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 Technician/Programmer Credit International
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