Feature or enhancement

It should be possible for decimal.Decimals (and potentially other non-integer numeric types) to be interpreted as integers if doing so would not cause a loss of precision.

Pitch

As of GH-15636 (part of Python from 3.10.0 onwards), it's no longer possible to use certain numeric types as arguments to standard library functions when an integer is expected due to the removal of type coercion logic. This is, of course, quite sensible: datetime.datetime(year=2023, month=3, day=17.5) is meaningless, and should result in a TypeError rather than a datetime object representing 2023-03-17 00:00:00. However, there are situations where certain numeric types might be coercible to integers - one example is a decimal.Decimal with no fractional part. In these situations, I think it makes sense for the standard library functions whose behaviour changed following GH-15636 to behave as they did before.

In the Decimal case, this would involve implementing __index__ in a way that returns an integer representation of the value if the number has no fractional part, or raising a TypeError if it does. I believe this complies with the general understanding of how __index__ is supposed to work - from the What's New entry discussing PEP 357:

The return value must be either a Python integer or long integer. The interpreter will check that the type returned is correct, and raises a TypeError if this requirement isn't met.

I've implemented this in both _decimal and _pydecimal, with the following results:

>>> from decimal import Decimal
>>> from datetime import datetime
>>> datetime(year=Decimal(2023), month=Decimal(3), day=Decimal(17))
datetime.datetime(2023, 3, 17, 0, 0)
>>> datetime(year=Decimal(2023), month=Decimal(3), day=Decimal(17.5))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot convert Decimal with fractional part to integer

I have a particular need for this to work with Decimal, but perhaps the same could be done for other types in the standard library if there's enough interest.

Previous discussion

In issue36048, Serhiy discusses the rationale for this behaviour being deprecated in the first place (in Python 3.8) - the comment identifies the problem with "dropping the fractional part if the object is not [an] integral number", but doesn't address the corollary of that (i.e. permitting type coercion if the object is an integral number), which is the purpose of this issue.

Linked PRs

• gh-102791: allow non-fractional decimal.Decimals to be interpreted as integers #102794