Spir and Peter, thanks for the specifics, super helpful. Alan, super thanks for the general advice, you guys are awesome!!
On Mon, Jan 20, 2014 at 5:34 AM, <tutor-requ...@python.org> wrote: > Send Tutor mailing list submissions to > tutor@python.org > > To subscribe or unsubscribe via the World Wide Web, visit > https://mail.python.org/mailman/listinfo/tutor > or, via email, send a message with subject or body 'help' to > tutor-requ...@python.org > > You can reach the person managing the list at > tutor-ow...@python.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Tutor digest..." > > > Today's Topics: > > 1. Re: Understanding Classes (Alan Gauld) > 2. Re: string indexing (Keith Winston) > 3. Re: Question on os.popen (Alan Gauld) > 4. Re: Question on os.popen (SM) > 5. Re: Question on os.popen (eryksun) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sun, 19 Jan 2014 23:50:59 +0000 > From: Alan Gauld <alan.ga...@btinternet.com> > To: tutor@python.org > Subject: Re: [Tutor] Understanding Classes > Message-ID: <lbhocn$vg3$1...@ger.gmane.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 19/01/14 21:59, Christian Alexander wrote: > > Looked all over the net for class tutorials > > Unable to understand the "self" argument > > Attempting to visual classes > > If you read my OOP tutorial there is a section there specifically about > self. > > And the v3 tutor includes an introduction to the formal visualisation > technique for OOP called UML. The diagrams illustrating the designs may > help. > > http://www.alan-g.me.uk/l2p/tutclass.htm > > > I have searched high and low, for easy to follow tutorials regarding > > classes. Although I grok the general concept of classes, > > Do you also grok the concept of objects? > Classes on their own are fairly useless (unless you are using Java) > it is only when you create a universe of objects from those classes that > they become useful. > > If you can repeat to us your understanding of classes and their > relationship with objects that will help us understand your > level and shape our responses accordingly. > > > to visually understand what exactly "self" does, or why it is even > > necessary. It seems very "magic" to me. > > When you define a class you define the data (attributes) that > the class instances will have. Each instance will have a copy of the > data defined in the __init__() method. > You also define a set of operations or methods that are associated > with the class. Those methods are shared by the instances. > > Note the difference. Instances get a copy of the attributes > but they all share the methods. > > Thus when you invoke a method on an instance the instance relays that > call to the class. For the class to know which instance is being > operated on, and for the method to be able to access the correct > instance's data it needs a reference to the instance. That reference > is typically called 'self' or 'this'. (In some languages it's fixed > but in Python self is only a convention, you can use any name you like). > > You can make the call to the class explicit and it will still work. > See below: > > # define a class > class MyClass: > def __init__(self,x): self.x = x > def myMethod(self): print(self.x) > > # create some instances > ObjA = MyClass(2) > ObjB = MyClass(4) > ObjC = MyClass(6) > > # send some messages/call methods > objA.myMethod() # call from the instance > MyClass.myMethod(ObjB) # call explicitly to the class > objC.myMethod() # direct again > > All 3 calls do the same thing except the middle one > passes the object identifier directly to the class > whereas the first and last both do that internally > within the object structure. > > > difficult with the "__init__()" method in classes, > > and why that is also required. > > It is not *required* as such. You can create a class > without an init but it's unusual. > > When you create an instance of a class it creates a > data structure in memory referenced by the name of > the instance. But that structure is empty, it has > no data. So to populate the data for the instances > you must initialize it. That's what __init__() does. > It takes the arguments you provide and applies them > to the instance along with any static data definitions > you may define. > > In the example we create an instance variable, x, > within the instances and assign the value of the > argument passed to init. Like any other method the > actual code lives in the class so we could initialize > it by calling init like so: > > MyClass.