🌐 Mailman 3 November 1999 - Python-list - python.org

TopSort in Python?
by Tim Peters Jan. 20, 2008

Jan. 20, 2008

From: "Tim Peters" <tim_one(a)email.msn.com> [Dinu C. Gherman] > Does anybody have a simple-minded-but-working full-Python > implementation of topsort, the topological sorting algorithm? > Or maybe *any* topsort? I remember only one such algorithm... > python.org/search doesn't reveal any... Searching for "topological" instead turns up two hits there, a few more on DejaNews. Apparently "the std" algorithm I posted years ago predates DejaNews, though. So here's a fancier version ... check-out-aaron's-kjbuckets-too-ly y'rs - tim # topsort takes a list of pairs, where each pair (x, y) is taken to # mean that x <= y wrt some abstract partial ordering. The return # value is a list, representing a total ordering that respects all # the input constraints. # E.g., # topsort( [(1,2), (3,3)] ) # may return any of (but nothing other than) # [3, 1, 2] # [1, 3, 2] # [1, 2, 3] # because those are the permutations of the input elements that # respect the "1 precedes 2" and "3 precedes 3" input constraints. # Note that a constraint of the form (x, x) is really just a trick # to make sure x appears *somewhere* in the output list. # # If there's a cycle in the constraints, say # topsort( [(1,2), (2,1)] ) # then CycleError is raised, and the exception object supports # many methods to help analyze and break the cycles. This requires # a good deal more code than topsort itself! from exceptions import Exception class CycleError(Exception): def __init__(self, sofar, numpreds, succs): Exception.__init__(self, "cycle in constraints", sofar, numpreds, succs) self.preds = None # return as much of the total ordering as topsort was able to # find before it hit a cycle def get_partial(self): return self[1] # return remaining elt -> count of predecessors map def get_pred_counts(self): return self[2] # return remaining elt -> list of successors map def get_succs(self): return self[3] # return remaining elements (== those that don't appear in # get_partial()) def get_elements(self): return self.get_pred_counts().keys() # Return a list of pairs representing the full state of what's # remaining (if you pass this list back to topsort, it will raise # CycleError again, and if you invoke get_pairlist on *that* # exception object, the result will be isomorphic to *this* # invocation of get_pairlist). # The idea is that you can use pick_a_cycle to find a cycle, # through some means or another pick an (x,y) pair in the cycle # you no longer want to respect, then remove that pair from the # output of get_pairlist and try topsort again. def get_pairlist(self): succs = self.get_succs() answer = [] for x in self.get_elements(): if succs.has_key(x): for y in succs[x]: answer.append( (x, y) ) else: # make sure x appears in topsort's output! answer.append( (x, x) ) return answer # return remaining elt -> list of predecessors map def get_preds(self): if self.preds is not None: return self.preds self.preds = preds = {} remaining_elts = self.get_elements() for x in remaining_elts: preds[x] = [] succs = self.get_succs() for x in remaining_elts: if succs.has_key(x): for y in succs[x]: preds[y].append(x) if __debug__: for x in remaining_elts: assert len(preds[x]) > 0 return preds # return a cycle [x, ..., x] at random def pick_a_cycle(self): remaining_elts = self.get_elements() # We know that everything in remaining_elts has a predecessor, # but don't know that everything in it has a successor. So # crawling forward over succs may hit a dead end. Instead we # crawl backward over the preds until we hit a duplicate, then # reverse the path. preds = self.get_preds() from random import choice x = choice(remaining_elts) answer = [] index = {} in_answer = index.has_key while not in_answer(x): index[x] = len(answer) # index of x in answer answer.append(x) x = choice(preds[x]) answer.append(x) answer = answer[index[x]:] answer.reverse() return answer def topsort(pairlist): numpreds = {} # elt -> # of predecessors successors = {} # elt -> list of successors for first, second in pairlist: # make sure every elt is a key in numpreds if not numpreds.has_key(first): numpreds[first] = 0 if not numpreds.has_key(second): numpreds[second] = 0 # if they're the same, there's no real dependence if first == second: continue # since first < second, second gains a pred ... numpreds[second] = numpreds[second] + 1 # ... and first gains a succ if successors.has_key(first): successors[first].append(second) else: successors[first] = [second] # suck up everything without a predecessor answer = filter(lambda x, numpreds=numpreds: numpreds[x] == 0, numpreds.keys()) # for everything in answer, knock down the pred count on # its successors; note that answer grows *in* the loop for x in answer: assert numpreds[x] == 0 del numpreds[x] if successors.has_key(x): for y in successors[x]: numpreds[y] = numpreds[y] - 1 if numpreds[y] == 0: answer.append(y) # following "del" isn't needed; just makes # CycleError details easier to grasp del successors[x] if numpreds: # everything in numpreds has at least one predecessor -> # there's a cycle if __debug__: for x in numpreds.keys(): assert numpreds[x] > 0 raise CycleError(answer, numpreds, successors) return answer

