It would help over using a regular dict as a default argument to a function by preventing accidental mutation of the default or constant mapping. This is a quickly contrived example of the convert_price function now having a side effect by changing the translation_map. from unicodedata import normalize prices = [{'croissant': 1}, {'coffee': 3}] translation_map = {'apple': 'pomme', 'coffee': 'café'} def normalize_text(s): return normalize('NFD', s).encode('ascii', 'ignore').decode("utf-8") def usd_to_eur(v): return v / 1.2 def passthrough(v): return v def convert_price(record, convert_map=translation_map): # remove accents for price mapping. Oops! for key, value in convert_map.items(): convert_map[key] = normalize_text(value) record = { convert_map[k]: usd_to_eur(v) for k, v in record.items() } return record On Wed, Oct 10, 2018 at 6:24 PM Steven D'Aprano <steve@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@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/