Comment on PEP-0238

[Terry Reedy]

> By the way (this is for Terry :-), int/int in Python does not do the
> same as it does in C for negative operands.
>
> --Guido van Rossum (home page: http://www.python.org/~guido/)

I may have noticed this when I switched several years ago, but indeed had
forgotten since.  Fortunately, I know enough not to rely on my memory for
something I would only do occasionally.
---

Tuesday afternoon's experiment: I asked my (non-programmer,
non-mathematical) wife, "What is eight divided by three."  Even with a hint
("eight, not eight point zero"), she replied 'two point something or
other."  So Python's current behavior would have initially urprised her.

On the other hand, when I said to think back to second grade, she easily
remembered quotient-remainder answers to division.problems.  After I
explained Python's notation, , she thought she could handle using '.0' or
not to indicate which type of answer she wanted.  It was unsurprising to
her that she should have a choice.  This took at most two minutes.
---

That said, I can imagine that it would have been better to have defined
Python's arithmetic a little differently back at the beginning.  If I had a
time machine, here is what I would propose:  use / for fractional division
and use div, mod, and divmod (as infix operators) for whole number,
remainder division.  A simple pair of rules that I think should be easy to
teach, learn, and remember.

Note 1: The omission of '.0' and 'float()' where now needed would more than
make up for the two extra chars in 'div' versus '/' and 'mod' versus '%'.

Note 2: 'mod' is the standard mathematical notation for the modulus
(remainer) operation.  C used '%' because all its operators are symbolic.
This is not true for Python so there is no need to copy C (or any other
language that uses '5'), especially when paired with 'div'.

What would I propose now?  If and when int division is changed (preferably
as a package of arithmetic changes -- see my response to Guido in Re:
Language change and code breaks), leave divmod a function if you want,
change / to div, and also change % to mod.
A) The three names obviously fit together very nicely, which the pair div,
% strikes me as a bit ugly.  B) The overloading of (format) % with the
unrelated or ever conflicting (mod) % (one adds, the other tosses away)
strikes me as a slight wart on the language.  Do beginners ever find this
confusing?  And yes, do add corresponding magic methods to classes.

Terry J. Reedy