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Dealing with faults matters (was: Is Python dying?)
by claird＠Starbase.NeoSoft.COM Aug. 28, 2003

Aug. 28, 2003

In article <3706f449.12085377(a)news.cybercity.dk>, Morten Christensen <mmc(a)vip.cybercity.dk> wrote: >On 31 Mar 1999 06:01:03 -0600, claird(a)Starbase.NeoSoft.COM (Cameron >Laird) wrote: >>*I* certainly discuss it. Anyone who understands >>that exception-handling is important I invite to >>realize that exception-handling on the sociologi- >>cal-organizational dimension has an even bigger >>multiplier. >> >>This is a plank of my "Visual C considered profes- >>sional malpractice" campaign. > >??? > >...I understand exception handling and regard it as very important, >although the (Visual) C++ implementation is really awful (as everyone >who have experienced "unexplainable" Visual C++ exceptions of unknown >origin and type knows ).... I also know a lot about how exception >handling is actually implemented in modern OO languages such as C++ >(this was the subject I wrote my master thesis on).... > >HOWEVER, I have NO clue about what you mean by exception-handling as a >multiplier on the sociological-organization dimension :-) I would like >to find out. Can you explain? . . . Happily. Excuse my giddiness, please; I see that that last message *did* wander too far into the land of obscurity. Here's what I was saying: exception-handling is important. Programs that intelligently use exception-handling mechanisms are better programs. Exception-handling is even more important to people. People and organizations that deal with exceptions intelligently, and don't pre- tend that everything should be without fault, are better people and organizations. In fact, a correct computer program often has over half its lines devoted to accomo- dating error. A wise person will have good ways to respond to criticism/problems/diffi- culties/...; dealing with success is relatively easier. -- Cameron Laird http://starbase.neosoft.com/~claird/home.html claird(a)NeoSoft.com +1 281 996 8546 FAX

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Bug in v1.5.2 Websucker GUI?
by dhfx＠realtime.net June 13, 2003

June 13, 2003

I'm running v1.5.2 on a Win95 box. I tried wsgui.py and got the following error message: ... File "C:\ ... \WSGUI.PY", line 165, in go self.sucker.rootdir = os.path.dirname( [ websucker.Sucker.savefilename(self.url)) ] TypeError: unbound method must be called with class instance 1st arg (the part in [ ] didn't appear in the message - I added it from the code) The object-instance self.sucker is an instantiation of class SuckerThread, which is derived from class Sucker of the websucker code module. The SuckerThread class includes a method savefilename() which overwrites the method in websucker.Sucker of the same name. Whoever coded wsgui evidently wanted to use the method of the parent class, rather than the method of the derived class. What seems to be giving trouble is that the arg of dirname() is a class method, rather than a method of an instance of the class. Here's what I did: I defined a new method savefilename1() in SuckerThread which just calls the savefilename() of the parent class: def savefilename1(self, url): return websucker.Sucker.savefilename(self, url) Then I changed the code at "line 165" to the following: self.sucker.rootdir = os.path.dirname( self.sucker.savefilename1(url)) and now it works. Note that the arg "self" of savefilename1() in the line above is not needed, it's implicit. In fact, when I had it in, I got an error message that I had too many args! David Friedman

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Re: automatic "lotjes trekken"
by François Pinard Aug. 26, 2002

Aug. 26, 2002

"Tim Peters" <tim_one(a)email.msn.com> writes: > [...] with a belly full of far too much turkey. Strange. I was imagining you rather slim. Are you? :-) > where p[:] ranges over all permutations of range(N) [...] By the way, I do not remember any standard library helper to enumerate all permutations (or combinations, or arrangements) of a given list (or tuple). Ideally one at a time, of course. Would this be useful? -- François Pinard http://www.iro.umontreal.ca/~pinard

