[Python-ideas] divmod(): fallback to __floordiv__ and __mod__?

[Mark Dickinson]

On Sat, Sep 17, 2016 at 10:01 AM, Spencer Brown <spencerb21 at live.com> wrote:
> Currently, calling divmod() on a class with __floordiv__ and __mod__
> defined, but not __divmod__ raises a TypeError. Is there any reason why it
> doesn't fallback to (self // x, self % x)?

It's an interesting idea. I wonder whether the falling back shouldn't
be in the other direction, though: that is, if a class defines
`__divmod__` but not `__floordiv__` or `__mod__`, perhaps the `//` and
`%` operations should use `__divmod__`? That way, if you're writing a
class that intends to support all three operations, you only have to
write one method. And it might make sense from an efficiency
perspective, too; it's common for a `divmod` computation to be cheaper
than doing separate computations of the quotient and remainder.

For the builtin int type, for example, in nontrivial cases Python
computes both the quotient and remainder when asked to do a % or //
operation, and then discards whichever part isn't needed. So in that
case it would be wasteful to build up the divmod result from two
separate % and // calls.

-- 
Mark