🌐 Mailman 3 June 2008 - Python-list - python.org

Re: Why does python not have a mechanism for data hiding?
by Lie June 3, 2008

June 3, 2008

On Jun 3, 5:07 pm, "BJörn Lindqvist" <bjou...(a)gmail.com> wrote: > On Mon, Jun 2, 2008 at 10:50 PM, Russ P. <Russ.Paie...(a)gmail.com> wrote: > > On Jun 2, 6:41 am, Carl Banks <pavlovevide...(a)gmail.com> wrote: > > >> You are not realizing that only useful(**) thing about data hiding is > >> that some code has access to the data, other code does not. If you > >> "hide" data equally from everyone it's just a useless spelling change. > > > I think you're missing the point. > > > As I see it, the primary value of data hiding is that it provides > > useful information on which data and methods are intended for the > > client and which are intended for internal use. It's like putting a > > front panel on a TV set with the main controls intended for the > > viewer. > > Here's my two cents. First of all, a TV is a bad analogy compared to > reusable software libraries. Really bad analogy. A TV is a horribly > complicated device which has to be dumbed down because otherwise it > would be to hard to use for ordinary people. I think it's actually quite a good analogy, a class may get quite complicated and it may do completely different things that the public interface seems to imply. Anyway, an analogy is an analogy, don't expect it to be exactly the same as the case itself, expect it to illustrate the point well enough and ignore the differences not being illustrated. TV is a good analogy since it illustrated the point quite well, that there are some things user may freely interact, some that users should not mess with, and things that is strictly not for you. Nevertheless, with the proper knowledge and proper tools, any users could open the case and get the special screwdriver to open the TV, if all else fails, he could always get a hammer to break the casing and gone his way through. Python does not enforce data-hiding because it expect people that gone his way to ignore the warning and get the special screwdriver to be knowledgeable enough to mess with it. C/C++ expects people to use hammer to break through their casings, and in the end, since the casings has already been broken, the device may never look as beautiful as before. In python, the device may appear to look just as beautiful. > A software developers relation to a third party library is more > similar to a TV repair man trying to repair a TV than to a random > person watching TV. For a repair man, the front panel is just useless > and in the way. No, for a knowledgable man (a TV repairman), he'd try first to fix the TV without opening the case (such as seeing whether the power cable is actually plugged), and if those attempts fails (or if he already know where the damage is from the beginning), he'd then open the screws. The public interface isn't "useless and in the way". > Oh, and to continue on the TV analogy, one of the reason why a TV is > complicated is because its interface is totally different from its > implementation. Channels are just a bad abstraction for tuning the > receiver to different frequencies and for switching inputs. Merely > using a TV doesn't teach you anything about how it actually works. Why couldn't a class have interface that's completely different thing than the implementation. > KISS: Keep It Simple Stupid. And it is always simpler to not implement > the gunk needed for data hiding than to do it. By keeping things > simple you keep your code easy to implement, easy to understand and > easy to reuse. > > Data hiding sacrifices implementation simplicity supposedly to make > the interface simpler and to keep backwards compatibility. It allows > you to change implementation details without affecting the > interface. But do you really want to do that? Consider this silly Java > example: > > class Foo { > private int bar; > public int getBar() { > return bar; > } > }; > > Then for some reason you decide that hm, "bar" is not a good attribute > name so you change it to "babar". And you can do that without changing > the public interface! Woho! So now you have a public getter named > "getBar" that returns an attribute named "babar". That's in reality > just bad and whoever is maintaining the implementation is going to be > annoyed that the getters name doesn't match the attribute name. > > What would have happened without data hiding? Renaming the public > attribute "bar" to "babar" probably cause some grief for someone > reusing your library, but you would keep your implementation pure. > > What about semantic changes? Data hiding doesn't protect you against > that, so you'll have to change your interface anyway. The interface > for a car hasn't changed much in the last 100 years, but the > implementation has. How easy is it to repair a car nowadays compared > to 30 years ago? > > And data hiding as a documentation aid is just a sham. "These methods > are public so you can call them, these aren't so hands off!" A reuser > of your library *will* want to know what happens on the inside, by > trying to make stuff impossible to reach you are just making that kind > of information much harder to come by. And we expect those people is ready that the car may blow off right in their face since they have violated the lines. If they broke the lines and still think that we're guilty for his burnt face, that's their problem. > The better method is to just write proper docstrings that tell the > user what the methods do and when they can be called. > > Another good way to see how useless data hiding is, is to try and unit > test a very encapsulated library. You'll see that it is almost > impossible to write good unit tests unless you publicly export > almost everything in the code. At which point you come to realize that > all the data hiding was for naught. > > -- > mvh Björn

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Compare 2 files and discard common lines
by loial June 3, 2008

June 3, 2008

I have a requirement to compare 2 text files and write to a 3rd file only those lines that appear in the 2nd file but not in the 1st file. Rather than re-invent the wheel I am wondering if anyone has written anything already?

