embedding python in python

[Maurice LING]

> 
>  >>> myglobals = {}
>  >>> mylocals = {}
>  >>> exec "b = 1 + 2" in myglobals, mylocals
>  >>> mylocals
> {'b': 3}
>  >>> exec "c = b * 2" in myglobals, mylocals
>  >>> mylocals
> {'c': 6, 'b': 3}
>  >>> >>>
> 
> The exec statement can take an optional pair of dictionaries that it 
> uses as the global and local namespaces to execute the given code in.  
> This is *much* preferable to a bare exec for several reasons, one of 
> them being exactly your issue, and another being safety -- since you 
> must explicitly include any names that you want exec to use, it makes 
> exec rather less likely to unintentionally stomp all over your namespace 
> (or for malicious code to do certain types of harm).
> 
> Jeff Shannon
> Technician/Programmer
> Credit International
> 

Please pardon me for my dumbness but what exactly goes on in


exec "b = 1 + 2" in myglobals, mylocals

?

Thanks
Maurice