🌐 Mailman 3 July 2010 - Python-list - python.org

Re: Performance ordered dictionary vs normal dictionary
by Chris Rebert Aug. 3, 2010

Aug. 3, 2010

On Wed, Jul 28, 2010 at 8:12 PM, Navkirat Singh <navkirats(a)gmail.com> wrote: > Sorry, I might have been a bit vague: > (Also, I am new to pythong) > I am trying to do construct my own web session tracking algorithm for a web > server (which also I have constructed). The book keeping is for the session > information I track. The dictionary object will do this for me. I was > initially using a plain dictionary object, but I read this in the > documentation that made me think about the question I asked :- Does your program actually make use of the ordering? If so, then yes, using OrderedDict is obviously preferable (Why reimplement something needlessly?). If not, then why bother with the extra complication? (You are aware that the "ordered" in OrderedDict means that its keys are ordered, and not that, say, a list containing OrderedDicts can be sorted, right?) Cheers, Chris -- http://blog.rebertia.com

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Normalizing A Vector
by Lawrence D'Oliveiro Aug. 3, 2010

Aug. 3, 2010

Say a vector V is a tuple of 3 numbers, not all zero. You want to normalize it (scale all components by the same factor) so its magnitude is 1. The usual way is something like this: L = math.sqrt(V[0] * V[0] + V[1] * V[1] + V[2] * V[2]) V = (V[0] / L, V[1] / L, V[2] / L) What I don’t like is having that intermediate variable L leftover after the computation. Here’s how to do it in one step: V = tuple \ ( x / math.sqrt ( reduce(lambda a, b : a + b, (y * y for y in V), 0) ) for x in V ) which, incidentally, also works for vectors with dimensions other than 3.

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Re: Builtn super() function. How to use it with multiple inheritance? And why should I use it at all?
by Steven D'Aprano Aug. 2, 2010

Aug. 2, 2010

On Fri, 30 Jul 2010 14:43:07 +0200, Jean-Michel Pichavant wrote: > Quoting Michele's article (I think he's still hanging around this list) > > "Readers familiar will single inheritance languages, such as Java or > Smalltalk, will have a clear concept of superclass in mind. This > concept, however, has /no useful meaning/ in Python or in other multiple > inheritance languages". I have read Michelle Simionato's articles on super in Python. He has taught me a lot. But on that specific matter, I think he is wrong. Of course, he is right to say that the concept of *the* superclass is meaningless in a MI language like Python. If MyClass inherits method spam() from class A, and inherits method ham() from class B, which is "the" superclass of MyClass? But Michelle is wrong to conclude that the problem lies with the concept of *superclass*. The problem lies with the idea that there is ONE superclass. By dismissing the entire concept, he is throwing out the baby with the bathwater. The useful, and I would argue *correct*, concept of superclass is very simple. It is a reflection of subclass: if Z is a subclass of A, then A is a superclass of Z. This follows e.g. superset and subset. We don't have any problem understanding that a class can have many subclasses. Why the resistance to the idea that a class can have many superclasses? Even in a single inheritance language, if we had a class hierarchy A -> B -> C -> ... -> Y -> Z it makes perfect sense to describe *all* of A-Y as superclasses of Z, just as we describe all of B-Z as subclasses of A. -- Steven

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Re: Accumulate function in python
by Paul Rubin Aug. 2, 2010

Aug. 2, 2010

Brian Victor <homeusenet4(a)brianhv.org> writes: > def running_sum(result, current_value): > return result + [result[-1]+current_value if result else current_value] > > reduce(running_sum, x, []) That is not really any good because Python lists are actually vectors, so result+[...] actually copies the whole old list, making your function take quadratic time. It would be ok in a FP language where lists were chains of cons nodes and result+[...] just allocated a single cons. I think Peter Otten's solution involving a generator is the one most in the current Python spirit. It's cleaner (for my tastes) than the ones that use things like list.append.

