How to make dictionary read-only in Python

Question

I have a class like:

class A:     def __init__(self):         self.data = {} 

and at some moment I want to prohibit self.data fields modification.

I've read in PEP-416 rejection notice that there are a lot of ways to do it. So I'd like to find what they are.

I tried this:

a = A() a.data = types.MappingProxyType(a.data) 

That should work but first, its python3.3+ and second, when I do this "prohibition" multiple times I get this:

>>> a.data = types.MappingProxyType(a.data) >>> a.data = types.MappingProxyType(a.data) >>> a.data mappingproxy(mappingproxy({})) 

though it would be much better to get just mappingproxy({}) as I am going to "prohibit" a lot of times. Check of isinstance(MappingProxyType) is an option, but I think that other options can exist.

Thanks

Answer 1

Use collections.Mapping e.g.

import collections  class DictWrapper(collections.Mapping):      def __init__(self, data):         self._data = data      def __getitem__(self, key):          return self._data[key]      def __len__(self):         return len(self._data)      def __iter__(self):         return iter(self._data)