Re: [Tutor] Builtin "property" decorator hiding exceptions
* Jacob Abraham <[EMAIL PROTECTED]> [070419 10:08]: > Hi Andreas Kostyrka, > >I am aware that property is a data descriptor. And I still don't see why > if can't be uses as a decorator since decorators were created to fix the > issue. > > getx= property(getx) Simple, they don't return function objects. Basically, function objects (methods) do the __get__ protocol that property is doing to make the self parameter binding. decorators, if you expect them to work like normal methods do have certain limits what they can return. Ever wondered why practically all (well, I did it for a closed source contract once differently) decorators are functions that return interior nested functions, instead of returning callable object instances? That's why. Andreas ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
Alan, Please let us know when you have a draft of your tutorial on Beautiful Soup and such. I'd be eager to have a look! Thanks, Nick ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Builtin "property" decorator hiding exceptions
Andreas Kostyrka wrote: > * Jacob Abraham <[EMAIL PROTECTED]> [070419 10:08]: >> Hi Andreas Kostyrka, >> >>I am aware that property is a data descriptor. And I still don't see why >> if can't be uses as a decorator since decorators were created to fix the >> issue. >> >> getx= property(getx) > > Simple, they don't return function objects. That is not a requirement, afaik. classmethod() and staticmethod() don't return function objects either and they are usable as decorators. @property def foo()... is equivalent to def foo()... foo = property(foo) which is the usual usage. To argue from authority :-) Ian Bicking agrees with me: http://blog.ianbicking.org/property-decorator.html Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Builtin "property" decorator hiding exceptions
Jacob Abraham wrote:
> Hi,
>
>The sample script below throws the exception "AttributeError: input"
> rather than the expected exception "AttributeError: non_existent_attribute".
>
>Please help me write a decorator or descriptor of my own that fixes the
> issue.
>
> class Sample(object):
> def __init__(self):
> self.arguments = {}
>
> def __getattr__(self, name):
> ret_val = self.arguments.get(name, None)
> if ret_val != None: return ret_val
> raise AttributeError, name
>
> @property
> def input(self):
> self.non_existent_attribute
>
>
> sample = Sample()
> sample.input
I don't fully understand what is going on here but it seems that it is
your __getattr__() that is hiding the exception.
If I take out __getattr__(), I get
AttributeError: 'Sample' object has no attribute 'non_existent_attribute'
I think this is what is happening:
- the input attribute is fetched
- input is a descriptor so its __get__ method is called
- __get__ tries to find non_existent_attribute by the usual methods,
which fail
- __get__ calls __getattr__ which raises AttributeError
- __get__ raises AttributeError
OK, so think about it - trying to access .input has raised an
AttributeError, i.e. attribute access through the normal mechanism has
failed.
Now __getattr__ is called for input and again AttributeError is raised;
this is the error you see.
If you put some print statements into __getattr__ you can see this is
the order of events.
This is not the behaviour you want but it does make a certain sense.
If you raise a different exception in __getattr__ it is propagated out.
