[Python-Dev] Add a frozendict builtin type (original) (raw)

Guido van Rossum guido at python.org
Fri Mar 2 01:50:06 CET 2012

On Thu, Mar 1, 2012 at 4:39 PM, Victor Stinner <victor.stinner at gmail.com> wrote:

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 typefinal.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.

I think you should provide stronger arguments in each case why the data needs to be truly immutable or read-only, rather than just using a convention or an "advisory" API (like __private can be circumvented but clearly indicates intent to the reader).

-- --Guido van Rossum (python.org/~guido)

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