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

[Steven Bethard]

Peter Hansen wrote:
Nick Coghlan wrote:
Generally, altering the contents of the dicts returned by locals() and 
globals() is unreliable at best.

Nick, could you please comment on why you say this about globals()?
I've never heard of any possibility of "unreliability" in updating
globals() and, as far as I know, a large body of code exists which
does in fact rely on this -- much of mine included. ;-)
Updating locals() is unreliable.  Updating globals() is fine, AFAIK.
http://docs.python.org/lib/built-in-funcs.html
I believe that the only time that locals() is updatable is when locals() 
is globals():

>>> locals() is globals()
True
>>> x
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
NameError: name 'x' is not defined
>>> locals()['x'] = 3
>>> x
3
>>> def f():
...     print locals() is globals()
...     locals()['x'] = 3
...     print x
...
>>> f()
False
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File "<interactive input>", line 4, in f
NameError: global name 'x' is not defined
Steve
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