__init__(objC, 66) > > which is almost the same as doing: > > objC = MyClass(66) > > The difference is that the first case requires the object ObjC > to already exist, the second example creates a new instance and > then calls init on that instance. > > > Keep in mind that I am a visual person (maybe I should have > > been a graphic designer), therefore most programming concepts flow > > irritatingly slow for me. > > Most programming concepts have visual representations, > its just that program code being text tends to lead programmers > to be verbally based. But algorithms, state machines, logic, data > structures, GUIs, formal requirements and OOP all have well > established visual representations, and in many cases they > are formalized so that, with the right tools, you can > create code purely visually. > > If the above doesn't clarify things, and I suspect it won't > entirely, then please do express your understanding so far > in your own words and we'll try to clarify things from there. > > -- > Alan G > Author of the Learn to Program web site > http://www.alan-g.me.uk/ > http://www.flickr.com/photos/alangauldphotos > > > > ------------------------------ > > Message: 2 > Date: Sun, 19 Jan 2014 19:19:46 -0500 > From: Keith Winston <keithw...@gmail.com> > Cc: Python Tutor Mailing List <tutor@python.org> > Subject: Re: [Tutor] string indexing > Message-ID: > <CAO5ffbZfZmJA3Ex5j6aDJ9K_qGRxu=p6tw3FxDZAa4R242= > h...@mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Sun, Jan 19, 2014 at 3:50 PM, Alan Gauld <alan.ga...@btinternet.com> > wrote: > >> How would Python know whether you want find for gettext, mmap, str, > >> xml.etree.ElementTree.Element or xml.etree.ElementTree.ElementTree? > > > > > > Absolutely, but a newbie doesn't even guess that more than one find would > > exist. Or even that there would need to be more than one. > > That's exactly it. I'm just getting to the point of being able to > understand how much I don't know, and (I'm only a little embarrassed > to admit) Alan's empty-string example was an "ah-ha" moment for me. I > expect Help will be a great deal more useful now (of course, as I type > that I realize I could have used the class name, help(str.find), > instead of an impromptu instance. Another little ah-ha). And of > course, the instant I understood all that, the point that Mark made > became obvious. But I didn't see it before. > > > -- > Keith > > > ------------------------------ > > Message: 3 > Date: Mon, 20 Jan 2014 01:20:20 +0000 > From: Alan Gauld <alan.ga...@btinternet.com> > To: tutor@python.org > Subject: Re: [Tutor] Question on os.popen > Message-ID: <lbhtk8$fjt$1...@ger.gmane.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 19/01/14 23:36, SM wrote: > > > I read about os.popen in > > http://docs.python.org/2/library/subprocess.html#subprocess.Popen > > This doesn't answer the question but I'm curious. > If you read about os.popen in the subprocess module docs why did > you use it? The subprocess module replaces all the os.popen calls > with the generally superior subprocess.Popen class. > > Now, I don't know if using subprocess instead of os would fix > things in this case but it seems an odd choice? I'm curious why > you chose to go with os.popen? > > BTW you don't tell us which OS you are using (or which Python > version), that may be pertinent to the answer. I'm assuming > it's some variant of *nix but which one? > > -- > Alan G > Author of the Learn to Program web site > http://www.alan-g.me.uk/ > http://www.flickr.com/photos/alangauldphotos > > > > ------------------------------ > > Message: 4 > Date: Sun, 19 Jan 2014 23:00:14 -0500 > From: SM <sunith...@gmail.com> > To: Alan Gauld <alan.ga...@btinternet.com> > Cc: "tutor@python.org" <tutor@python.org> > Subject: Re: [Tutor] Question on os.popen > Message-ID: > <CAOeFuMSKTcfJ2t1PwM=b4m=yJfhby7snGGDCrmA= > qrokwb6...@mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Hi Alan, > Thanks for your reply. > My answer to why I am using os.popen could be a lame one - I have used it > extensively in various places before and it has been working well and so > was hung up on using it. Now I replaced it by subprocess.check_output with > appropriate parameters and it seems to be working. Does that mean this is > one of the limitations of os.popen? I am not sure. > Sorry for not giving details on the OS and python version I am using: > Ubuntu and Python3. > Thanks, > SM > > > On Sun, Jan 19, 2014 at 8:20 PM, Alan Gauld <alan.ga...@btinternet.com > >wrote: > > > On 19/01/14 23:36, SM wrote: > > > > I read about os.popen in > >> http://docs.python.org/2/library/subprocess.html#subprocess.Popen > >> > > > > This doesn't answer the question but I'm curious. > > If you read about os.popen in the subprocess module docs why did > > you use it? The subprocess module replaces all the os.popen calls > > with the generally superior subprocess.Popen class. > > > > Now, I don't know if using subprocess instead of os would fix > > things in this case but it seems an odd choice? I'm curious why > > you chose to go with os.popen? > > > > BTW you don't tell us which OS you are using (or which Python > > version), that may be pertinent to the answer. I'm assuming > > it's some variant of *nix but which one? > > > > -- > > Alan G > > Author of the Learn to Program web site > > http://www.alan-g.me.uk/ > > http://www.flickr.com/photos/alangauldphotos > > > > _______________________________________________ > > Tutor maillist - Tutor@python.org > > To unsubscribe or change subscription options: > > https://mail.python.org/mailman/listinfo/tutor > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://mail.python.org/pipermail/tutor/attachments/20140119/5b5bb634/attachment-0001.html > > > > ------------------------------ > > Message: 5 > Date: Sun, 19 Jan 2014 23:33:46 -0500 > From: eryksun <eryk...@gmail.com> > To: SM <sunith...@gmail.com> > Cc: "tutor@python.org" <tutor@python.org> > Subject: Re: [Tutor] Question on os.popen > Message-ID: > <CACL+1avKNFbE+TuAwUpkz=kZMSSDngJZX_Wp7cpWCaq1= > tb...@mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > On Sun, Jan 19, 2014 at 6:36 PM, SM <sunith...@gmail.com> wrote: > > > > This time it probably ran for a few more iterations than before and > stopped > > with the same error message. This time it also output the following > > messages: > > > > IOError: [Errno 4] Interrupted system call > > Attribute not found in file (tsk_fs_attrlist_get: Attribute 128 not > found) > > Attribute not found in file (tsk_fs_attrlist_get: Attribute 128 not > found) > > Attribute not found in file (tsk_fs_attrlist_get: Attribute 128 not > found) > > I can't help with these (NTFS?) Attribute errors from "The Sleuth Kit" > digital forensics tools. > > In Python 3.3, `IOError` is an alias for `OSError`, and EINTR (i.e. > errno.errorcode[4]) is exposed directly as `InterruptedError`. So you > must be running a previous version. I see you're using `print` as a > function, so I'll guess you're using 3.2. > > In 3.2, `os.popen` is implemented via `subprocess.Popen`: > > http://hg.python.org/cpython/file/cef745775b65/Lib/os.py#l776 > > For example, it uses the following for 'r' mode: > > proc = subprocess.Popen(cmd, > shell=True, > stdout=subprocess.PIPE, > bufsize=buffering) > return _wrap_close(io.TextIOWrapper(proc.stdout), proc) > > If you're sticking to `os.popen`, you'll need to retry the read in > case of an interrupted system call. > > I recommend you switch to `Popen` directly and call `communicate`. > This retries reading `stdout` using the helper function > `_eintr_retry_call`: > > http://hg.python.org/cpython/file/cef745775b65/Lib/subprocess.py#l452 > > def _eintr_retry_call(func, *args): > while True: > try: > return func(*args) > except (OSError, IOError) as e: > if e.errno == errno.EINTR: > continue > raise > > More simply, use `subprocess.check_output`, which calls `communicate` > for you. You can pass `shell=True` if you really need it. > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Tutor maillist - Tutor@python.org > https://mail.python.org/mailman/listinfo/tutor > > > ------------------------------ > > End of Tutor Digest, Vol 119, Issue 91 > ************************************** > -- many thanks mat
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