🌐 Mailman 3 December 2006 - Python-list - python.org

do design patterns still apply with Python?
by John Salerno Jan. 27, 2008

Jan. 27, 2008

Since Python does so many things different, especially compared to compiled and statically typed languages, do most of the basic design patterns still apply when writing Python code? If I were to read a design pattern book (such as Head First Design Patterns), could I apply their Java examples to Python easily enough, or does Python require a different perspective when applying patterns?

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TopSort in Python?
by Tim Peters Jan. 21, 2008

Jan. 21, 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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fun with unicode files
by Thomas Heller May 19, 2007

May 19, 2007

I want to use ConfigParser with both NT4-style .reg files, which are ascii (or ansi?) files, and XP-stype .reg files which seem to be UTF-16 encoded unicode-files (hope that's the correct terminology). [And yes, I have read the warning in the manual that ConfigParser doesn't interpret the value-type prefixes in the reg files] Here's the start of the method I wrote to detect the encoding and read the file: def _parse_regfile(self, filename): ifi = open(filename, "r") import codecs, StringIO if ifi.read(2) in (codecs.BOM_LE, codecs.BOM_BE): ifi.close() ifi = codecs.open(filename, "r", "utf-16") I wonder: do I really have to check for the BOM manually, or is there a Python function which does that? Continuing the code: # ConfigParser calls .readline(), but: # NotImplementedError: '.readline() is not implemented for UTF-16' # so we need to put the data into a StringIO instance. # Um, cStringIO doesn't handle unicode correctly, so we'll have # to use the slower StringIO ifi = StringIO.StringIO(ifi.read()) ifi.readline() # skip the first two lines ifi.readline() c = ConfigParser() c.readfp(ifi) return c Is there a better way to do this? Why doesn't the UTF-16 codec implement readline()? Thomas

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Uninstall speed with Python 2.4 MSI
by David Bolen May 19, 2007

May 19, 2007

Although it isn't often necessary, I tend to be in the habit of uninstalling Python before updating to new minor releases, or especially during the alpha/beta cycle. I'm in the process of uninstalling 2.4a2 to install 2.4a3 and the uninstall is running absolutely dog slow and burning 100% cpu while doing it (all going to mshta.exe). Watching the progress bar it almost seems to be doing a whole bunch of operations per file or something. The mshta process went away after the removal so I don't believe there was any other scripting running on my system simultaneously. It probably took 5+ minutes for the removal. I can't recall if removing 2.4a1 was like this, but certainly the non-MSI versions weren't, even when all the docs were separate files. I was wondering if anyone else has experience this? I may not have the fastest system at the moment, but this has to have been one of the slowest (proceeeding, if not absolute time) uninstalls I've done in a long while. -- David

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Re: LRU cache (and other things missing from the standard library ...)
by Evan Prodromou March 30, 2007

March 30, 2007

Gerhard, I agree that LRU caches are a data structure too frequently re-implemented in Python. I'd also like to see a "standard" cache package, either as part of the standard library or elsewhere. To that end, I've just uploaded an LRU cache package to the Python Package Index: http://www.python.org/pypi?:action=display&name=lrucache&version=0.2 Please, let me know if there are features you'd need from a cache class that aren't present, implementation improvements that could be made, or what have you. I'm working on a version 0.3 that includes a transparent cache wrapper and LRU memoization for functions. ~ESP -- Evan Prodromou .O. http://bad.dynu.ca/~evan/ ..O evan(a)bad.dynu.ca OOO

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DOS, UNIX and tabs
by Ben Jan. 26, 2007

Jan. 26, 2007

Hi, I have a python script on a unix system that runs fine. I have a python script on a windows system that runs fine. Both use tabs to indent sections of the code. I now want to run them on the same system, actually in the same script by combining bits and pieces. But whatever I try my windows tabs get converted to spaces when I transfer it to the unix system and the interpreter complains that the indentation style is not consistent throughout the file. Short of going through 350 lines of code and manually replacing spaces with tabs what an I do? I'm thinking there surely must be a simple solution I have missed here! Cheers, Ben

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Re: loose methods: Smalltalk asPython
by Carl Banks Jan. 26, 2007

Jan. 26, 2007

Jan Theodore Galkowski wrote: > > We've not had > > an excellent dynamic OO language since Smalltalk, IMO. I would say that "excellence" in object oriented programming is not a strong design goal of Python. Python tries to support OOP well, but not to enhance OOP to the detriment of other, more important goals. [snip] > >Guido was opposed to modifying builtin types before Java existed. It's > >similar to his opposition to dynamic syntax. > > Opposition or not, the language definition is there. Surely Smalltalk's > OO style can be respected. Smalltalkers have been doing OO in a dynamic > context longer than many. There are things to learn there. Well, that Smalltalk and Ruby do this at least suggests that it won't be the end of the world as we know it if it were to happen in Python. Personally, I'm with Guido here. Allowing this might make Python's OOP more "excellent", but it would be to the detriment of other, more important goals, like minimizing surprise. > There are two things here that need to be kept straight, I think. > > One is the primacy of builtin types. > > The other is placing those builtins at the top of their own object > hierarchy. Is "int", properly speaking, a descendent of "object"? No, > it's the root of its own tree. Are you saying that it shouldn't be a descendant of "object", or that it isn't? "int" is certainly a subclass of "object" in Python (as are all objects). > Smalltalk has "int" as well, as a > *subclass* of Number. It's primitive. Number is primitive? I thought all types in Smalltalk had a common base type. Anyways, Python 3000 is exploring the idea of an abstract hierarchy that has classes (interfaces?) like Number and Sequence. In fact, I even saw some talk about somehow being able to retrofit classes to support one of these interfaces/ABCs, though it was pretty conjectural. > In any case, as respectful of Mr/Dr van Rossum as I am, simply because > so-and-so says something is bad and therefore it must be true is a > fallacious response. The same could be said of Smalltalk, i.e., because Smalltalk does it therefore it is good is just as fallacious. Regardless, whatever Guido pronounces is what happens. If he's adamantly against something, it's pretty much an academic discussion. Carl Banks

