Command exec in a function, name error - python3

Question

I'm begginer in Python and I'm in front of a problem that I can't understand. I tried to just define a variable with a exec() and then just print it. And it's working well. BUT when I do the same code in a function, it's not working...

Example :

def fonct():
    possibilite = [0,1,2,3]
    testc = 0
    testl = 1
    commande = "proba"+str(testc+1)+str(testl)+" = possibilite"
    exec(commande)
    print(commande)
    print(proba11)

The same thing but not in a function has for result, that the command print(proba11) returns [0,1,2,3] so it works. But for the example I got this :

proba11 = possibilite
NameError: name 'proba11' is not defined

There is no stories about globals or locals, everything is local...

There is no variable named proba11, that's why you have this error. First, define proba11 if you want to print it — PCM
– PCM, Commented Jun 8, 2021 at 5:48
But when I do the same code, without defining proba11 but not in a function it's working. So I was thinking that it was ok to not define it, I will try, thank you. — LeoZ
– LeoZ, Commented Jun 8, 2021 at 5:53
proba11 = possibilite came because of this code commande = "proba"+str(testc+1)+str(testl)+" = possibilite". It is just a string, not a variable — PCM
– PCM, Commented Jun 8, 2021 at 5:55
Ok, I tried, in the first lines of the function, I wrote proba11 = [0]. And the print(proba11) returns [0] and not [0,1,2,3].... So I think the exec() is not working as expected but I don't know why... I used it in other conditions and this is the only place I got problems.... — LeoZ
– LeoZ, Commented Jun 8, 2021 at 5:59

rdas · Accepted Answer · 2021-06-08 06:02:39Z

1

Updating the local variables with exec() in Python 3 is tricky due to the way local variables are stored. It used to work in Python 2.7 and earlier.

To workaround this, you need to

Pass an explicit locals dictionary to exec
Grab the newly defined variable from the updated locals dictionary

Like so:

def fonct():
    possibilite = [0, 1, 2, 3]
    testc = 0
    testl = 1
    varname = "proba" + str(testc + 1) + str(testl)
    commande = varname + " = possibilite"
    _locals = locals()
    exec(commande, globals(), _locals)
    proba11 = _locals[varname]
    print(proba11)

Which works as expected.

You can read more about it here:

answered Jun 8, 2021 at 6:02

rdas

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1 Comment

LeoZ Over a year ago

Thank you very much ! It was just a trick a didn't know about, thank you for teaching me this !

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