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: $Revision$ Last-Modified: $Date$ Author: Heiko Wundram <[EMAIL PROTECTED]> 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(): <do something with node> as syntactic sugar for: for node in tree: if not node.haschildren(): continue <do something with node> 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: for_stmt: 'for' exprlist 'in' testlist_safe ('if' old_test)* ':' suite ['else' ':' suite] This change would replace testlist with testlist_safe 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 SETUP_LOOP 42 (to 45) 3 LOAD_GLOBAL 0 (range) 6 LOAD_CONST 1 (10) 9 CALL_FUNCTION 1 12 GET_ITER >> 13 FOR_ITER 28 (to 44) 16 STORE_FAST 0 (x) 19 LOAD_FAST 0 (x) 22 LOAD_CONST 2 (1) 25 COMPARE_OP 2 (==) 28 JUMP_IF_FALSE 9 (to 40) 31 POP_TOP 3 32 LOAD_FAST 0 (x) 35 PRINT_ITEM 36 PRINT_NEWLINE 37 JUMP_ABSOLUTE 13 >> 40 POP_TOP 41 JUMP_ABSOLUTE 13 >> 44 POP_BLOCK >> 45 LOAD_CONST 0 (None) 48 RETURN_VALUE 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&group_id=5470&atid=305470 Copyright This document has been placed in the public domain. === --- Heiko. -- http://mail.python.org/mailman/listinfo/python-list