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
I also have a series of unit tests for it if you're interested in them.
--
Steven
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