On Aug 13, 2019, at 15:01, Eric V. Smith <e...@trueblade.com> wrote:
>
>> On 8/13/2019 5:49 PM, Andrew Barnert via Python-ideas wrote:
>> But I think the lazy-import-decimal-on-first-dump-with-use_decimal solves
>> that, and solves it even better than __json__, even besides the fact that
>> it’s a better API than exposing “dump raw text into any JSON, and it’s up to
>> you to get it right”.
>> No import time if you’re not using it, just setting a global to None. Even
>> if you are using it, the cost of importing it from the sys.modules cache is
>> pretty tiny. (After all, you won’t have any Decimal objects without having
>> imported decimal, unless you do some nasty tricks—at which point
>> monkeypatching json to fake the import isn’t any nastier.)
>
> dataclasses does something similar: it wants to know if something is a
> typing.ClassVar, but it doesn't want to import typing to find out. So it has:
>
> typing = sys.modules.get('typing')
> if typing:
> if (_is_classvar(a_type, typing)
> or (isinstance(f.type, str)
> and _is_type(f.type, cls, typing, typing.ClassVar,
> _is_classvar))):
That’s clever, but I don’t think it’s actually needed here. We can just do the
usual lazy import thing: set it to None, and then when we need it (whenever you
construct a JSONEncoder object with use_decimal*), if it’s None, import it.
That will automatically use the sys.modules cache anyway, and it only costs a
few extra nanoseconds, and we only do it once in your entire program. Surely
it’s not worth optimizing away a few ns one time in exchange for making every
single test more complicated, and probably slower?
What I was referring to in the parens was that it is technically possible to
get objects that would pass isinstance(decimal.Decimal) except that decimal
isn’t in sys.modules, but I don’t think we have to worry about that. For
example, anyone who uses importlib to get decimal imported into a local that’s
not stored in the cache ought to know how to monkeypatch json.decimal =
my_secret_decimal_import if they want to avoid the cost of a second import.
—-
* If you’re worried about the None test on each JSONEncoder construction where
use_decimal, given that every call to dump or dumps constructs an encoder,
consider this: that construction takes microseconds, and adding a None test
makes about an 0.1% difference. Anyone who’s dumping zillions of objects to
JSON already has to explicitly construct and reuse an encoder, so that extra
cost only happens once in their program.
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