python-peps/pep-0238.txt

PEP: 238
Title: Non-integer Division
Version: $Revision$
Author: pep@zadka.site.co.il (Moshe Zadka)
Status: Draft
Type: Standards Track
Created: 11-Mar-2001
Python-Version: 2.2
Post-History: 16-Mar-2001


Abstract

    Dividing integers currently returns the floor of the quantities.
    This behavior is known as integer division, and is similar to what
    C and FORTRAN do.  This has the useful property that all
    operations on integers return integers, but it does tend to put a
    hump in the learning curve when new programmers are surprised that

        1/2 == 0

    This proposal shows a way to change this while keeping backward
    compatibility issues in mind.


Rationale

    The behavior of integer division is a major stumbling block found
    in user testing of Python.  This manages to trip up new
    programmers regularly and even causes the experienced programmer
    to make the occasional mistake.  The workarounds, like explicitly
    coercing one of the operands to float or use a non-integer
    literal, are very non-intuitive and lower the readability of the
    program.


// Operator

    A `//' operator which will be introduced, which will call the
    nb_intdivide or __intdiv__ slots.  This operator will be
    implemented in all the Python numeric types, and will have the
    semantics of

        a // b == floor(a/b)

    Except that the type of a//b will be the type a and b will be
    coerced into.  Specifically, if a and b are of the same type, a//b
    will be of that type too.


Changing the Semantics of the / Operator

    The nb_divide slot on integers (and long integers, if these are a
    separate type, but see PEP 237[1]) will issue a warning when given
    integers a and b such that

        a % b != 0

    The warning will be off by default in the 2.2 release, and on by
    default for in the next Python release, and will stay in effect
    for 24 months.  The next Python release after 24 months, it will
    implement

        (a/b) * b = a (more or less)

    The type of a/b will be either a float or a rational, depending on
    other PEPs[2, 3].


__future__

    A special opcode, FUTURE_DIV will be added that does the
    equivalent of:

        if type(a) in (types.IntType, types.LongType):
           if type(b) in (types.IntType, types.LongType):
               if a % b != 0:
                    return float(a)/b
        return a/b

    (or rational(a)/b, depending on whether 0.5 is rational or float).

    If "from __future__ import non_integer_division" is present in the
    module, until the IntType nb_divide is changed, the "/" operator
    is compiled to FUTURE_DIV.


Open Issues

    Should the // operator be renamed to "div"?

    Should the // be made into a function called "div"?

References

    [1] PEP 237, Unifying Long Integers and Integers, Zadka,
        http://python.sourceforge.net/peps/pep-0237.html

    [2] PEP 239, Adding a Rational Type to Python, Zadka,
        http://python.sourceforge.net/peps/pep-0239.html

    [3] PEP 240, Adding a Rational Literal to Python, Zadka,
        http://python.sourceforge.net/peps/pep-0240.html


Copyright

    This document has been placed in the public domain.



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