Concise idiom to initialize dictionaries

[Frohnhofer, James]

exarkun at intarweb.us wrote:

>   This is actually not the problem being experienced.  The 
> actual problem is much more subtle and insidious.  Consider 
> this example:
> 
>     >>> locals()['x'] = 'y'
>     >>> print x
>     y
>     >>> 
> 
>   All appears well!  Mutating the object returned by locals() 
> lets us change arbitrary names in our local scope.  Great.  
> Now, let's go write a program:
> 
>     exarkun at boson:~$ cat > foo.py
>     def foo():
>         locals()['x'] = 'y'
>         print x
>     foo()
>     exarkun at boson:~$ python foo.py
>     Traceback (most recent call last):
>       File "foo.py", line 4, in ?
>         foo()
>       File "foo.py", line 3, in foo
>         print x
>     NameError: global name 'x' is not defined
>     exarkun at boson:~$ 
> 
>   "_Whaaaaat_?" you say.  "It's doing exactly the same 
> thing!" you say.  Well, not quite.

Exactly what happened.

> 
>   Mutating the objects returned by both locals() and 
> globals() is a no-no.  They are intended to provide _read 
> only_ access to those two scopes.  Since they return 
> dictionaries, you _can_ actually mutate them, though.  And 
> mutating globals() even works as expected; however, mutating 
> locals() does not.
> 
>   So why did it work in the interactive interpreter?  Well, 
> because of this nifty little fact:
> 
>     >>> locals() is globals()
>     True
>     >>> 
>

Ahhh. Now I see.
 
>   Oops.  At the top-level of the interactive interpreter, 
> there is no distinct local scope, just like at the top-level 
> of a module.  So the locals() function just returns the 
> globals dictionary, which happens to respect modifications.
> 
>   Kind of surprising, eh?
> 
>   Luckily for the original poster, namespaces aren't the only 
> dicts in town.  What he really wants to do is something like this:
> 

The number of dictionaries I need is 12.  I started out with a dictionary of
dictionaries, but my expressions (where I was actually using the dictionaries)
were pushed over the readibility edge by the extra level of indirection.  I
will probably stick with initializing them individually, preferring clear code
to compact code (while always trying for both).

Thanks to all who helped.

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