133 lines
3.7 KiB
Plaintext
133 lines
3.7 KiB
Plaintext
PEP: 238
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Title: Non-integer Division
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Version: $Revision$
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Author: pep@zadka.site.co.il (Moshe Zadka)
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Status: Draft
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Type: Standards Track
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Created: 11-Mar-2001
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Python-Version: 2.2
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Post-History: 16-Mar-2001
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Abstract
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Dividing integers currently returns the floor of the quantities.
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This behavior is known as integer division, and is similar to what
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C and FORTRAN do. This has the useful property that all
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operations on integers return integers, but it does tend to put a
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hump in the learning curve when new programmers are surprised that
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1/2 == 0
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This proposal shows a way to change this while keeping backward
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compatibility issues in mind.
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Rationale
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The behavior of integer division is a major stumbling block found
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in user testing of Python. This manages to trip up new
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programmers regularly and even causes the experienced programmer
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to make the occasional mistake. The workarounds, like explicitly
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coercing one of the operands to float or use a non-integer
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literal, are very non-intuitive and lower the readability of the
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program.
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// Operator
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A `//' operator which will be introduced, which will call the
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nb_intdivide or __intdiv__ slots. This operator will be
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implemented in all the Python numeric types, and will have the
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semantics of
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a // b == floor(a/b)
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Except that the type of a//b will be the type a and b will be
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coerced into. Specifically, if a and b are of the same type, a//b
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will be of that type too.
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Changing the Semantics of the / Operator
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The nb_divide slot on integers (and long integers, if these are a
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separate type, but see PEP 237 [1]) will issue a warning when given
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integers a and b such that
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a % b != 0
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The warning will be off by default in the 2.2 release, and on by
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default for in the next Python release, and will stay in effect
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for 24 months. The next Python release after 24 months, it will
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implement
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(a/b) * b = a (more or less)
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The type of a/b will be either a float or a rational, depending on
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other PEPs[2, 3].
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__future__
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A special opcode, FUTURE_DIV will be added that does the
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equivalent of:
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if type(a) in (types.IntType, types.LongType):
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if type(b) in (types.IntType, types.LongType):
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if a % b != 0:
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return float(a)/b
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return a/b
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(or rational(a)/b, depending on whether 0.5 is rational or float).
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If "from __future__ import non_integer_division" is present in the
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module, until the IntType nb_divide is changed, the "/" operator
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is compiled to FUTURE_DIV.
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This is not yet implemented in the Python 2.2 release.
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FAQ
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Should the // operator be renamed to "div"?
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No. There are problems with new keywords.
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Should the // be made into a function called "div"?
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No. People expect to be able to write math expressions directly
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in Python.
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Implementation
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A mostly-complete implementation (not exactly following the above
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spec, but close enough except for the lack of a warning for
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truncated results from old division) is available from the
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SourceForge patch manager:
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http://sourceforge.net/tracker/index.php?func=detail&aid=443474&group_id=5470&atid=305470
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References
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[1] PEP 237, Unifying Long Integers and Integers, Zadka,
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http://www.python.org/peps/pep-0237.html
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[2] PEP 239, Adding a Rational Type to Python, Zadka,
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http://www.python.org/peps/pep-0239.html
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[3] PEP 240, Adding a Rational Literal to Python, Zadka,
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http://www.python.org/peps/pep-0240.html
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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