On Friday, April 6, 2018 at 9:03:05 PM UTC-4, Raymond Hettinger wrote: > > > On Friday, April 6, 2018 at 8:14:30 AM UTC-7, Guido van Rossum wrote: > > On Fri, Apr 6, 2018 at 7:47 AM, Peter O'Connor <peter.ed...@gmail.com> > wrote: > >> So some more humble proposals would be: > >> > >> 1) An initializer to itertools.accumulate > >> functools.reduce already has an initializer, I can't see any > controversy to adding an initializer to itertools.accumulate > > > > See if that's accepted in the bug tracker. > > It did come-up once but was closed for a number reasons including lack of > use cases. However, Peter's signal processing example does sound > interesting, so we could re-open the discussion. > > For those who want to think through the pluses and minuses, I've put > together a Q&A as food for thought (see below). Everybody's design > instincts are different -- I'm curious what you all think think about the > proposal. > > > Raymond > > --------------------------------------------- > > Q. Can it be done? > A. Yes, it wouldn't be hard. > > _sentinel = object() > > def accumulate(iterable, func=operator.add, start=_sentinel): > it = iter(iterable) > if start is _sentinel: > try: > total = next(it) > except StopIteration: > return > else: > total = start > yield total > for element in it: > total = func(total, element) > yield total > > Q. Do other languages do it? > A. Numpy, no. R, no. APL, no. Mathematica, no. Haskell, yes. >
Isn't numpy a yes? https://docs.scipy.org/doc/numpy/reference/generated/numpy.ufunc.accumulate.html They definitely support it for add and multiply. It's defined, but doesn't seem to work on custum ufuncs (the result of frompyfunc). > > * > http://docs.scipy.org/doc/numpy/reference/generated/numpy.ufunc.accumulate.html > > * https://stat.ethz.ch/R-manual/R-devel/library/base/html/cumsum.html > * http://microapl.com/apl/apl_concepts_chapter5.html > \+ 1 2 3 4 5 > 1 3 6 10 15 > * https://reference.wolfram.com/language/ref/Accumulate.html > * https://www.haskell.org/hoogle/?hoogle=mapAccumL > > > Q. How much work for a person to do it currently? > A. Almost zero effort to write a simple helper function: > > myaccum = lambda it, func, start: accumulate(chain([start], it), func) > > > Q. How common is the need? > A. Rare. > > > Q. Which would be better, a simple for-loop or a customized itertool? > A. The itertool is shorter but more opaque (especially with respect > to the argument order for the function call): > > result = [start] > for x in iterable: > y = func(result[-1], x) > result.append(y) > > versus: > > result = list(accumulate(iterable, func, start=start)) > > > Q. How readable is the proposed code? > A. Look at the following code and ask yourself what it does: > > accumulate(range(4, 6), operator.mul, start=6) > > Now test your understanding: > > How many values are emitted? > What is the first value emitted? > Are the two sixes related? > What is this code trying to accomplish? > > > Q. Are there potential surprises or oddities? > A. Is it readily apparent which of assertions will succeed? > > a1 = sum(range(10)) > a2 = sum(range(10), 0) > assert a1 == a2 > > a3 = functools.reduce(operator.add, range(10)) > a4 = functools.reduce(operator.add, range(10), 0) > assert a3 == a4 > > a4 = list(accumulate(range(10), operator.add)) > a5 = list(accumulate(range(10), operator.add, start=0)) > assert a5 == a6 > > > Q. What did the Python 3.0 Whatsnew document have to say about reduce()? > A. "Removed reduce(). Use functools.reduce() if you really need it; > however, 99 percent of the time an explicit for loop is more readable." > > > Q. What would this look like in real code? > A. We have almost no real-world examples, but here is one from a > StackExchange post: > > def wsieve(): # wheel-sieve, by Will Ness. > ideone.com/mqO25A->0hIE89 > wh11 = [ 2,4,2,4,6,2,6,4,2,4,6,6, 2,6,4,2,6,4,6,8,4,2,4,2, > 4,8,6,4,6,2,4,6,2,6,6,4, 2,4,6,2,6,4,2,4,2,10,2,10] > cs = accumulate(cycle(wh11), start=11) > yield( next( cs)) # cf. ideone.com/WFv4f > ps = wsieve() # > codereview.stackexchange.com/q/92365/9064 > p = next(ps) # 11 > psq = p*p # 121 > D = dict( zip( accumulate(wh11, start=0), count(0))) # start > from > sieve = {} > for c in cs: > if c in sieve: > wheel = sieve.pop(c) > for m in wheel: > if not m in sieve: > break > sieve[m] = wheel # sieve[143] = wheel@187 > elif c < psq: > yield c > else: # (c==psq) > # map (p*) (roll wh from p) = roll (wh*p) from (p*p) > x = [p*d for d in wh11] > i = D[ (p-11) % 210] > wheel = accumulate(cycle(x[i:] + x[:i]), start=psq) > p = next(ps) ; psq = p*p > next(wheel) ; m = next(wheel) > sieve[m] = wheel > _______________________________________________ > Python-ideas mailing list > python...@python.org <javascript:> > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ >
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