362021_5524

Works on 2.7, fails on everything above 3.x

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[issue36426] exec() issue when used inside function

[Domen Jurkovič]

New submission from Domen Jurkovič :

I've reported a stack overflow question and no reasonable explation was 
offered. Here is what I've discovered:
.. code-block:: python
def func():
varName = 'bar'
varValue = 42
localVarToEvaluate = varName + ' = varValue'

try:
exec(localVarToEvaluate)
except Exception as err:
print(str(err))

if 'bar' in locals():
# print(locals()['bar']) # (1) OK
# print(bar)  # (2) ERR
#print("'bar' OK:", bar)  # (3) ERR
pass # uncomment any line above

func()

After ``exec()`` is executed, ``bar`` can be seen in ``locals()``, but not 
accessible by intereter. Also, It can be accessed by directly calling 
``print(locals()['bar'](``, but not ``print(bar)``.

This is the problem as long as the code is wrapped in function. If the same 
code is placed in the module body, works as expected. 

Is there any exaplanation for such behaviour, or is this a bug?

--
components: Interpreter Core, Windows
messages: 338812
nosy: paul.moore, schperplata, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: exec() issue when used inside function
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6, Python 3.7

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