Status of PEP's?
James_Althoff at i2.com
James_Althoff at i2.com
Wed Feb 27 18:52:34 EST 2002
Below is a revision (not yet submitted) of PEP 276.
Rather than restart the entire debate on PEP 276, please send me
suggestions for specific changes to the PEP.
Thanks,
Jim
===============================================================
PEP: 276
Title: Simple Iterator for ints
Version: $Revision: 1.1 $
Last-Modified: $Date: 2001/11/13 20:52:37 $
Author: james_althoff at i2.com (Jim Althoff)
Status: Draft
Type: Standards Track
Created: 12-Nov-2001
Python-Version: 2.3
Post-History:
Abstract
Python 2.1 added new functionality to support iterators[1].
Iterators have proven to be useful and convenient in many coding
situations. It is noted that the implementation of Python's
for-loop control structure uses the iterator protocol as of
release 2.1. It is also noted that Python provides iterators for
the following builtin types: lists, tuples, dictionaries, strings,
and files. This PEP proposes the addition of an iterator for the
builtin type int (types.IntType). Such an iterator would simplify
the coding of certain for-loops in Python.
Specification
Define an iterator for types.intType (i.e., the builtin type
"int") that is returned from the builtin function "iter" when
called with an instance of types.intType as the argument.
The returned iterator has the following behavior:
- Assume that object i is an instance of types.intType (the
builtin type int) and that i > 0
- iter(i) returns an iterator object
- said iterator object iterates through the sequence of ints
0,1,2,...,i-1
Example:
iter(5) returns an iterator object that iterates through the
sequence of ints 0,1,2,3,4
- if i <= 0, iter(i) returns an "empty" iterator, i.e., one that
throws StopIteration upon the first call of its "next" method
In other words, the conditions and semantics of said iterator is
consistent with the conditions and semantics of the range() and
xrange() functions.
Note that the sequence 0,1,2,...,i-1 associated with the int i is
considered "natural" in the context of Python programming because
it is consistent with the builtin indexing protocol of sequences
in Python. Python lists and tuples, for example, are indexed
starting at 0 and ending at len(object)-1 (when using positive
indices). In other words, such objects are indexed with the
sequence 0,1,2,...,len(object)-1
Rationale
A common programming idiom is to take a collection of objects and
apply some operation to each item in the collection in some
established sequential order. Python provides the "for in"
looping control structure for handling this common idiom. Cases
arise, however, where it is necessary (or more convenient) to
access each item in an "indexed" collection by iterating through
each index and accessing each item in the collection using the
corresponding index.
For example, one might have a two-dimensional "table" object where one
requires the application of some operation to the first column of
each row in the table. Depending on the implementation of the table
it might not be possible to access first each row and then each
column as individual objects. It might, rather, be possible to
access a cell in the table using a row index and a column index.
In such a case it is necessary to use an idiom where one iterates
through a sequence of indices (indexes) in order to access the
desired items in the table. (Note that the commonly used
DefaultTableModel class in Java-Swing-Jython has this very protocol).
Another common example is where one needs to process two or more
collections in parallel. Another example is where one needs to
access, say, every second item in a collection.
There are many other examples where access to items in a
collection is facilitated by a computation on an index thus
necessitating access to the indices rather than direct access to
the items themselves.
Let's call this idiom the "indexed for-loop" idiom. Some
programming languages provide builtin syntax for handling this
idiom. In Python the common convention for implementing the
indexed for-loop idiom is to use the builtin range() or xrange()
function to generate a sequence of indices as in, for example:
for rowcount in range(table.getRowCount()):
print table.getValueAt(rowcount, 0)
or
for rowcount in xrange(table.getRowCount()):
print table.getValueAt(rowcount, 0)
From time to time there are discussions in the Python community
about the indexed for-loop idiom. It is sometimes argued that the
need for using the range() or xrange() function for this design
idiom is:
- Not obvious (to new-to-Python programmers),
- Error prone (easy to forget, even for experienced Python
programmers)
- Confusing and distracting for those who feel compelled to understand
the differences and recommended usage of xrange() vis-a-vis range()
- Unwieldy, especially when combined with the len() function,
i.e., xrange(len(sequence))
- Not as convenient as equivalent mechanisms in other languages,
- Annoying, a "wart", etc.
And from time to time proposals are put forth for ways in which
Python could provide a better mechanism for this idiom. Recent
examples include PEP 204, "Range Literals", and PEP 212, "Loop
Counter Iteration".
Most often, such proposal include changes to Python's syntax and
other "heavyweight" changes.
Part of the difficulty here is that advocating new syntax implies
a comprehensive solution for "general indexing" that has to
include aspects like:
- starting index value
- ending index value
- step value
- open intervals versus closed intervals versus half opened intervals
Finding a new syntax that is comprehensive, simple, general,
Pythonic, appealing to many, easy to implement, not in conflict
with existing structures, not excessively overloading of existing
structures, etc. has proven to be more difficult than one might
anticipate.
The proposal outlined in this PEP tries to address the problem by
suggesting a simple "lightweight" solution that helps the most
common case by using a proven mechanism that is already available
(as of Python 2.1): namely, iterators.
