Classes derived from dict and eval

[Jeremy Sanders]

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(glo bals())
self['val'] = 42

def __getitem__(sel f, 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.

Thanks

Jeremy

[Robert Kern]

Re: Classes derived from dict and eval

Jeremy Sanders wrote:[color=blue]
> 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(glo bals())
> self['val'] = 42
>
> def __getitem__(sel f, 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.[/color]

In [1]: eval?
Type: builtin_functio n_or_method
Base Class: <type 'builtin_functi on_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@ucsd.edu

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

[Kent Johnson]

Re: Classes derived from dict and eval

Jeremy Sanders wrote:[color=blue]
> 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(glo bals())
> self['val'] = 42
>
> def __getitem__(sel f, 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.[/color]

Try Python 2.4.1:
Python 2.4.1 (#65, Mar 30 2005, 09:13:57) [MSC v.1310 32 bit (Intel)] on win32
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> class Foo(dict):[/color][/color][/color]
... def __init__(self):
... self.update(glo bals())
... self['val'] = 42
... def __getitem__(sel f, item):
... # this doesn't get called from the eval statement
... print "*", item
... return dict.__getitem_ _(self, item)
...[color=blue][color=green][color=darkred]
>>> a = Foo()
>>>
>>> print a['val'][/color][/color][/color]
* val
42[color=blue][color=green][color=darkred]
>>> print eval('val*2+6', a)[/color][/color][/color]
* val
90

Kent

[Jeremy Sanders]

Re: Classes derived from dict and eval

On Tue, 20 Sep 2005 13:59:50 -0700, Robert Kern wrote:
[color=blue]
> 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.[/color]

It seems that on Python 2.3 then neither globals or locals accessing by
eval calls the __getitem__ member of the dicts.

Jeremy