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I want to subclass dict in python such that all the dictionaries of the sub-class are immutable.
I don't understand how does __hash__ affects the immutability, since in my understanding it just signifies the equality or non-equality of objects !
So, can __hash__ be used to implement immutability ? How ?
Update:
Objective is that common response from an API is available as a dict, which has to be shared as a global variable. So, that needs to be intact no matter what ?
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Values can be any type of object, but keys must be immutable. This means keys could be integers, strings, or tuples, but not lists, because lists are mutable. Dictionaries themselves are mutable, so entries can be added, removed, and changed at any time.
All you have to do is sub-class from Freezer . After initialization of the Python, I “freeze” the object with my overriding of the __delattr__ and __setattr__ methods on the object. I set _frozen = True which indicates the instance is now “frozen”. objects become immutable.
Second, a dictionary key must be of a type that is immutable. For example, you can use an integer, float, string, or Boolean as a dictionary key. However, neither a list nor another dictionary can serve as a dictionary key, because lists and dictionaries are mutable.
I found a Official reference : suggestion contained in a rejected PEP.
class imdict(dict):
def __hash__(self):
return id(self)
def _immutable(self, *args, **kws):
raise TypeError('object is immutable')
__setitem__ = _immutable
__delitem__ = _immutable
clear = _immutable
update = _immutable
setdefault = _immutable
pop = _immutable
popitem = _immutable
Attribution : http://www.python.org/dev/peps/pep-0351/
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