updating locals() and globals() (WAS: How do I do this? (eval() on the left hand side))

Jeff Shannon jeff at ccvcorp.com
Thu Dec 9 00:53:18 CET 2004

Steven Bethard wrote:

> I remember hearing an explanation of why locals() is not writable that 
> had to do with something about efficiency and the call stack (or 
> something like that)...

IIRC, the function local namespace (in CPython) is kept internally as a 
static array, rather than as a dictionary, which dramatically speeds 
(function-local) name lookups.  However, this also means that all local 
names must be determinable at function-definition time.  In contrast, 
the global namespace is a true Python dictionary.  Where globals() 
returns a reference to that dictionary, locals() (when locals() is not 
globals()) constructs a separate dictionary from the static array 
containing the local namespace.  In effect, globals() gives a reference 
while locals() gives a copy, with standard Python reference/copy 
semantics applying.

(Note also that functions which use exec cannot use the static namespace 
optimization, and thus tend to be *much* slower than normal functions 
(in addition to being a huge security problem).  I don't know, however, 
whether locals() can update the local namespace in such un-optimized 
functions.  Whether it actually does so or not, relying on such 
special-case behavior would be extremely foolish to do...)

Of course, I'm just going off of what I remember having been said 
elsewhere on c.l.p., and I wouldn't recommend anyone betting their life 
savings on the reliability of my memory... :)

Jeff Shannon
Credit International

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