Kent
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Re: [Tutor] Questions of Maths
Thank you very much guys, this has been most helpful. At the moment I run the program as Alan suggests. However, the goal is to incorporate the program into Glade2 GUI and I am not sure that it will work with Tk, although I must admit I have not tried it out yet. Abu Ismail On 4/18/07, János Juhász <[EMAIL PROTECTED]> wrote: > Hi Abu, > > > Question: how to determine whether point C is to the left or to the > > right of the line AB? > > When the line given by A(Xa,Ya) and B(Xb, Yb), > the area of the A-B-C triangle can be calculated with the value of next > determinant / 2 > > | Xa, Ya, 1 | > | Xb, Yb, 1 | > | Xc, Yc, 1 | / 2 > > So: > > Area = ( Xa(Yb-Yc) - Xb(Ya-Yc) + Xc(Ya-Yb) ) / 2 > > Area > 0 | the points are clockwise (C is on the left) > Area < 0 | the points are counterclockwise (C is on the right) > Area = 0 | the points are on the same line > > > It can be written in a python function > > ### > def TriArea(a, b, c): > #Area = (Xa(Yb-Yc) - Xb(Ya-Yc) + Xc(Ya-Yb)) /2 > return ( a[0] * (b[1]-c[1]) - b[0] * (a[1]-c[1]) + c[0] * (a[1]-b[1]) > )/2 > > ### > # to test it. > print TriArea((0,0), (10,0), (2,2)) > print TriArea((0,0), (10,0), (2,-2)) > print TriArea((0,0), (10,0), (2,0)) > ### > > > The biggest advantage of this calculation is that, > it never goes to zero division exception. > > > > Best regards, > Janos > ___ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
"Switanek, Nick" <[EMAIL PROTECTED]> wrote Please let us know when you have a draft of your tutorial on Beautiful Soup and such. I'd be eager to have a look! Here is the current draft, the concepts bits are all there (nothing on Beautiful Soup as yet) but the example is giving me some headaches so it could change significantly over the next weeks. But early feedback is always welcome! The idea is to do the example using only the standard library tools then repeat with BS to show how much easier it is. But I'm hitting problems getting the standard tools to work! Alan G. Title: Web Client Programming Web Client Programming What will we cover? A brief history of the web The fundamentals of http Sending requests to a web server Processing html Note: In this topic I refer to a lot of conceptual ideas for which only limited knowledge is required to write the code discussed. I try to provide links, mainly to Wikipedia articles, but if there is no link you will find most of the subjects discussed in more depth on Wikipedia. A Brief History of the World Wide Web The story of the world wide web and its invention by Tim Berners-Lee is probably one of the best known in computing. However it is worth revisiting some of the key points in the story to provide a background to why the web and its technology are the way they are today. The web was invented by Berners-Lee to solve a real problem that he and his colleagues at CERN were experiencing namely, document sharing. They had a large number of documents in various locations and in different formats. Each required understanding of concepts that were explained in other documents. Many were works in progress, being developed by multiple authors. Berners-Lee realised that a techology existed which could greatly ease the situation, it was Hypertext. Hypertext had been around in various forms for many years, in fact the idea pre-dated the inventionof the modern computer! There were several Hypertext solutions available on computers when Berners-Lee studied the problem, but they were either expensive and proprietary or only capable of running on a single computer. Berners-Lee's big contribution was to take the idea and put it on a network. This was what enabled the collaboration of many workers at once and the access to many diversely located documents. By making the network access transparent to the user it was as if the library was one gigantic document all cross-linked and seamlessly joined together, truly a world wide web of information. Hypertext, HTML and static content Berners-Lee had already built a type of hypertext system and he had experience with the internet so it was fairly natural for him to take these two ideas and join them together. In doing so he invented several component parts which together form the web. The Hypertext concept required a mechanism for linking documents together, and so Berners-Lee invented a text formatting, or markup, system which he called Hyper Text Markup Language (HTML) based on an existing standard called Standard Generalised Markup Language (SGML). All web pages are written in HTML, either by a human author or by a computer program. Web browsers display their pages by interpreting the HTML markup tags and presenting the formatted content. This is not the place to try and teach HTML, so if you don't already know how to create simple HTML documents then I suggest you find a good reference or tutorial, like the one here. Having