eval and dict

[Anton Muhin]

Steffen Ries wrote:
> Hi,
> 
> I am trying to implement lazy evaluation of user provided expressions.
> 
> I'm using "eval(expression, dictionary)" with multiple calls
> evaluating different expressions using the same context. It turned out
> that creating the dictionary up front is a performance bottleneck.
> 
> Since I typically don't need all possible value in the dictionary, I
> was trying to defer creating the entries until an expression actually
> needs the entry.
> 
> I tried to implement a delegating dictionary, but "eval(exp,
> lazyDict)" complains "TypeError: eval() argument 2 must be dict, not
> instance"
> 
> I was using a class like this:
> 
> class LazyDict:
>     def __init__(self, source):
>         self.source = source
>         self.dict = {}
>     def __getitem__(self, key):
>         if self.dict.has_key(key):
>             return self.dict[key]
>         # lookup key in self.source, cache entry in self.dict[key]
>         # return real value
> 
> For my second try I was extending dict, but that did not work
> either. In this case I got around the TypeError, but my overloaded
> __getitem__ was not called, e.g. (just to demonstrate, I drop the
> pretense of getting the real value)
> 
> class LazyDict(dict):
>     def __init__(self):
>         dict.__init__(self)
>     def __getitem__(self, key):
>         print "__getitem__", key
>         return 1
> 
> 
>>>>d = LazyDict()
>>>>eval('a+b', d)
> 
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   File "<string>", line 0, in ?
> NameError: name 'a' is not defined
> 
>>>>d['a']
> 
> __getitem__ a
> 1
> 
> Is there a way to bypass this behavior of eval()?
> 
> At the end of the day, I need to do this in Jython, where it actually
> works the way I expect it, but I would like to understand what's going
> on.
> 
> TIA
> /steffen

A couple of weeks ago the similar question arised and someone said that 
it's well-known limitation of eval. As a workaround I can suggest to use 
compile function, analyse co_names and create Proxy objects.

hth,
anton.