262 lines
9.8 KiB
Plaintext
262 lines
9.8 KiB
Plaintext
PEP: 284
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Title: Integer for-loops
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Version: $Revision$
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Last-Modified: $Date$
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Author: eppstein@ics.uci.edu (David Eppstein),
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greg@cosc.canterbury.ac.nz (Greg Ewing)
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Status: Draft
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Type: Standards Track
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Created: 1-Mar-2002
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Python-Version: 2.3
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Post-History:
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Abstract
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This PEP proposes to simplify iteration over intervals of
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integers, by extending the range of expressions allowed after a
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"for" keyword to allow three-way comparisons such as
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for lower <= var < upper:
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in place of the current
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for item in list:
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syntax. The resulting loop or list iteration will loop over all
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values of var that make the comparison true, starting from the
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left endpoint of the given interval.
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Rationale
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One of the most common uses of for-loops in Python is to iterate
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over an interval of integers. Python provides functions range()
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and xrange() to generate lists and iterators for such intervals,
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which work best for the most frequent case: half-open intervals
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increasing from zero. However, the range() syntax is more awkward
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for open or closed intervals, and lacks symmetry when reversing
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the order of iteration. In addition, the call to an unfamiliar
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function makes it difficult for newcomers to Python to understand
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code that uses range() or xrange().
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The perceived lack of a natural, intuitive integer iteration
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syntax has led to heated debate on python-list, and spawned at
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least four PEPs before this one. PEP 204 [1] (rejected) proposed
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to re-use Python's slice syntax for integer ranges, leading to a
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terser syntax but not solving the readability problem of
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multi-argument range(). PEP 212 [2] (deferred) proposed several
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syntaxes for directly converting a list to a sequence of integer
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indices, in place of the current idiom
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range(len(list))
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for such conversion, and PEP 281 [3] proposes to simplify the same
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idiom by allowing it to be written as
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range(list).
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PEP 276 [4] proposes to allow automatic conversion of integers to
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iterators, simplifying the most common half-open case but not
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addressing the complexities of other types of interval.
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Additional alternatives have been discussed on python-list.
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The solution described here is to allow a three-way comparison
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after a "for" keyword, both in the context of a for-loop and of a
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list comprehension:
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for lower <= var < upper:
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This would cause iteration over an interval of consecutive
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integers, beginning at the left bound in the comparison and ending
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at the right bound. The exact comparison operations used would
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determine whether the interval is open or closed at either end and
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whether the integers are considered in ascending or descending
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order.
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This syntax closely matches standard mathematical notation, so is
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likely to be more familiar to Python novices than the current
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range() syntax. Open and closed interval endpoints are equally
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easy to express, and the reversal of an integer interval can be
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formed simply by swapping the two endpoints and reversing the
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comparisons. In addition, the semantics of such a loop would
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closely resemble one way of interpreting the existing Python
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for-loops:
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for item in list
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iterates over exactly those values of item that cause the
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expression
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item in list
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to be true. Similarly, the new format
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for lower <= var < upper:
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would iterate over exactly those integer values of var that cause
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the expression
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lower <= var < upper
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to be true.
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Specification
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We propose to extend the syntax of a for statement, currently
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for_stmt: "for" target_list "in" expression_list ":" suite
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["else" ":" suite]
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as described below:
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for_stmt: "for" for_test ":" suite ["else" ":" suite]
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for_test: target_list "in" expression_list |
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or_expr less_comp or_expr less_comp or_expr |
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or_expr greater_comp or_expr greater_comp or_expr
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less_comp: "<" | "<="
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greater_comp: ">" | ">="
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Similarly, we propose to extend the syntax of list comprehensions,
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currently
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list_for: "for" expression_list "in" testlist [list_iter]
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by replacing it with:
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list_for: "for" for_test [list_iter]
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In all cases the expression formed by for_test would be subject to
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the same precedence rules as comparisons in expressions. The two
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comp_operators in a for_test must be required to be both of
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similar types, unlike chained comparisons in expressions which do
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not have such a restriction.