defined a markup language Berners-Lee could now create basic hypertext documents, like this tutorial. The pages could be formatted, images inserted and links to other documents created. So far no networking is involved. The next step was to make these documents accessible over the network and to do that required the definition of a standard address format for content. This is the now ubiquitous Uniform Resource Locator or URL. The first part of a URL identifies the protocol to be used, usually http for the web. The next part uses standard internet naming to identify a server (and optional port, the default being 80) and the final part is the logical location of the content on the server. I say logical because, although the specification looks like an absolute directory path in Unix, it is in fact relative to some fixed location known to the server, and indeed may not be a real location at all, since the server may translate the location into an application invokation or other form of data source. So if we look at the full URL of this page it is: protocolIP addressportlocation http://www.freenetpages.co.uk :80/hp/alan.gauld/tutwebc.htm One problem with the static hypertext content that this scheme provided was that it was read-only. There was no way to interact with the content, nor indeed could the content be easily modified. Berners-Lee wanted more, he wanted the ability to interact with the co
Re: [Tutor] screen scraping web-based email (Alan Gauld)
Alan Gauld wrote: > > "Switanek, Nick" <[EMAIL PROTECTED]> wrote > >> Please let us know when you have a draft of your tutorial on Beautiful >> Soup and such. I'd be eager to have a look! > > Here is the current draft, the concepts bits are all there (nothing > on Beautiful Soup as yet) but the example is giving me some headaches > so it could change significantly over the next weeks. But early feedback > is always welcome! FWIW most real-world HTML parsers (including Beautiful Soup) seem to be based directly on SMTPlib, not htmllib or HTMLParser. On a lawyerly note, I think your Google example violates Google's terms of service which prohibits any automated access. http://www.google.com/accounts/TOS Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
Kent Johnson wrote: > FWIW most real-world HTML parsers (including Beautiful Soup) seem to be > based directly on SMTPlib, not htmllib or HTMLParser. I'm assuming you mean sgmllib here? TJG ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
Tim Golden wrote: > Kent Johnson wrote: >> FWIW most real-world HTML parsers (including Beautiful Soup) seem to >> be based directly on SMTPlib, not htmllib or HTMLParser. > > I'm assuming you mean sgmllib here? :-) yes, I mean sgmllib.SGMLParser Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
"Kent Johnson" <[EMAIL PROTECTED]> wrote > On a lawyerly note, I think your Google example violates Google's > terms > of service which prohibits any automated access. > http://www.google.com/accounts/TOS Thanks for pointing that out Kent. It probably explains why they send me a Restricted HTTP error. I was going to try fooling them by messing with the http headers but I'll find a friendlier web site... The reason for trying to use Google was that they do offer an XML/RPC route in and I was intending to go on to show how much easier a proper API was. Oh well, another bright idea bites the dust. Alan G. PS I didn't actually intend to send the draft to the group but hit ReplyAll out of habit. It was only meant to go to Nick... But in this case it's probably as well I did! ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
"Kent Johnson" <[EMAIL PROTECTED]> wrote in > FWIW most real-world HTML parsers (including Beautiful Soup) seem to > be > based directly on SMTPlib, not htmllib or HTMLParser. Yes, I noticed that, although htmllib is itself based on sgmllib... And it has a better event based parsing model but unfortunately it doesn't throw errors when you get an http error back, which makes HTMLParser much more user friendly for beginners... Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
"Alan Gauld" <[EMAIL PROTECTED]> wrote > it doesn't throw errors when you get an http error back, Oops, confusing myself here. Its urllib2 that throws errors and nothing to do with HTMLParser. The advantage of HTMLParser is the lack of a need for a formatter and writer object. Sorry for any confusion. Alan G ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] screen scraping web-based email (Alan Gauld)
Alan Gauld wrote: > "Alan Gauld" <[EMAIL PROTECTED]> wrote > >> it doesn't throw errors when you get an http error back, > > Oops, confusing myself here. > Its urllib2 that throws errors and nothing to do with HTMLParser. > The advantage of HTMLParser is the lack of a need for a > formatter and writer object. sgmllib doesn't need a formatter either, that is added by htmllib. Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] Custom metaclass?
