PEP: 284 Title: Integer for-loops Author: David Eppstein
<eppstein@ics.uci.edu>, Gregory Ewing <greg.ewing@canterbury.ac.nz>
Status: Rejected Type: Standards Track Content-Type: text/x-rst Created:
01-Mar-2002 Python-Version: 2.3 Post-History:

Abstract

This PEP proposes to simplify iteration over intervals of integers, by
extending the range of expressions allowed after a "for" keyword to
allow three-way comparisons such as :

    for lower <= var < upper:

in place of the current :

    for item in list:

syntax. The resulting loop or list iteration will loop over all values
of var that make the comparison true, starting from the left endpoint of
the given interval.

Pronouncement

This PEP is rejected. There were a number of fixable issues with the
proposal (see the fixups listed in Raymond Hettinger's python-dev post
on 18 June 2005[1]). However, even with the fixups the proposal did not
garner support. Specifically, Guido did not buy the premise that the
range() format needed fixing, "The whole point (15 years ago) of range()
was to avoid needing syntax to specify a loop over numbers. I think it's
worked out well and there's nothing that needs to be fixed (except
range() needs to become an iterator, which it will in Python 3.0)."

Rationale

One of the most common uses of for-loops in Python is to iterate over an
interval of integers. Python provides functions range() and xrange() to
generate lists and iterators for such intervals, which work best for the
most frequent case: half-open intervals increasing from zero. However,
the range() syntax is more awkward for open or closed intervals, and
lacks symmetry when reversing the order of iteration. In addition, the
call to an unfamiliar function makes it difficult for newcomers to
Python to understand code that uses range() or xrange().

The perceived lack of a natural, intuitive integer iteration syntax has
led to heated debate on python-list, and spawned at least four PEPs
before this one. PEP 204 (rejected) proposed to re-use Python's slice
syntax for integer ranges, leading to a terser syntax but not solving
the readability problem of multi-argument range(). PEP 212 (deferred)
proposed several syntaxes for directly converting a list to a sequence
of integer indices, in place of the current idiom :

    range(len(list))

for such conversion, and PEP 281 proposes to simplify the same idiom by
allowing it to be written as :

    range(list).

PEP 276 proposes to allow automatic conversion of integers to iterators,
simplifying the most common half-open case but not addressing the
complexities of other types of interval. Additional alternatives have
been discussed on python-list.

The solution described here is to allow a three-way comparison after a
"for" keyword, both in the context of a for-loop and of a list
comprehension:

    for lower <= var < upper:

This would cause iteration over an interval of consecutive integers,
beginning at the left bound in the comparison and ending at the right
bound. The exact comparison operations used would determine whether the
interval is open or closed at either end and whether the integers are
considered in ascending or descending order.

This syntax closely matches standard mathematical notation, so is likely
to be more familiar to Python novices than the current range() syntax.
Open and closed interval endpoints are equally easy to express, and the
reversal of an integer interval can be formed simply by swapping the two
endpoints and reversing the comparisons. In addition, the semantics of
such a loop would closely resemble one way of interpreting the existing
Python for-loops:

    for item in list

iterates over exactly those values of item that cause the expression :

    item in list

to be true. Similarly, the new format :

    for lower <= var < upper:

would iterate over exactly those integer values of var that cause the
expression :

    lower <= var < upper

to be true.

Specification

We propose to extend the syntax of a for statement, currently :

    for_stmt: "for" target_list "in" expression_list ":" suite
          ["else" ":" suite]

as described below:

    for_stmt: "for" for_test ":" suite ["else" ":" suite]
    for_test: target_list "in" expression_list |
            or_expr less_comp or_expr less_comp or_expr |
            or_expr greater_comp or_expr greater_comp or_expr
    less_comp: "<" | "<="
    greater_comp: ">" | ">="

Similarly, we propose to extend the syntax of list comprehensions,
currently :

    list_for: "for" expression_list "in" testlist [list_iter]

by replacing it with:

    list_for: "for" for_test [list_iter]

In all cases the expression formed by for_test would be subject to the
same precedence rules as comparisons in expressions. The two
comp_operators in a for_test must be required to be both of similar
types, unlike chained comparisons in expressions which do not have such
a restriction.

