[Python-Dev] Add a frozendict builtin type

[Victor Stinner]

Le 01/03/2012 19:07, Guido van Rossum a écrit :
> What other use cases are there?

frozendict could be used to implement "read-only" types: it is not 
possible to add or remove an attribute or set an attribute value, but 
attribute value can be a mutable object. Example of an enum with my 
type_final.patch (attached to issue #14162).

 >>> class Color:
...   red=1
...   green=2
...   blue=3
...   __final__=True
...
 >>> Color.red
1
 >>> Color.red=2
TypeError: 'frozendict' object does not support item assignment
 >>> Color.yellow=4
TypeError: 'frozendict' object does not support item assignment
 >>> Color.__dict__
frozendict({...})

The implementation avoids the private PyDictProxy for read-only types, 
type.__dict__ gives directly access to the frozendict (but 
type.__dict__=newdict is still blocked).

The "__final__=True" API is just a proposition, it can be anything else, 
maybe a metaclass.

Using a frozendict for type.__dict__ is not the only possible solution 
to implement read-only types. There are also Python implementation using 
properties. Using a frozendict is faster than using properties because 
getting an attribute is just a fast dictionary lookup, whereas reading a 
property requires to execute a Python function. The syntax to declare a 
read-only class is also more classic using the frozendict approach.

Victor