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making the Python case to my dept.
by William Ahern Dec. 23, 1999

Dec. 23, 1999

Hello, I have never used Python, but I'm currently attempting to persuade a department at my university (Geroge Washington) to use Python as an intro language. I have read that it was good for this. From what I hear, the current options on the table are Delphi and Java. I don't know much about Delphi, but I doubt Java is any good for intro classes. Could anybody suggest some substantive info to make the case (or not make it). Also, the department will need an IDE (Win95/Mac), so if anybody knows where to find one.... I know Python is being used on a broad range of applications. Complaints from students these days, a professor told me, is that the current language, Pascal, is outdated and doesn't have much applicability in the real world. I tend to agree, though I'm an International Affairs major, not CS. I've seen all of the energy and spirit behind the Python community, and I can't fathom why any cash strapped organization would drop a few grand for something that is freely available on the internet. Anyhow, how much support coukd somebody need for an IDE (and a prgramming language). Not to mention the fact that a free and OSS language implementation and IDE would preclude students from secretly passing around the only CD in class w/ the applications on it. I figure using Python would be a start to getting people to think outside of the Microsoft culture (for better or worse). In any event, please at least CC: responses to wahern(a)jinsa.org. I'm not subscribed to this list. Thanx a bunch, Bill Ahern MIS, JINSA

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Re: eff-bot Python book ... revisited
by François Pinard Dec. 23, 1999

Dec. 23, 1999

Ivan Van Laningham <ivanlan(a)callware.com> writes: > (And last time I checked, the PDF viewer for Linux sucked; maybe that's > changed by now, maybe Adobe offers their own 4.0 viewer for Linux. ...) I'm far from PDF, but it happened that Ghostscript helped me at viewing it conveniently on Linux -- through `gv', I rarely use Ghostscript directly. However, sending to the printer from `gv' (a Tkinter introduction, if I remember well?) was less happy, as the inter-letter spacing was all wrong, making the printed result especially difficult to read. Even if it helps me (?!) at reading more slowly, the fatigue grows faster. I have no idea of the cause of this random spacing, far less the solution. -- François Pinard http://www.iro.umontreal.ca/~pinard

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Order of keyword arguments
by Andres Corrada Dec. 21, 1999

Dec. 21, 1999

Hi, Can one access the order in which keyword arguments were passed to a function? I want to call a function with the keyword syntax: f( foo='theFooString', bar='theBarString' ) and use the fact that foo was listed first so I can do something like: def f( **kw ): Process keyword arguments remembering the order in which the were used to call the function. Print first keyword Print second keyword . . . ------------------------------------------------------ Andres Corrada-Emmanuel Email: andres(a)corrada.com Owner http://www.corrada.com/mamey Mamey Phone: (413) 587-9595 ------------------------------------------------------

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python apache module?
by Rob Nikander Dec. 19, 1999

Dec. 19, 1999

I am just beginning to learn python and so far I think it is awesome. I was wondering if there is an Apache module analogous to mod_perl for python? I don't see anything on the Apache page. If not, is someone working on one? Rob

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Newbie question : Info on Python under DOS/Windows
by endgamer71＠hotmail.com Dec. 19, 1999

Dec. 19, 1999

Okay, I'm really new to Python, and have limited experience of real programming (yes, the age-old story....crippled by BASIC at about the age of 10 <g>). I'm learning from texts, this froup and O'Reilley's "Programming Python" and as a starting project I'm writing a Y2K mini-fix for Wintel machines - a program which prompts the user for the current date and feeds it into "date", for machines which won't hold the date on reboot after the millenium. My question is: I've found it hard to locate resources or information on Python under DOS/Windows - are there any special groups, sites or FAQs/Manuals with general information? Much as I hate MS environments, in my line of work I have to work with them almost constantly (back on my lovely Linux machine whenever I get a chance <g>) and I suspect I'll need this information again... Endgamer Spider Warrior (Endgamer71(a)spamtrap.hotmail.com) ##################################### Not a quote, just an on-the-spot observation: You know you're a geek when your PC costs exactly 15 times as much as your stereo. And gives better sound quality.<sigh> Endgamer #####################################

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Python + JavaSpaces?
by Greg Wilson Dec. 17, 1999

Dec. 17, 1999

Has anyone built (or thought about building) a Python interfaces to JavaSpaces? Thanks, Greg

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