pep-0204.txt

PEP: 204
Title: Range Literals
Version: $Revision$
Last-Modified: $Date$
Author: thomas@python.org (Thomas Wouters)
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 14-Jul-2000
Python-Version: 2.0
Post-History:


Introduction
============

This PEP describes the "range literal" proposal for Python 2.0.
This PEP tracks the status and ownership of this feature, slated
for introduction in Python 2.0.  It contains a description of the
feature and outlines changes necessary to support the feature.
This PEP summarizes discussions held in mailing list forums, and
provides URLs for further information, where appropriate.  The CVS
revision history of this file contains the definitive historical
record.


List ranges
===========

Ranges are sequences of numbers of a fixed stepping, often used in
for-loops.  The Python for-loop is designed to iterate over a
sequence directly::

   >>> l = ['a', 'b', 'c', 'd']
   >>> for item in l:
   ...     print item
   a
   b
   c
   d

However, this solution is not always prudent.  Firstly, problems
arise when altering the sequence in the body of the for-loop,
resulting in the for-loop skipping items.  Secondly, it is not
possible to iterate over, say, every second element of the
sequence.  And thirdly, it is sometimes necessary to process an
element based on its index, which is not readily available in the
above construct.

For these instances, and others where a range of numbers is
desired, Python provides the ``range`` builtin function, which
creates a list of numbers.  The ``range`` function takes three
arguments, *start*, *end* and *step*.  *start* and *step* are
optional, and default to 0 and 1, respectively.

The ``range`` function creates a list of numbers, starting at
*start*, with a step of *step*, up to, but not including *end*, so
that ``range(10)`` produces a list that has exactly 10 items, the
numbers 0 through 9.

Using the ``range`` function, the above example would look like
this::

   >>> for i in range(len(l)):
   ...     print l[i]
   a
   b
   c
   d

Or, to start at the second element of ``l`` and processing only
every second element from then on::

   >>> for i in range(1, len(l), 2):
   ...     print l[i]
   b
   d

There are several disadvantages with this approach:

- Clarity of purpose: Adding another function call, possibly with
  extra arithmetic to determine the desired length and step of the
  list, does not improve readability of the code.  Also, it is
  possible to "shadow" the builtin ``range`` function by supplying a
  local or global variable with the same name, effectively
  replacing it.  This may or may not be a desired effect.

- Efficiency: because the ``range`` function can be overridden, the
  Python compiler cannot make assumptions about the for-loop, and
  has to maintain a separate loop counter.

- Consistency: There already is a syntax that is used to denote
  ranges, as shown below.  This syntax uses the exact same
  arguments, though all optional, in the exact same way.  It seems
  logical to extend this syntax to ranges, to form "range
  literals".


Slice Indices
=============

In Python, a sequence can be indexed in one of two ways:
retrieving a single item, or retrieving a range of items.
Retrieving a range of items results in a new object of the same
type as the original sequence, containing zero or more items from
the original sequence.  This is done using a "range notation"::

   >>> l[2:4]
   ['c', 'd']

This range notation consists of zero, one or two indices separated
by a colon.  The first index is the *start* index, the second the
*end*.  When either is left out, they default to respectively the
start and the end of the sequence.

There is also an extended range notation, which incorporates
*step* as well.  Though this notation is not currently supported
by most builtin types, if it were, it would work as follows::

   >>> l[1:4:2]
   ['b', 'd']

The third "argument" to the slice syntax is exactly the same as
the *step* argument to ``range()``.  The underlying mechanisms of the
standard, and these extended slices, are sufficiently different
and inconsistent that many classes and extensions outside of
mathematical packages do not implement support for the extended
variant.  While this should be resolved, it is beyond the scope of
this PEP.

Extended slices do show, however, that there is already a
perfectly valid and applicable syntax to denote ranges in a way
that solve all of the earlier stated disadvantages of the use of
the ``range()`` function:

- It is clearer, more concise syntax, which has already proven to
  be both intuitive and easy to learn.

- It is consistent with the other use of ranges in Python
  (e.g. slices).

- Because it is built-in syntax, instead of a builtin function, it
  cannot be overridden.  This means both that a viewer can be
  certain about what the code does, and that an optimizer will not
  have to worry about ``range()`` being "shadowed".