Greetings. I've been looking at O'Reilly's "Python Cookbook", 2nd edition, just for my own edification. In particular, I'm looking at chapter 6, "Object-Oriented Programming". Recipe 6.3, "Restricting Attribute Setting", intends to solve the problem that, by default, users can add and modify attributes of class instances, or even classes themselves, as in the following example: >>> class Foo(object): pass ... >>> f = Foo() >>> f.age = 42 >>> f.age 42 >>> Foo.loc = 'Tralfamadore' >>> g = Foo() >>> g.loc 'Tralfamadore' The approach, in essence, is a fairly standard one: redefine the __setattr__ method to add some logic to enforce whatever restrictions you choose, but the details are a bit convoluted. Copyright law probably prohibits me from posting the entire recipe, but here's the piece that leads to my question (see below): class NoNewAttrs(object): """ subclasses of NoNewAttrs inhibit addition of new attributes, while allowing existing attributed to be set to new values. """ # block the addition new attributes to instances of this class __setattr__ = no_new_attributes(object.__setattr__) class __metaclass__(type): "simple custom metaclass to block adding new attrs to this class" __setattr__ = no_new_attributes(type.__setattr__) (Beware line breaks introduced by email software.) Finally, my question: can somebody enlighten me as to how and why the "custom metaclass", class __metaclass__(type): does something useful? Thanks. - Mike -- Michael Hannonmailto:[EMAIL PROTECTED] Dept. of Physics 530.752.4966 University of California 530.752.4717 FAX Davis, CA 95616-8677 ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Custom metaclass?
Michael Hannon physics.ucdavis.edu> writes:
> The approach, in essence, is a fairly standard one: redefine the
> __setattr__ method to add some logic to enforce whatever restrictions
> you choose, but the details are a bit convoluted. Copyright law
> probably prohibits me from posting the entire recipe, but here's the
Well, here's a dummy that simply ignores requests to add attributes, instead
printing a message about the attempt to the console, so we have a concrete
example to look at:
def no_new_attributes(*args, **kwargs):
print 'called ', args, kwargs
return no_new_attributes
class NoNewAttrs(object):
__setattr__ = no_new_attributes(object.__setattr__)
class __metaclass__(type):
__setattr__ = no_new_attributes(type.__setattr__)
nna = NoNewAttrs()
#NoNewAttrs.__setattr__ = object.__setattr__
nna.x = 5
try:
print 'object nna.x =' + str(nna.x)
except:
print 'object nna.x doesn\'t exist'
NoNewAttrs.x = 5
try:
print 'class nna.x =', nna.x
except:
print 'class nna.x doesn\'t exist'
> Finally, my question: can somebody enlighten me as to how and why the
> "custom metaclass",
>
> class __metaclass__(type):
>
> does something useful?
A metaclass is to a class like a class is to an object. The way a class
customizes the behavior of an object, a metaclass customizes the behavior of a
class. It's all a bit mind-bending and I don't think I'm an expert on the
subject, but let's see how far we get.
Try running the example above with the __metaclass__ code and without: you'll
see that if you leave it out, you can still add attributes to the class itself
(even though not to the object), so your console will say:
object nna.x doesn't exist
class nna.x = 5
If however you put the metaclass in, you'll get:
object nna.x doesn't exist
called (, 'x', 5) {}
class nna.x = class nna.x doesn't exist
Without the metaclass code, it would be trivial to work around the __setattr__
limitation. Just try uncommenting the line "NoNewAttrs.__setattr__ =
object.__setattr__". If you disabled the metaclass, you'll see that nna.x will
be set. If the metaclass is enabled, it will not be set.
This being Python, you can still work around all this metaclass safety by adding
using this code:
NoNewAttrs.__metaclass__.__setattr__ = type.__setattr__
NoNewAttrs.__setattr__ = object.__setattr__
nna.x = 5
print 'After reset: nna.x =', nna.x
Run this and you'll see it will have restored the attribute-setting
functionality even if it's theoretically blocked. I'm not sure there is a way to
absolutely completely stop users from adding attributes.
Yours,
Andrei
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