[Python-Dev] Dictionary subclasses and exec

[Guido van Rossum]

> The following bit of code defines a dictionary that folds all its keys
> to lowercase before inserting them, but 'exec "Name = 1" in
> LowerCaseDict()' doesn't seem to call my __getitem__(), as the listed
> keys include 'Name'.  Should this be expected to work?
> 
> --amk
> 
> class LowerCaseDict(dictionary):
>     def _fold_key (self, key):
>         if not isinstance(key, str):
>             raise TypeError, "All keys must be strings"
>         return key.lower()
>     
>     def __getitem__ (self, key):
>         key = self._fold_key(key)
>         return dictionary.__getitem__(self, key)
> 
>     def __setitem__ (self, key, value):
>         key = self._fold_key(key)
>         dictionary.__setitem__(self, key, value)
> 
>     def __delitem__ (self, key):
>         key = self._fold_key(key)
>         dictionary.__delitem__(self, key, value)
> 
> d = LowerCaseDict()
> exec 'Name = 1' in d
> print d.keys()

Alas, this is one of the things that don't work yet.  To set and get
local variables, exec uses lower-level APIs (PyDict_SetItem and
PyDict_GetItem) that you can't override.  I've thought about what it
would take to make this work as desired, but I haven't found a way yet
that wouldn't (a) slow down the normal case, or (b) create subtle
reference count bugs.

--Guido van Rossum (home page: http://www.python.org/~guido/)