Mailman 3 April 2013 - Edu-sig

explaining about decorators
by Kirby Urner 17 Apr '13

17 Apr '13

I'm sure this could be improved upon and/or is discussed more eloquently elsewhere. I'm inviting links, comments. I recall our own discussion here on edu-sig where we played with @D def f(x):... as taking a Derivative of f. Guido didn't think it was such a great idea as I recall. http://aroberge.blogspot.com/2005/04/computing-derivatives-using-python.html Kirby === Excerpt from a note to a student: Why do we need decorators you ask? We should go back to Guido's original complaint about the old way of doing it, which was to define a method, say m, and then go m = staticmethod(m) at the end, to apply a transformation to the method, which would not change its name, but would make it a static method, meaning it did not require self, nor even the class as a first argument: >>> class Foo: ... def add(a,b): ... return a + b ... add = staticmethod(add) ... >>> obj = Foo() >>> obj.add(2,3) 5 Likewise property( ) and classmethod( ): these were applied after method m had already been defined, retroactively as it were. That was Guido's complaint, the fact that transformations were applied as a footnote, as it were, rather than boldly up front (at the top). All a decorator is, is syntax for saying "the function or class defined below is going to be put through @this and @that ringer where 'ringer' means other classes or functions". It's a way of indicating the transformations up front, versus waiting until the bottom / end of method, which Guido thought a place of relative obscurity, maybe on some next page. Buried. Harder to parse. These two say the same thing: class A: pass # could go on for pages A = foo(bar(A)) @foo @bar class A: pass Like so much of Python, such syntax may be put to nefarious uses, i.e. Python gives you more than enough rope to hang yourself with its liberal / powerful syntax. Misuses of decorators are certainly possible, where a "misuse" might be anything that complicates, obscures, confuses. Back to the original example: >>> class Foo: ... @staticmethod ... def add(a,b): ... return a + b has a certain elegance about it. In retrospect, the innovation has proved powerful. As Raymond Hettinger recounts in his recent Pycon keynotes, it was a case of Guido asking everyone for feedback, their coming to near consensus in a rare moment, and him doing something else anyway. Something like that. -Kirby

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another Python generator (Bernoulli numbers)
by kirby urner 17 Apr '13

17 Apr '13

from fractions import Fraction as R def choose(n, k): """ chosen / adapted from: http://www.velocityreviews.com/forums/t502438-combination-function-in-pytho… """ ntok = 1 for t in range(min(k, n-k)): ntok = ntok*(n-t)//(t+1) return ntok def Bernoulli(): """ Bernoulli Numbers using Fraction type numbers http://en.wikipedia.org/wiki/Bernoulli_number#Recursive_definition http://oeis.org/A027641 http://oeis.org/A027642 """ B = [R(1,1)] def Sum(m): total = R(0,1) for k in range(0,m): total += choose(m, k) * R(B[k],m-k+1) return 1 - total m = 1 while True: B.append(Sum(m)) yield B[-1] m += 1 ==== >>> thegen = Bernoulli() >>> [next(thegen) for i in range(20)] [Fraction(1, 2), Fraction(1, 6), Fraction(0, 1), Fraction(-1, 30), Fraction(0, 1), Fraction(1, 42), Fraction(0, 1), Fraction(-1, 30), Fraction(0, 1), Fraction(5, 66), Fraction(0, 1), Fraction(-691, 2730), Fraction(0, 1), Fraction(7, 6), Fraction(0, 1), Fraction(-3617, 510), Fraction(0, 1), Fraction(43867, 798), Fraction(0, 1), Fraction(-174611, 330)] >>> import pprint >>> pprint.pprint([next(thegen) for i in range(20)], width=80) [Fraction(0, 1), Fraction(854513, 138), Fraction(0, 1), Fraction(-236364091, 2730), Fraction(0, 1), Fraction(8553103, 6), Fraction(0, 1), Fraction(-23749461029, 870), Fraction(0, 1), Fraction(8615841276005, 14322), Fraction(0, 1), Fraction(-7709321041217, 510), Fraction(0, 1), Fraction(2577687858367, 6), Fraction(0, 1), Fraction(-26315271553053477373, 1919190), Fraction(0, 1), Fraction(2929993913841559, 6), Fraction(0, 1), Fraction(-261082718496449122051, 13530)]

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PyOhio 2013 Call For Proposals
by Brian Costlow 16 Apr '13

16 Apr '13

PyOhio 2013, the annual Python programming conference for Ohio and the surrounding region, is now accepting proposals for scheduled talks, tutorials, and panels. This year's PyOhio will will take place Saturday, July 27th, and Sunday, July 28th, 2013 at the Ohio Union, on the campus of The Ohio State University in Columbus, Ohio. One of the areas we would like to emphasize this year is education: using Python to teach about computing, using Python to build tools to help teach other subjects, and educating people about the usefulness of Python itself. Deadline for submitting a proposal is June, 1st 2013 Details about submitting a proposed talk are here: http://pyohio.org/call-for-proposals/ The 2013 PyOhio Organizing Committee

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another Python generator (logarithmic numbers)
by kirby urner 15 Apr '13

15 Apr '13

from fractions import Fraction as R def Gregory(): """ Gregory Coefficients ( http://en.wikipedia.org/wiki/Euler-Mascheroni_constant) 1/2, 1/12, 1/24, 19/720, 3/160, 863/60480... using fraction module to keep it rational. """ G = [R(-1,1)] def Sum(n): total = R(0,1) for k in range(0,n): total += R(G[k],n+1-k) return -total n = 1 while True: G.append(Sum(n)) yield G[-1] n += 1 ==== >>> from anyproject import Gregory >>> thegen = Gregory() >>> next(thegen) Fraction(1, 2) >>> next(thegen) Fraction(1, 12) >>> next(thegen) Fraction(1, 24) >>> next(thegen) Fraction(19, 720) >>> next(thegen) Fraction(3, 160) >>> next(thegen) Fraction(863, 60480) >>> next(thegen) Fraction(275, 24192) >>> next(thegen) Fraction(33953, 3628800) >>> next(thegen) Fraction(8183, 1036800) ... Kirby

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import turtle ERROR
by Karine Laidley 12 Apr '13

12 Apr '13

Hi python experts, I am teaching python turtle graphics now and in class, most students have been successful using it. However, several students today got the same error message complaining about "module does not exist". If they run the same code in the shell (I am using version 3.3), it works just fine. But the second they copy the code into the code editor window (file), it fails with that error. Then I thought maybe it is because the file is saved on the school network under the student's "My Documents" set up on the server side. So I copied the file to the desktop and test it from there but that failed too. I eventually copied the file to the python directory (c:\python33) and that worked. So I guess the location of the file decided to act out just for those few students... Other students using the same computers were fine. Is there a way to make this work CONSISTENTLY for all users from their own "My Documents" on the server? I would hate to make them all save on the C drive, for backup issues and security issues (they can see other people's files there)... I hope this is a simple fix!! Please let me know... Thank you, Karine Laidley Computer Science Teacher (6th and 7th grade) AMSA Charter School Marlborough, MA (508)597-2400 k.laidley(a)amsacs.org

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