We refer to the two or_expr's occurring on the left and right sides of
the for-loop syntax as the bounds of the loop, and the middle or_expr as
the variable of the loop. When a for-loop using the new syntax is
executed, the expressions for both bounds will be evaluated, and an
iterator object created that iterates through all integers between the
two bounds according to the comparison operations used. The iterator
will begin with an integer equal or near to the left bound, and then
step through the remaining integers with a step size of +1 or -1 if the
comparison operation is in the set described by less_comp or
greater_comp respectively. The execution will then proceed as if the
expression had been :

    for variable in iterator

where "variable" refers to the variable of the loop and "iterator"
refers to the iterator created for the given integer interval.

The values taken by the loop variable in an integer for-loop may be
either plain integers or long integers, according to the magnitude of
the bounds. Both bounds of an integer for-loop must evaluate to a real
numeric type (integer, long, or float). Any other value will cause the
for-loop statement to raise a TypeError exception.

Issues

The following issues were raised in discussion of this and related
proposals on the Python list.

-   Should the right bound be evaluated once, or every time through the
    loop? Clearly, it only makes sense to evaluate the left bound once.
    For reasons of consistency and efficiency, we have chosen the same
    convention for the right bound.

-   Although the new syntax considerably simplifies integer for-loops,
    list comprehensions using the new syntax are not as simple. We feel
    that this is appropriate since for-loops are more frequent than
    comprehensions.

-   The proposal does not allow access to integer iterator objects such
    as would be created by xrange. True, but we see this as a
    shortcoming in the general list-comprehension syntax, beyond the
    scope of this proposal. In addition, xrange() will still be
    available.

-   The proposal does not allow increments other than 1 and -1. More
    general arithmetic progressions would need to be created by range()
    or xrange(), or by a list comprehension syntax such as :

        [2*x for 0 <= x <= 100]

-   The position of the loop variable in the middle of a three-way
    comparison is not as apparent as the variable in the present :

        for item in list

    syntax, leading to a possible loss of readability. We feel that this
    loss is outweighed by the increase in readability from a natural
    integer iteration syntax.

-   To some extent, this PEP addresses the same issues as PEP 276. We
    feel that the two PEPs are not in conflict since PEP 276 is
    primarily concerned with half-open ranges starting in 0 (the easy
    case of range()) while this PEP is primarily concerned with
    simplifying all other cases. However, if this PEP is approved, its
    new simpler syntax for integer loops could to some extent reduce the
    motivation for PEP 276.

-   It is not clear whether it makes sense to allow floating point
    bounds for an integer loop: if a float represents an inexact value,
    how can it be used to determine an exact sequence of integers? On
    the other hand, disallowing float bounds would make it difficult to
    use floor() and ceiling() in integer for-loops, as it is difficult
    to use them now with range(). We have erred on the side of
    flexibility, but this may lead to some implementation difficulties
    in determining the smallest and largest integer values that would
    cause a given comparison to be true.

-   Should types other than int, long, and float be allowed as bounds?
    Another choice would be to convert all bounds to integers by int(),
    and allow as bounds anything that can be so converted instead of
    just floats. However, this would change the semantics: 0.3 <= x is
    not the same as int(0.3) <= x, and it would be confusing for a loop
    with 0.3 as lower bound to start at zero. Also, in general int(f)
    can be very far from f.

Implementation

An implementation is not available at this time. Implementation is not
expected to pose any great difficulties: the new syntax could, if
necessary, be recognized by parsing a general expression after each
"for" keyword and testing whether the top level operation of the
expression is "in" or a three-way comparison. The Python compiler would
convert any instance of the new syntax into a loop over the items in a
special iterator object.

References

Copyright

This document has been placed in the public domain.

[1] Raymond Hettinger, Propose updating PEP 284 -- Integer for-loops
https://mail.python.org/pipermail/python-dev/2005-June/054316.html