In <pan.2009.08.04.03.23...@remove.this.cybersource.com.au> Steven D'Aprano
<ste...@remove.this.cybersource.com.au> writes:
>On Mon, 03 Aug 2009 21:07:32 +0000, kj wrote:
>> I use the term "no-clobber dict" to refer to a dictionary D with the
>> especial property that if K is in D, then
>>
>> D[K] = V
>>
>> will raise an exception unless V == D[K]. In other words, D[K] can be
>> set if K doesn't exist already among D's keys, or if the assigned value
>> is equal to the current value of D[K]. All other assignments to D[K]
>> trigger an exception.
>Coincidentally, I just built something very close to what you ask. Here
>it is:
>class ConstantNamespace(dict):
> """Dictionary with write-once keys."""
> def __delitem__(self, key):
> raise TypeError("can't unbind constants")
> def __setitem__(self, key, value):
> if key in self:
> raise TypeError("can't rebind constants")
> super(ConstantNamespace, self).__setitem__(key, value)
> def clear(self):
> raise TypeError('cannot unbind constants')
> def pop(self, key, *args):
> raise TypeError("cannot pop constants")
> def popitem(self):
> raise TypeError("cannot pop constants")
> def update(self, other):
> for key in other:
> if key in self:
> raise TypeError('cannot update constants')
> # If we get here, then we're not modifying anything,
> # so okay to proceed.
> super(ConstantNamespace, self).update(other)
> def copy(self):
> c = self.__class__(**self)
> return c
Thanks. As you note this not quite what I'm looking for, but it's
a good template for it.
kynn
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