Mailman 3 February 2000 - Python-list

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

8 9

Blank ASP pages
by python Dec. 10, 2004

Dec. 10, 2004

My server is spitting out blank .asp pages which use python. If Python is not declared, the asp pages work fine. I just yesterday stepped up to Python 1.52 and the new win32all After the install, the blanks started appearing. All the paths look fine in registry. Any clues? ( Intel dual PIII 550 Xeon, 1G RAM, NT 4.0 SP 6a, IIS 4) ************************************************** Jason S. Nadler Lead Programmer Channel21 Productions, Inc. ... What are you doing right now? **************************************************

3 2

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

3 2

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

5 8

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

4 5

Circular references and python
by Jason Stokes Jan. 27, 2002

Jan. 27, 2002

Well, my adventures with the Python language are moving along. One thing that worries me is the lack of garbage collection in Python; the application I'm thinking of using Python for demands many data structures that use circular references, but Python only implements a reference counting scheme in the way of memory management. How do I use such data structures in Python?

17 27

New Python Books
by python＠commandprompt.com Nov. 2, 2001

Nov. 2, 2001

Hello, I recently posted this message to comp.os.python: Hello, I represent LinuxPorts.Com (http://www.linuxports.com/) and I have been approached by a printer to possibly publish OpenSource books. To start these books would just be paper versions of the actual documentation. In this case, the Python Documentation and we were wondering if this would be of interest to the Python Community. I spoke with the printer and we both have agreed that if we do this that we will be providing a portion of the Gross Profits back to the individual projects. In this case the Python project. Anyway, if this of interest to you please go to http://www.linuxports.com/ and make your vote. It will help us determine the demand for such a book. ----- We received about 8 responses with feedback that we much appreciated and would now like to solicit some more. If the people that read this email could just think about a few of the following questions and provide some feedback we would be very thankful. We are considering publishing the current set of Python documentation as a single volume versus several volumes such as toExcel does. Would this be of interest to the Python community? One person made mention of the "Vaults of Parnassus" if we were to include a snapshot of this would this be of interest? We are also considering "paying" someone to write a book about Python. Unfortunately, unlike traditional publishers the author would not get paid until the book was done, and then it would be on the basis of per book sold. We are aware that this is not the normal way of doing things, but we are not Prentice Hall and this would be the only way "currently" that we could pull this off. Is there anyone on this list that feels confident enough in their Python skills and writing ability to be able to perform such a task? Last but definitely not least, no matter what we do, if we publish a Python book, 20% to 40% of the Gross Profits would go back to the PSA. Thanks for your time: Joshua Drake Webmaster - LinuxPorts.Com WebMaster - LinuxDoc.org

3 3

does pythonwin support adsi`
by test Jan. 16, 2001

Jan. 16, 2001

and were to I get examples? thanks tr

4 3

Customize keybindings in IDLE??
by Thomas Weholt May 24, 2000

May 24, 2000

Hallo, Idle is coming along fine now, but the key-bindings suck. How can I change this to the good-old CTRL+C for copy, CTRL+V for paste etc. ??? And would the changes be saved between sessions?? ( Saw another post asking about saving syntax-coloring setup yearlier so ... ) Thomas

2 1

map.yahoo.com uses Python?
by Aaron J Reichow April 5, 2000

April 5, 2000

I went to maps.yahoo.com to look something up, and lo and behold, it looks like they use Python for their map CGI -- http://maps.yahoo.com redirects you to http://maps.yahoo.com/py/maps.py. Just an interesting note, I suppose. Aaron

3 2