[Python-Dev] Add a frozendict builtin type (original) (raw)

Victor Stinner victor.stinner at gmail.com
Thu Mar 1 15:49:29 CET 2012

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")  ...

I would like to implement frozendict in C to be able to pass it to PyDict_GetItem(), PyDict_SetItem() and PyDict_DelItem(). Using such Python implementation, you would get surprising result:

d = frozendict() dict.setitem(d, 'x', 1) # this is what Python does internally when it expects a dict (e.g. in ceval.c for builtins) 'x' in d => False

(Python is not supposed to use the PyDict API if the object is a dict subclass, but PyObject_Get/SetItem.)

Victor

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