pythondictionaryreadonlyreadonly-collection

How to make dictionary read-only?

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

Solution

  • 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)