[issue44547] fraction.Fraction does not implement __int__.

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

FWIW, there's some history here: there's a good reason that fractions.Fraction didn't originally implement __int__.

Back in the Bad Old Days, many Python functions that expected an integer would accept anything whose type implemented __int__ instead, and call __int__ to get the required integer. For example:

Python 3.7.10 (default, Jun  1 2021, 23:43:35) 
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from decimal import Decimal
>>> chr(Decimal("45.67"))
'-'

Effectively, __int__ was being used to mean two different things: (1) this value can be used as an integer, and (2) this value can be truncated to an integer. The solution was to introduce two new dedicated magic methods __index__ and __trunc__ for these two different meanings. See PEP 357 and PEP 3141 for some of the details. So adding __int__ to fractions.Fraction would have made things like `chr(Fraction("5/2"))` possible, too.

The behaviour above is still present (with a deprecation warning) in Python 3.9, and `chr(Decimal("45.67"))` has only finally been made a TypeError in Python 3.10.

We may now finally be in a state where ill-advised uses of __int__ in internal functions have all been deprecated and removed, so that it's safe to re-add __int__ methods.

But still, this seems mostly like an issue with the typing library. What is typing.SupportsInt intended to indicate? That an object can be used _as_ an integer, or that an object can be _truncated_ to an integer?

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nosy: +mark.dickinson
type: behavior -> enhancement
versions:  -Python 3.10, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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