`types.MappingProxyType` is documented as 'Read-only proxy of a mapping'.
But if used with a `collections.defaultdict` mapping, it can modify the underlying mapping.
```
import collections, types
dd = collections.defaultdict(set)
mpt = types.MappingProxyType(dd)
mpt['__getitem__'] # key inserted
mpt.get('get') # key not inserted
print(dd.items()) # dict_items([('__getitem__', set())])
```

> `types.MappingProxyType` is documented as 
> 'Read-only proxy of a mapping'.  But if used with a 
> `collections.defaultdict` mapping, it can modify 
> the underlying mapping.

Technically, the underlying mapping is modifying itself.  That is allowed.

Also, there isn't really anything that the mapping proxy can do about it.  All MappingProxy can do it forward calls to methods that are usually non-mutating.