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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Dynamic variable creation from string
Date: Wed, 07 Dec 2011 17:59:33 -0500
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On 12/7/2011 12:09 PM, Massi wrote:

> in my script I have a dictionary whose items are couples in the form
> (string, integer values), say
>
> D = {'a':1, 'b':2, 'c':3}
>
> This dictionary is passed to a function as a parameter, e.g. :
>
> def Sum(D) :
>      return D['a']+D['b']+D['c']
>
> Is there a way to create three variables dynamically inside Sum in
> order to re write the function like this?
>
> def Sum(D) :
>      # Here some magic to create a,b,c from D
>      return a+b+c

No. The set of local names for a function is determined when the 
definition is executed and the body is compiled. Python 2 had an 
exception -- 'from x import *' -- that required an alternate 
complilation pathway. That possibility was eliminated in Python 3 and is 
now a syntax error.

Within functions, 'locals().update(D)' is more or less guaranteed to 
*not* add local names to the local namespace, even if it does add keys 
to the locals() dict that shadows the local namespace.

 > It is really important that the scope of a,b,c is limited to the Sum
 > function, they must not exisit outside it or inside any other nested
 > functions.

Local names, by definition, are lexically scoped to the function 
definition and are not visible without. Since nested definitions are 
part of that lexical scope, local names are always visible within nested 
definitions. You cannot stop that. The association of local names is 
usually dynamically limited to one function call. The two exceptions are 
enclosure by a nested function that survives the function call and 
generators in a paused state.

-- 
Terry Jan Reedy