Classes derived from dict and eval

[Robert Kern]

Jeremy Sanders wrote:
> Hi -
> 
> I'm trying to subclass a dict which is used as the globals environment of
> an eval expression. For instance:
> 
> class Foo(dict):
>     def __init__(self):
>         self.update(globals())
>         self['val'] = 42
>                  
>     def __getitem__(self, item):
>         # this doesn't get called from the eval statement
>         print "*", item
>         return dict.__getitem__(self, item)
> 
> a = Foo()
> 
> print a['val']
> print eval('val*2+6', a)
> 
> The first print statements also prints "* val", but __getitem__ is never
> called by the evaluation in the eval statement.
> 
> Is this a bug? Does anyone have an idea for a workaround? I'm using
> Python 2.3.3.

In [1]: eval?
Type:           builtin_function_or_method
Base Class:     <type 'builtin_function_or_method'>
String Form:    <built-in function eval>
Namespace:      Python builtin
Docstring:
    eval(source[, globals[, locals]]) -> value

    Evaluate the source in the context of globals and locals.
    The source may be a string representing a Python expression
    or a code object as returned by compile().
    The globals must be a dictionary and locals can be any mappping,
    defaulting to the current globals and locals.
    If only globals is given, locals defaults to it.

globals needs to be a real dictionary. The implementation uses the C
API, it doesn't use the overridden __getitem__. The locals argument,
apparently can be some other kind of mapping.

-- 
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
 Are the graves of dreams allowed to die."
  -- Richard Harter