[Python-Dev] Add a frozendict builtin type

Thu Mar 1 14:44:18 CET 2012

01.03.12 11:11, Victor Stinner написав(ла):
>> You can redefine dict.__setitem__.
> Ah? It doesn't work here.
>>>>> dict.__setitem__=lambda key, value: None
> Traceback (most recent call last):
> File "<stdin>", line 1, in<module>
> TypeError: can't set attributes of built-in/extension type 'dict'

Hmm, yes, it's true. It was too presumptuous of me to believe that you 
have not considered such simple approach.
But I will try to suggest another approach. `frozendict` inherits from 
`dict`, but data is not stored in the parent, but in the internal 
dictionary. And even if dict.__setitem__ is used, it will have no 
visible effect.
 class frozendict(dict):
 def __init__(self, values={}):
 self._values = dict(values)
 def __getitem__(self, key):
 return self._values[key]
 def __setitem__(self, key, value):
 raise TypeError ("expect dict, got frozendict")
 ...
 >>> a = frozendict({1: 2, 3: 4})
 >>> a[1]
 2
 >>> a[5]
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 5, in __getitem__
 KeyError: 5
 >>> a[5] = 6
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 7, in __setitem__
 TypeError: expect dict, got frozendict
 >>> dict.__setitem__(a, 5, 6)
 >>> a[5]
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 5, in __getitem__
 KeyError: 5