Feature Requests item #1673203, was opened at 2007-03-03 19:21 Message generated for change (Comment added) made by twobitsprite You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=355470&aid=1673203&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: None Group: None Status: Open Resolution: None Priority: 5 Private: No Submitted By: paul rubin (phr) Assigned to: Raymond Hettinger (rhettinger) Summary: add identity function Initial Comment: Requested and assigned to Raymond at his suggestion: http://groups.google.com/group/comp.lang.python/msg/603870361743c85c There should be an identify function identity(x)=x. I suggest it also take and ignore an optional second arg: identity(x1,x2)=x1. The second arg can be useful in some generator expressions: foo = (x for x in bar if condition(x) and identity(True, memoize(x)) That allows calling memoize (or some other function) on the selected elements in the genexp, and disposing of the returned value. It's sort of like the const function (K combinator) to go along with the identity function's I combinator. OK, the above is not really in the functional spirit, but it's been useful. There could conceivably be also an actual const function const(k)=partial(identity,k) but I can't remember needing that in Python code. The two-arg identity function (uncurried version of const) is probably enough. ---------------------------------------------------------------------- Comment By: Memotype (twobitsprite) Date: 2007-03-22 09:06 Message: Logged In: YES user_id=1679533 Originator: NO I also would like to have a built-in identity function (in fact, I found this by googling "python identity function"). My use-case is a bit different. I ofter find myself wanting to simply specify a function for be used in a loop, something like: def f(items): if something: wrapper = int else: wrapper = identity for item in items: yield wrapper(item) of course, usually it's a bit more complex than that, but you get the idea... and I supposed its actually more like the previous use-case than I thought. I realize I could just use "lambda x: x", but I feel that comes with an unnecessary performance impact for something so trivial. I don't know how much python does to compile built-in functions, but I imagine that the identity function can be mostly optimized out at compile time if it were built-in. Just my two-cents. ---------------------------------------------------------------------- Comment By: Alexander Belopolsky (belopolsky) Date: 2007-03-19 15:57 Message: Logged In: YES user_id=835142 Originator: NO I have just realized that the requested functionality is known in C as the comma operator. I often find myself writing "return Py_INCREF(o),o;" in my C code, but I cannot really defend that idiom against "Py_INCREF(o); return o;" alternative. My personal reason is entirely C-specific, if followed an if(), the first form does not require curly braces. In any case, comma operator can be emulated in python as exp1,expr2,expr3 -> (exp1,expr2,expr3)[-1] Since multi-argument "identity" is likely to be rejected, my proposal to alter the order of arguments is moot. My other suggestion that with identity, map(None, ..) should be deprecated in favor of map(identity, ..) is probably an arument against the identity proposal now. ---------------------------------------------------------------------- Comment By: Collin Winter (collinwinter) Date: 2007-03-19 15:05 Message: Logged In: YES user_id=1344176 Originator: NO I can see adding the 1-argument form to operator or functools (as it's useful in functional programming), but the 2-argument form you've suggested is right out. If you really feel the need to torture a "for" loop into a genexp/listcomp like that, foo = (x for x in bar if condition(x) and [memoize(x)]) does the same thing using today's capabilities. ---------------------------------------------------------------------- Comment By: Josiah Carlson (josiahcarlson) Date: 2007-03-12 16:06 Message: Logged In: YES user_id=341410 Originator: NO Not all x line functions should be built into Python. Further, Python's standard syntax offers an infix operator that does the same thing (though in slightly different order as described below, you can reorder with minimal effort). identity(X, Y) -> (Y and False) or X Also, the only use-case that you are provided and that I can imagine, are examples like you provide where one is changing state within a statement (if, elif, while, etc.) or expression (generator, list comprehension, conditional, etc.). ---------------------------------------------------------------------- Comment By: Alexander Belopolsky (belopolsky) Date: 2007-03-05 09:21 Message: Logged In: YES user_id=835142 Originator: NO 1. If this proposal is accepted, it will make sense to deprecate the use of None as an identity function in map: >>> map(None, range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 2. Some other languages have an dyadic identity function that returns the *second* argument. For example, K has : primitive: identity:(:) identity[1;2] 2 The rationale in K is that it is useful in an ammed function that replaces entries of an an array with a result of a dyadic function applied to the old and the supplied value and it is natural to have old value first: @[1 2 3;1;-;20] 1 -18 3 @[1 2 3;1;:;20] 1 20 3 This rationale does not apply to Python, but in the absence of other reasons to choose the order of arguments, Python may as well follow the precedent. Does anyone know a less exotic language that has a dyadic identity? ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=355470&aid=1673203&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