Because for-loops already use "iterator" protocol as of Python
2.1, adding an iterator for types.IntType as proposed in this PEP
would enable by default the following shortcut for the indexed
for-loop idiom:
for rowcount in table.getRowCount():
print table.getValueAt(rowcount, 0)
The following benefits for this approach vis-a-vis the current
mechanism of using the range() or xrange() functions are claimed
to be:
- Simpler,
- Less cluttered,
- Focuses on the problem at hand without the need to resort to
secondary implementation-oriented functions (range() and
xrange())
And compared to other proposals for change:
- Requires no new syntax
- Requires no new keywords
- Takes advantage of the new and well-established iterator mechanism
And generally:
- Is consistent with iterator-based "convenience" changes already
included (as of Python 2.1) for other builtin types such as:
lists, tuples, dictionaries, strings, and files.
Backwards Compatibility
The proposed mechanism is generally backwards compatible as it
calls for neither new syntax nor new keywords. All existing,
valid Python programs should continue to work unmodified.
However, this proposal is not perfectly backwards compatible in
the sense that certain statements that are currently invalid
would, under the current proposal, become valid.
Tim Peters has pointed out two such examples:
1) The common case where one forgets to include range() or
xrange(), for example:
for rowcount in table.getRowCount():
print table.getValueAt(rowcount, 0)
in Python 2.2 raises a TypeError exception.
Under the current proposal, the above statement would be valid
and would work as (presumably) intended. Presumably, this is a
good thing.
As noted by Tim, this is the common case of the "forgotten
range" mistake (which one currently corrects by adding a call
to range() or xrange()).
2) The (hopefully) very uncommon case where one makes a typing
mistake when using tuple unpacking. For example:
x, = 1
in Python 2.2 raises a TypeError exception.
Under the current proposal, the above statement would be valid
and would set x to 0. The PEP author has no data as to how
common this typing error is nor how difficult it would be to
catch such an error under the current proposal. He imagines
that it does not occur frequently and that it would be
relatively easy to correct should it happen.
Issues:
Extensive discussions concerning PEP 276 on the Python interest
mailing list suggests a range of opinions: some in favor, some
neutral, some against. Those in favor tend to agree with the
claims above of the usefulness, convenience, and simplicity of
a simple iterator for integers.
Issues with PEP 276 include:
- Some feel that iterating over the sequence "0, 1, 2, ..., n-1"
for an integer n is not intuitive. "for i in 5:" is considered
(by some) to be "non-obvious", for example.
Response: This seems to be largely a matter of preference.
Some like the proposed idiom and see it as simple and
elegant. Some are neutral on this issue. Others, as noted,
dislike it.
- Is it obvious that iter(5) maps to the sequence 0,1,2,3,4?
Response: Given, as noted above, that Python has a strong
convention for indexing sequences starting at 0 and stopping at
(inclusively) the index whose value is one less than the length
of the sequence, it is argued that the proposed sequence is
reasonably intuitive to a Python programmer while being useful
and practical. More importantly, it is argued that once learned
this convention is very easy to remember.
- Possible ambiguity
for i in 10: print i
might be mistaken for
for i in (10,): print i
Response: This is exactly the same situation with strings in
current Python (replace 10 with 'spam' in the above, for
example).
- Might dissuade newbies from using the indexed for-loop idiom when
the standard "for item in collection:" idiom is clearly better.
Response: The standard idiom is so nice when it fits that it
needs neither extra "carrot" nor "stick". On the other hand,
one does notice cases of overuse/misuse of the standard idiom
(due, most likely, to the awkwardness of the indexed for-loop
idiom), as in:
for item in sequence:
print sequence.index(item)
The majority of disagreement with PEP 276 came from those who
favor much larger changes to Python to address the more general
problem of specifying a sequence of integers where such
a specification is general enough to handle the starting value,
ending value, and stepping value of the sequence and also
addresses variations of open, closed, and half-open (half-closed)
integer intervals. Many suggestions of such were discussed.
These include:
- adding Haskell-like notation for specifying a sequence of
integers in a literal list,
- various uses of slicing notation to specify sequences,
- changes to the syntax of for-in loops to allow the use of
relational operators in the loop header,
- creation of an integer-interval class along with methods that
overload relational operators or division operators
to provide "slicing" on integer-interval objects,
- and more.
It should be noted that there was much debate but not an
overwhelming concensus for any of these larger-scale suggestions.
Clearly, PEP 276 does not propose such a large-scale change
and instead focuses on a specific problem area. Towards the
end of the discussion period, several posters expressed favor
for the narrow focus and simplicity of PEP 276 vis-a-vis the more
ambitious suggestions that were advanced. There did appear to be
concensus for the need for a PEP for any such larger-scale,
alternative suggestion. In light of this recognition, details of
the various alternative suggestions are not discussed here further.
Implementation
An implementation is not available at this time but is expected
to be straightforward.
References
[1] PEP 234, Iterators
http://python.sourceforge.net/peps/pep-0234.html
[2] PEP 204, Range Literals
http://python.sourceforge.net/peps/pep-0204.html
[3] PEP 212, Loop Counter Iteration
http://python.sourceforge.net/peps/pep-0212.html
Copyright
This document has been placed in the public domain.
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