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Misuse of list comprehensions?
by John Salerno June 3, 2008

June 3, 2008

I posted this code last night in response to another thread, and after I posted it I got to wondering if I had misused the list comprehension. Here's the two examples: Example 1: -------------------- def compress(s): new = [] for c in s: if c not in new: new.append(c) return ''.join(new) ---------------------- Example 2: ------------------------ def compress(s): new = [] [new.append(c) for c in s if c not in new] return ''.join(new) -------------------------- In example 1, the intention to make an in-place change is explicit, and it's being used as everyone expects it to be used. In example 2, however, I began to think this might be an abuse of list comprehensions, because I'm not assigning the result to anything (nor am I even using the result in any way). What does everyone think about this? Should list comprehensions be used this way, or should they only be used to actually create a new list that will then be assigned to a variable/returned/etc.?

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Re: PyInstaller: problem to build exe with PyQt4
by Giovanni Bajo June 3, 2008

June 3, 2008

On Tue, 03 Jun 2008 08:35:37 +0200, Mark Delon wrote: > Hi, > > I need to generate single EXEcutable via PyInstaller. It will be > genereated -> i get one single executable. > > AFTER CALL (exe) I get an error: "no module named _gt" > > Build command: > 1. Configure.py > 2. Makespec.py -F <script>.py > 3. Build.py <script>.spec > ----------------------------------------------------------------- after > exe-call ""no module named _gt" (is a part of PyQt) > ----------------------------------------------------------------- > > HOW CAN I INCLUDE PyQt explicite in build process OR HOW CAN I solve my > problem. > > Thank You for every idea or help Please try again with PyInstaller from SVN trunk. -- Giovanni Bajo Develer S.r.l. http://www.develer.com

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Formatting Output
by victor.herasme＠gmail.com June 3, 2008

June 3, 2008

Hi, i am building a little script and i want to output a series of columns more or less like this: 1 5 6 2 2 8 2 9 5 The matter is that i don't know in advance how many columns there will be. By the way, each column will be actually FLOATs, not INTs. How can i do this ? Any help welcome. Regards, Victor

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Re: Database Query Contains Old Data
by giraffeboy＠gmail.com June 3, 2008

June 3, 2008

On May 21, 1:49 pm, "Jerry Hill" <malaclyp...(a)gmail.com> wrote: > Did you remember to commit your changes before re-running the report? > Python's DB API requires that any auto-commit feature of the > underlying database be turned off by default, so you are required to > commit changes yourself. If you're used to having auto-commit turned > on, this can be confusing. I did and I confirmed this by modifying the data, selecting it from the mysql command line client to verify the changes, then running the report again. If I exit the application and then start it again, everything works as expected until the second instance of the report is run. I have a feeling it has something to do with there already being an instance of the report class, but I can't work out how to destroy it and aren't really sure how to remove the references to it either. I added a print statement to the code and it displays a row that doesn't exist any more. Code, output of print statement and result from the mysql client shown below. Andrew Code: #Get all the mileages and add them together for the month: curs.execute('SELECT date, mileage, name FROM mileage, agents WHERE mileage.agent = agent.rowid AND date >= CAST(%s AS DATE) AND date < CAST(%s AS DATE) AND mileage.agent=%s ORDER BY date', (fromdate, todate, agentid)) for row in curs: month = str(row[0])[:7] print row[1], month Output: 100.0 2008-03 MySQL Output just prior to running the report: mysql> select * from mileage; +-------+------------+---------+---------+-------+ | rowid | date | mileage | vehicle | agent | +-------+------------+---------+---------+-------+ | 1 | 2008-04-28 | 875.63 | 3 | 3 | | 2 | 2008-04-28 | 1188.13 | 6 | 6 | | 3 | 2008-04-28 | 676.88 | 4 | 4 | | 4 | 2008-04-21 | 1111.25 | 6 | 6 | | 5 | 2008-04-21 | 1126.88 | 3 | 3 | | 6 | 2008-04-28 | 1029.38 | 7 | 8 | | 7 | 2008-04-21 | 953.13 | 7 | 8 | | 8 | 2008-04-21 | 675.63 | 4 | 4 | | 9 | 2008-04-14 | 891.88 | 3 | 3 | +-------+------------+---------+---------+-------+ 9 rows in set (0.00 sec) As you can see there's no row with a mileage of 100 with the correct month.

