Mark Dickinson <[email protected]> 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?
----------
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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<https://bugs.python.org/issue44547>
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