[Python-Dev] Add a frozendict builtin type

[Yury Selivanov]

On 2012-03-01, at 7:50 PM, Guido van Rossum wrote:
> 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).


Here's one more argument to support frozendicts.

For last several months I've been thinking about prohibiting coroutines
(generators + greenlets in our framework) to modify the global state.
If there is a guarantee that all coroutines of the whole application, 
modules and framework are 100% safe from that, it's possible to do some 
interesting stuff.  For instance, dynamically balance jobs across all 
application processes:

@coroutine
def on_generate_report(context):
    data = yield fetch_financial_data(context)
    ...

In the above example, 'fetch_financial_data' may be executed in the
different process, or even on the different server, if the coroutines'
scheduler of current process decides so (based on its load, or a low 
priority of the coroutine being scheduled).

With built-in frozendict it will be easier to secure modules or
functions' __globals__ that way, allowing to play with features closer
to the ones Erlang and other concurrent languages provide.

-
Yury