[Python-ideas] Revisiting Immutable Mappings

Philip Martin philip.martin2007 at gmail.com
Wed Oct 10 21:25:38 EDT 2018

Steven, that's a great idea, and I would be 100% up for your suggestion to
have types.MappingProxyType renamed to frozendict. However, the differences
in the behavior of MappingProxyType's constructor versus dict's would make
the API's behavior confusing IMO. For example, MappingProxyType(x=5, y=10)
throws a TypeError. I don't think most people would expect this.

MappingProxyType to me though does seem to be a non-obvious name compared
to say frozenset as you have mentioned. Plus, it's included in a module
that I would say is very low level alongside functions like prepare_class,
new_class, etc.

On Wed, Oct 10, 2018 at 6:24 PM Steven D'Aprano <steve at pearwood.info> wrote:

> Hi Philiip, and welcome,
> On Wed, Oct 10, 2018 at 12:04:48PM -0500, Philip Martin wrote:
> > I generally have used MappingProxyType as a way to set default mapping
> > to a function or to set an empty mapping to a function.
> > I've created a gist with an example use case:
> >
> > https://gist.github.com/pmart123/493edf84d9aa61691ca7321325ebb6ab
> Please try to keep the discussion in one place (i.e. here), for the
> benefit of the archives and for those who have email access but not
> unrestricted web access.
> Can you explain (in English) your use-case, and why MappingProxyType
> isn't suitable? If it *is* suitable, how does your proposal differ?
> If the only proposal is to rename types.MappingProxyType to a builtin
> "frozendict", that's one thing; if the proposal is something else, you
> should explain what.
> > I've included an example of what code typically looks like when using
> > MappingProxyType and what it could look like with a
> > frozendict implementation.
> Wouldn't that be just:
>     from types import MappingProxyType as frozendict
>     d = frozendict({'spam': 1, 'eggs': 2})
> versus:
>     d = frozendict({'spam': 1, 'eggs': 2})
> Apart from the initial import, how would they be different? You want a
> frozendict; the existing MappingProxyType provides a frozendict (with a
> surprising name, but never mind...). Wouldn't you use them exactly the
> same way? They both (I presume) offer precisely the same read-only
> access to the mapping interface.
> --
> Steve
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20181010/d29ef3be/attachment.html>

More information about the Python-ideas mailing list