[Python-Dev] PEP-xxx: Unification of for statement and list-comp syntax (original) (raw)

Guido van Rossum guido at python.org
Sun May 21 17:38:49 CEST 2006

-1. The contraction just makes it easier to miss the logic.

Also, it would be a parsing conflict for the new conditional expressions (x if T else y).

This was proposed and rejected before.

--Guido

On 5/21/06, Heiko Wundram <me+python-dev at modelnine.org> wrote:

Hi all!

The following PEP tries to make the case for a slight unification of for statement and list comprehension syntax. Comments appreciated, including on the sample implementation. === PEP: xxx Title: Unification of for-statement and list-comprehension syntax Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Heiko Wundram <me at modelnine.org> Status: Active Type: Standards Track Content-Type: text/plain Created: 21-May-2006 Post-History: 21-May-2006 17:00 GMT+0200

Abstract When list comprehensions were introduced, they added the ability to add conditions which are tested before the expression which is associated with the list comprehension is evaluated. This is often used to create new lists which consist only of those items of the original list which match the specified condition(s). For example: [node for node in tree if node.haschildren()] will create a new list which only contains those items of the original list (tree) whose items match the havechildren() condition. Generator expressions work similarily. With a standard for-loop, this corresponds to adding a continue statement testing for the negated expression at the beginning of the loop body. As I've noticed that I find myself typing the latter quite often in code I write, it would only be sensible to add the corresponding syntax for the for statement: for node in tree if node.haschildren(): as syntactic sugar for: for node in tree: if not node.haschildren(): continue There are several other methods (including generator-expressions or list-comprehensions, the itertools module, or the builtin filter function) to achieve this same goal, but all of them make the code longer and harder to understand and/or require more memory, because of the generation of an intermediate list. Implementation details The implementation of this feature requires changes to the Python grammar, to allow for a variable number of 'if'-expressions before the colon of a 'for'-statement: forstmt: 'for' exprlist 'in' testlistsafe ('if' oldtest)* ':' suite ['else' ':' suite] This change would replace testlist with testlistsafe as the 'in'-expression of a for statement, in line with the definition of list comprehensions in the Python grammar. Each of the 'if'-expressions is evaluated in turn (if present), until one is found False, in which case the 'for'-statement restarts at the next item from the generator of the 'in'-expression immediately (the tests are thus short-circuting), or until all are found to be True (or there are no tests), in which case the suite body is executed. The behaviour of the 'else'-suite is unchanged. The intermediate code that is generated is modelled after the byte-code that is generated for list comprehensions: def f(): for x in range(10) if x == 1: print x would generate: 2 0 SETUPLOOP 42 (to 45) 3 LOADGLOBAL 0 (range) 6 LOADCONST 1 (10) 9 CALLFUNCTION 1 12 GETITER >> 13 FORITER 28 (to 44) 16 STOREFAST 0 (x) 19 LOADFAST 0 (x) 22 LOADCONST 2 (1) 25 COMPAREOP 2 (==) 28 JUMPIFFALSE 9 (to 40) 31 POPTOP 3 32 LOADFAST 0 (x) 35 PRINTITEM 36 PRINTNEWLINE 37 JUMPABSOLUTE 13 >> 40 POPTOP 41 JUMPABSOLUTE 13 >> 44 POPBLOCK >> 45 LOADCONST 0 (None) 48 RETURNVALUE where all tests are inserted immediately at the beginning of the loop body, and jump to a new block if found to be false which pops the comparision from the stack and jumps back to the beginning of the loop to fetch the next item. Implementation issues The changes are backwards-compatible, as they don't change the default behaviour of the 'for'-loop. Also, as the changes that this PEP proposes don't change the byte-code structure of the interpreter, old byte-code continues to run on Python with this addition unchanged. Implementation A sample implementation (with updates to the grammar documentation and a small test case) is available at: http://sourceforge.net/tracker/index.php?func=detail&aid=1492509&groupid=5470&atid=305470 Copyright This document has been placed in the public domain. === --- Heiko.

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-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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