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New os.path.exists() behavior - bug or feature?
by klappnase Jan. 24, 2007

Jan. 24, 2007

Hi all, yesterday I installed Python-2.5 and python-2.4.4 on my windows2k box. When trying to use os.path.exists() to detect which drives are present on the system I noticed that these behave differently: Python 2.4.4 (#71, Oct 18 2006, 08:34:43) [MSC v.1310 32 bit (Intel)] on win32 Type "copyright", "credits" or "license()" for more information. >>> import os >>> for char in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ': if os.path.exists('%s:\\' % char): print '%s:\\' % char A:\ C:\ D:\ E:\ >>> ########################################### Python 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)] on win32 Type "copyright", "credits" or "license()" for more information. >>> import os >>> for char in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ': if os.path.exists('%s:\\' % char): print '%s:\\' % char C:\ >>> When I insert a floppy into A: os.path.exists('A:\\') will return True on Python-2.5, too. Does anyone know, is this inconsistent behavior a bug or a new feature? I also noticed that the Tix binaries are no longer present in 2.5, does anyone know if it is planned to remove them pemanently? Thanks in advance Michael

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Xah's Edu Corner: Introduction to 3D Graphics Programing
by Xah Lee Jan. 22, 2007

Jan. 22, 2007

Of Interest: Introduction to 3D Graphics Programing http://xahlee.org/3d/index.html Currently, this introduction introduces you to the graphics format of Mathematica, and two Java Applet utilities that allows you to view them with live rotation in a web browser. Also, it includes a introductory tutorial to POV-Ray. Once you understand any one of these pages, you can use your favorite programing language to start doing 3D Graphics Programing. In the coming months, i will also introduce the .obj format that are used by many geometric modeling programs. And, a tutorial on how to use Python (or Perl, lisp) to do 3D Graphics programing, by setting up functions that spits out any of 3D-Geometry Formats (such as Mathematica Graphics or POV-Ray or .inc). These are the imminent plans for the coming weeks. Other potential subjects are introduction or tutorials on various utilities or programing lang libraries that does conversion between different 3D graphics formats, dedicated tutorial on how to generate mathematical surfaces, more elaborate study on POV-Ray's abilities and sample cases, etc. The focus of these pages, will be 3D-Graphics programing with the goal of Algorithmic Mathematical Art. (see http://xahlee.org/Periodic_dosage_dir/t1/20040113_cmaci_larcu.html ) In particular, we focus on creating geometric objects and their arrangement that are esthetic in a mathematical way. (for example, regular tilings in 3D, nested structures in 3D, symmetric structures in 3D, elaborate 3D maze tunnels, beehive sculpting, regular polyhedrons and their decorations, projection and slices of higher dimensional symmetries, 3D-manifolds ... etc.) This mathematical esthetic is opposed to, for example, rendering techniques or technologies (e.g. fogs, sceneries, fast algorithms...etc) that are quite common in computer graphics literatures. Xah xah(a)xahlee.org ∑ http://xahlee.org/

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ANN: Advanced Python training course, Nov 8-10, San Francisco
by wesley chun Jan. 12, 2007

Jan. 12, 2007

FINAL REMINDER... we still have some seats left! What: Advanced Python Programming When: Nov 8-10 2006 Where: San Francisco (SFO/San Bruno), CA, USA http://cyberwebconsulting.com (click on "Python Training") This course, meant to follow our in-depth introduction class, adds new tools to the Python programmer's toolkit. We explore advanced topics such as: network programming with sockets, Internet clients, GUI development, Web/CGI, databases/SQL, Extending Python with C, threads, etc. Lectures and lab will get attendees comfortable developing applications in these areas. Come join us in beautiful Northern California for another rigorous Python training event taught by software engineer, "Core Python Programming" author, and technical instructor, Wesley Chun. This course will take place in San Bruno right near the San Francisco International Airport at the: Staybridge Suites San Francisco Airport 1350 Huntington Ave San Bruno, CA 94066 USA +1-650-588-0770 LOCALS: accessible from the entire Bay Area: parking + easy 101/280/380 access, across the street from San Bruno BART and up the street from San Bruno CalTrain stations VISITORS: free shuttle to/from the airport, lots of free food and wireless The cost is $1295 per attendee. Discounts available. For more information and registration, go to the website above. 2007 CALENDAR: Feb 7-9 (Intro), May 16-18 (Advanced), Aug 20-22 (Intro) hope to see you in class! -- wesley - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - "Core Python Programming", Prentice Hall, (c)2007,2001 http://corepython.com wesley.j.chun :: wescpy-at-gmail.com python training and technical consulting cyberweb.consulting : silicon valley, ca http://cyberwebconsulting.com

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