The Proposed Solution
=====================

The proposed implementation of range-literals combines the syntax
for list literals with the syntax for (extended) slices, to form
range literals::

   >>> [1:10]
   [1, 2, 3, 4, 5, 6, 7, 8, 9]
   >>> [:5]
   [0, 1, 2, 3, 4]
   >>> [5:1:-1]
   [5, 4, 3, 2]

There is one minor difference between range literals and the slice
syntax: though it is possible to omit all of *start*, *end* and
*step* in slices, it does not make sense to omit *end* in range
literals.  In slices, *end* would default to the end of the list,
but this has no meaning in range literals.


Reference Implementation
========================

The proposed implementation can be found on SourceForge [1]_.  It
adds a new bytecode, ``BUILD_RANGE``, that takes three arguments from
the stack and builds a list on the bases of those.  The list is
pushed back on the stack.

The use of a new bytecode is necessary to be able to build ranges
based on other calculations, whose outcome is not known at compile
time.

The code introduces two new functions to ``listobject.c``, which are
currently hovering between private functions and full-fledged API
calls.

``PyList_FromRange()`` builds a list from start, end and step,
returning NULL if an error occurs.  Its prototype is::

    PyObject * PyList_FromRange(long start, long end, long step)

``PyList_GetLenOfRange()`` is a helper function used to determine the
length of a range.  Previously, it was a static function in
``bltinmodule.c``, but is now necessary in both ``listobject.c`` and
``bltinmodule.c`` (for ``xrange``).  It is made non-static solely to avoid
code duplication.  Its prototype is::

    long PyList_GetLenOfRange(long start, long end, long step)


Open issues
===========

- One possible solution to the discrepancy of requiring the *end*
  argument in range literals is to allow the range syntax to
  create a "generator", rather than a list, such as the ``xrange``
  builtin function does.  However, a generator would not be a
  list, and it would be impossible, for instance, to assign to
  items in the generator, or append to it.

  The range syntax could conceivably be extended to include tuples
  (i.e. immutable lists), which could then be safely implemented
  as generators.  This may be a desirable solution, especially for
  large number arrays: generators require very little in the way
  of storage and initialization, and there is only a small
  performance impact in calculating and creating the appropriate
  number on request.  (TBD: is there any at all? Cursory testing
  suggests equal performance even in the case of ranges of length
  1)

  However, even if idea was adopted, would it be wise to "special
  case" the second argument, making it optional in one instance of
  the syntax, and non-optional in other cases ?

- Should it be possible to mix range syntax with normal list
  literals, creating a single list?  E.g.::

     >>> [5, 6, 1:6, 7, 9]

  to create::

     [5, 6, 1, 2, 3, 4, 5, 7, 9]

- How should range literals interact with another proposed new
  feature, "list comprehensions" [2]_?  Specifically, should it be
  possible to create lists in list comprehensions?  E.g.::

     >>> [x:y for x in (1, 2) y in (3, 4)]

  Should this example return a single list with multiple ranges::

     [1, 2, 1, 2, 3, 2, 2, 3]

  Or a list of lists, like so::

     [[1, 2], [1, 2, 3], [2]_, [2, 3]]

  However, as the syntax and semantics of list comprehensions are
  still subject of hot debate, these issues are probably best
  addressed by the "list comprehensions" PEP.

- Range literals accept objects other than integers: it performs
  ``PyInt_AsLong()`` on the objects passed in, so as long as the
  objects can be coerced into integers, they will be accepted.
  The resulting list, however, is always composed of standard
  integers.

  Should range literals create a list of the passed-in type?  It
  might be desirable in the cases of other builtin types, such as
  longs and strings::

     >>> [ 1L : 2L<<64 : 2<<32L ]
     >>> ["a":"z":"b"]
     >>> ["a":"z":2]

  However, this might be too much "magic" to be obvious.  It might
  also present problems with user-defined classes: even if the
  base class can be found and a new instance created, the instance
  may require additional arguments to ``__init__``, causing the
  creation to fail.

- The ``PyList_FromRange()`` and ``PyList_GetLenOfRange()`` functions need
  to be classified: are they part of the API, or should they be
  made private functions?


Rejection
=========

After careful consideration, and a period of meditation, this
proposal has been rejected. The open issues, as well as some
confusion between ranges and slice syntax, raised enough questions
for Guido not to accept it for Python 2.0, and later to reject the
proposal altogether. The new syntax and its intentions were deemed
not obvious enough.

[ TBD: Guido, amend/confirm this, please. Preferably both; this
is a PEP, it should contain *all* the reasons for rejection
and/or reconsideration, for future reference. ]


Copyright
=========

This document has been placed in the Public Domain.


References
==========

.. [1] http://sourceforge.net/patch/?func=detailpatch&patch_id=100902&group_id=5470
.. [2] PEP 202, List Comprehensions



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