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We refer to the two or_expr's occurring on the left and right
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sides of the for-loop syntax as the bounds of the loop, and the
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middle or_expr as the variable of the loop. When a for-loop using
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the new syntax is executed, the expressions for both bounds will
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be evaluated, and an iterator object created that iterates through
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all integers between the two bounds according to the comparison
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operations used. The iterator will begin with an integer equal or
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near to the left bound, and then step through the remaining
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integers with a step size of +1 or -1 if the comparison operation
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is in the set described by less_comp or greater_comp respectively.
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The execution will then proceed as if the expression had been
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for variable in iterator
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where "variable" refers to the variable of the loop and "iterator"
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refers to the iterator created for the given integer interval.
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The values taken by the loop variable in an integer for-loop may
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be either plain integers or long integers, according to the
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magnitude of the bounds. Both bounds of an integer for-loop must
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evaluate to a real numeric type (integer, long, or float). Any
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other value will cause the for-loop statement to raise a TypeError
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exception.
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Issues
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The following issues were raised in discussion of this and related
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proposals on the Python list.
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- Should the right bound be evaluated once, or every time through
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the loop? Clearly, it only makes sense to evaluate the left
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bound once. For reasons of consistency and efficiency, we have
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chosen the same convention for the right bound.
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- Although the new syntax considerably simplifies integer
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for-loops, list comprehensions using the new syntax are not as
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simple. We feel that this is appropriate since for-loops are
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more frequent than comprehensions.
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- The proposal does not allow access to integer iterator objects
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such as would be created by xrange. True, but we see this as a
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shortcoming in the general list-comprehension syntax, beyond the
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scope of this proposal. In addition, xrange() will still be
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available.
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- The proposal does not allow increments other than 1 and -1.
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More general arithmetic progressions would need to be created by
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range() or xrange(), or by a list comprehension syntax such as
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[2*x for 0 <= x <= 100]
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- The position of the loop variable in the middle of a three-way
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comparison is not as apparent as the variable in the present
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for item in list
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syntax, leading to a possible loss of readability. We feel that
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this loss is outweighed by the increase in readability from a
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natural integer iteration syntax.
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- To some extent, this PEP addresses the same issues as PEP 276
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[4]. We feel that the two PEPs are not in conflict since PEP
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276 is primarily concerned with half-open ranges starting in 0
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(the easy case of range()) while this PEP is primarily concerned
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with simplifying all other cases. However, if this PEP is
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approved, its new simpler syntax for integer loops could to some
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extent reduce the motivation for PEP 276.
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- It is not clear whether it makes sense to allow floating point
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bounds for an integer loop: if a float represents an inexact
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value, how can it be used to determine an exact sequence of
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integers? On the other hand, disallowing float bounds would
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make it difficult to use floor() and ceiling() in integer
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for-loops, as it is difficult to use them now with range(). We
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have erred on the side of flexibility, but this may lead to some
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implementation difficulties in determining the smallest and
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largest integer values that would cause a given comparison to be
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true.
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- Should types other than int, long, and float be allowed as
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bounds? Another choice would be to convert all bounds to
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integers by int(), and allow as bounds anything that can be so
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converted instead of just floats. However, this would change
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the semantics: 0.3 <= x is not the same as int(0.3) <= x, and it
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would be confusing for a loop with 0.3 as lower bound to start
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at zero. Also, in general int(f) can be very far from f.
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Implementation
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An implementation is not available at this time. Implementation
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is not expected to pose any great difficulties: the new syntax
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could, if necessary, be recognized by parsing a general expression
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after each "for" keyword and testing whether the top level
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operation of the expression is "in" or a three-way comparison.
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The Python compiler would convert any instance of the new syntax
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into a loop over the items in a special iterator object.
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References
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[1] PEP 204, Range Literals
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http://www.python.org/peps/pep-0204.html
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[2] PEP 212, Loop Counter Iteration
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http://www.python.org/peps/pep-0212.html
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[3] PEP 281, Loop Counter Iteration with range and xrange
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http://www.python.org/peps/pep-0281.html
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[4] PEP 276, Simple Iterator for ints
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http://www.python.org/peps/pep-0276.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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fill-column: 70
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End:
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