Function internally using exec(astring) with a variable definition inside astring can not return the variable in python 3

Question

I have a python 3 function that takes a string of commands, say 'np.pi' and then tries to define a variable with that string. Then I try to return the variable, this however does not work.

input = np.pi astring = 'funcD(funcC(funcB(funcA(input))))' def function(astring): astring= 'variable = ' + astring exec(astring) return variable In: a = function(astring) Out: NameError: global name 'variable' is not defined

Nothing seems to have happened. What I would like is to have the function return the output of the command in the string. The string contains several functions who have each other as input like below. I tried putting the exec after return without adding the variable = and then call the function with a = function(astring) but that did not work either. I believe I cant use eval because my string has functions in it.

Works for me, python 2.7.5 on Fedora 19. Do you actually call your function? — Andrey Sobolev
– Andrey Sobolev, Commented Sep 9, 2013 at 10:50
@AndreySobolev I did call it, I will add it to the question. I use python3, maybe thats the difference. — Leo
– Leo, Commented Sep 9, 2013 at 11:44
@JanneKarila Thank you for that link. I searched the site but did not find that one. Did not do a search with the term 'locals' as I did not know it before. It answers my question. I will flag my question as a duplicate. — Leo
– Leo, Commented Sep 9, 2013 at 11:46
The output? And assignment has no output. If you just want the assigned thing, don't use exec to assign it, just return its eval instead: return eval(astring). — Alfe
– Alfe, Commented Sep 9, 2013 at 12:09

Alfe · Accepted Answer · 2013-09-09 12:26:54Z

You didn't state what you expected from the answer, so I take a guess: You want to make this work.

Try it using eval:

def function(astring): astring= 'variable = ' + astring exec(astring) return eval('variable') function('42')

returns as expected 42.

Or simply strip that assignment:

def function(astring): return eval(astring)

Thank you. I added further explanation as per your request. I understood from other SO questions that I cant use eval because the commands in my string call functions.

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