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Writing Empty folders into ZipFile
by otaeris＠o2.pl June 3, 2008

June 3, 2008

Hello. I'm having situation writing folders structure into a zip file. Folders contain no files. Is it possible to do in python ? Regards, Bartek

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ConfigObj quoting issues
by Roopesh June 3, 2008

June 3, 2008

Hi, I am using ConfigObj to write email addresses, as a list. I am using email module functions to extract email addresses: to_address = header.get_all('To', []) address_list = getaddresses(to_address) to = map(lambda address: '"'+address[0]+'" <'+address[1]+'>' ,address_list) conf_obj['to'] = to But quite often I am getting the error "cannot be safely quoted.". This error is because of the presence of \', \", \n etc. I had to do the following to make it work. address[i].replace("\'",'').replace('\"','').replace('\n','') [making list_all=False doesn't work properly(it fails when the first character is a double quote). ] I don't want to do the above, as it modifies the email address (name). In brief, my question is how to save a list of strings which might have quotes, double quotes to a file using ConfigObj. Any idea what is to be done. Thanks Roopesh

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Re: Better performance
by Paul Boddie June 3, 2008

June 3, 2008

On 2 Jun, 20:16, David <wizza...(a)gmail.com> wrote: > > PHP: Easy to make web pages. > Perl: Lots of libraries, good text processing support > Python: Easy to read and maintain PHP: For the security vulnerabilities. Perl: For the maintenance problem. Python: To the rescue! ;-) > You could even use all 3, in their strong areas. eg, PHP for the web > page, perl for quick & dirty text-processing utils, and Python for the > longer, more complicated scripts which you have to maintain. Yes, it depends on how big the problem really is and how complicated the solution has to be. Are the files all in the same format? Do they have to be parsed repeatedly? Will new versions of the files be released? Will the parser output go straight to a Web page or will it be stored somewhere? Paul

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Re: Database Query Contains Old Data
by James A. Donald June 3, 2008

June 3, 2008

On Mon, 02 Jun 2008 20:59:09 -0700, <wlfraed(a)ix.netcom.com> wrote: James A. Donald > > If one has transactions open for a long time, or transactions that > > involve a great deal of data, this will result in poor performance or > > poor scalability. But one may have such large transactions without > > being aware of it. Is there any way to make transaction size salient > > to the developer? Any way to make sure one is committing as early and > > often as possible? Dennis Lee Bieber > I would tend to suggest that one code to collect all needed > information for a complete transaction /before/ even starting a > transaction; then one can start it and feed all the operations in short > order. > > Now, if the application requires doing something like: > > select set of data > repeat until user is done > display by record to user > wait for user response > issue update for record > commit transaction > > this becomes problematic. Well obviously one should do the former and not the latter, but one may well start out doing the former and gradually drift into doing the latter without entirely being aware of it. > A more complex method would be: > > select set of data; save in local storage; commit > repeat until user is done > display saved record to user > wait for user response > save modified record (second storage) > repeat until no conflicts > select original set again > repeat until done > if saved original <> reselected original > if saved original had been modified by user > User interaction: override other changes > or redisplay and get edits from user > transaction begin > issue all updates > commit Having drifted into doing things the wrong way without being aware of it, one would then wait until ones application comes to a grinding halt due to scaling problems, figure out what was causing the problem, and then recode by the more complex method you describe. At that point, however, one would belatedly discover that one's schema made the more complex method hideously complex for the programmer and incomprehensible to the user, and that it was far too late to change the schema. I suppose there is no help for it but to be mindful of scaling and transactions from the beginning. -- ---------------------- We have the right to defend ourselves and our property, because of the kind of animals that we are. True law derives from this right, not from the arbitrary power of the omnipotent state. http://www.jim.com/ James A. Donald

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