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First draft of the range-literals PEP. Not sprinkeled with explicit 'TBD's, 

and the one TBD in there should be an 'XXX' in my opinion ;) but with a lot
of issues under 'Open issues'.

Please, comment freely, on anything from specific arguments and spelling
errors to style issues and argument-flow. I wrote all this on the train to
and from work, listening to UB40 on the way to work, and to my girlfriend
complaining about her work and her colleagues on the way home ;) So it might
be a bit incoherent in places.

One TBD that's going to come up a lot in PEPs, I think: how to wrap long
URLs ? I think whoever invents a good way to do that can become a
millionaire. Can we define/steal a good, standard way to wrap URLs, please ?
This commit is contained in:
Thomas Wouters 2000-07-18 10:01:12 +00:00
parent be3c33389e
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PEP: 203
PEP: 204
Title: Range Literals
Version: $Revision$
Owner: thomas@xs4all.net (Thomas Wouters)
Python-Version: 2.0
Status: Incomplete
Status: Draft
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 functioncall, possibly with
extra arithmatic 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 seperate 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 seperated
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
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, and this should definately be resolved, but this 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
(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.
PyObject * PyList_FromRange(long start, long end, long step)
builds a list from start, end and step, returning NULL if an error
occurs.
long PyList_GetLenOfRange(long start, long end, long step) is a
helper function to determine the length of a range. It was
previously 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.
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,
immutable lists, which could then be safely implemented as
generators. Especially for large number arrays, this may be a
desirable solution: 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, like so:
>>> [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', PEP-202 ? In specific, should it
be possible to create lists in list comprehensions, like so:
>>> [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 ?
References:
[1]
http://sourceforge.net/patch/?func=detailpatch&patch_id=100902&group_id=5470
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