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
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