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Re: The untimely dimise of a weak-reference
by Peter Otten Aug. 2, 2010

Aug. 2, 2010

Vincent van Beveren wrote: > Hi everyone, > > I was working with weak references in Python, and noticed that it was > impossible to create a weak-reference of bound methods. Here is a little > python 3.0 program to prove my point: > > import weakref > > print("Creating object...") > class A(object): > > def b(self): > print("I am still here") > > a = A() > > def d(r): > print("Aaah! Weakref lost ref") > > print("Creating weak reference") > > r = weakref.ref(a.b, d) The instance doesn't keep a reference of its bound method. Rather the bound method keeps a reference of its instance. Every time you say a.b you get a different bound method. What do you think should keep it alive? > print("Oh, wait, its already gone!") > print("Ref == None, cause of untimely demise: %s" % r()) > print("Object is still alive: %s" % a) > print("Function is still exists: %s" % a.b) > print("See:") > a.b() > > I also tried this in Python 2.5 and 2.6 (with minor modifications to the > syntax of course), and it yielded the exact same behavior. Why is this, > and is there anything I can do about it? I wish to reference these bound > functions, but I do not want to keep them in memory once the object they > belong to is no longer referenced. I fear you have to manage the methods' lifetime explicitly. Peter

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how python works
by Mahmood Naderan Aug. 1, 2010

Aug. 1, 2010

I want to know how python executes a .py file. Sometimes when I run a file, I get an error that there is a syntax error. This shows that the parser read and parse the whole file. Sometimes in the middle of the run I get an error that another line has problem. So how does it work? if it doesn't compile and read line by line them why some times I get error just before run? // Naderan *Mahmood;

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Re: Why is there no platform independent way of clearing a terminal?
by Tim Harig Aug. 1, 2010

Aug. 1, 2010

On 2010-07-28, Thomas Jollans <thomas(a)jollans.com> wrote: > It might be possible to write a curses-compatible library that works > with cmd.exe. Maybe. But, even if it's possible, I don't think it's > easy, and I especially don't think it would be particularly rewarding. http://pdcurses.sourceforge.net/ It would be rewarding as it would make writing cross-platform charactor mode applications possible. Using curses for the interface makes a lot of sense because it is already supported by almost every Unix/POSIX platorm, so re-implementation is only required for those odd platforms (Windows) where it does not exist natively,; because well documented on the internet and in print; and because a huge number of people are already familiar with its API. If licensing permits, I don't think it would be too difficult difficult to embed something like pdcurses into the existing curses module to use as a backup for platforms where curses does not already exist. If not, there cannot be much difference to writing a terminal emulator that runs inside the windows console and there are a *huge* number of terminal emulators available for ansi/vt100 terminals. Add a termcap database mechanism and an embedded emulator can easily convert the excape codes into actions for the console.

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Re: Builtn super() function. How to use it with multiple inheritance? And why should I use it at all?
by Steven D'Aprano Aug. 1, 2010

Aug. 1, 2010

On Fri, 30 Jul 2010 21:40:21 -0600, Ian Kelly wrote: > I have to chime in and agree that the name "super" is problematic. I'm > reading this thread with a sense of alarm because I apparently never > read the super() documentation too closely (why would I? "Oh, it just > accesses an attribute from a superclass. Moving on.") and have been > writing code for the past four years under the impression that super() > will always refer to a superclass of the current class. In Python 2.x, super() doesn't know what the current class is. You have to explicitly tell it. If you tell it a lie, surprising things will happen. Assuming you accurately tell it the current class, can you give an example where super() doesn't refer to a superclass of the current class? [...] > On a tangent, is it just me, or is the super() documentation incorrect, > or at least unclear? Quoting from the first two paragraphs: Yes, it's a bit unclear, because it's a complex function for dealing with a complicated situation. But if you have single inheritance, it's simple. Anywhere you would write class C(B): def method(self, *args): B.method(*args) you can write class C(B): def method(self, *args): super(C, self).method(*args) and it will Just Work. > super(type[, object-or-type]) > > Return a proxy object that delegates method calls to a parent or > sibling class of type. I think that the bit about sibling class refers to super(type, type2) calls, rather than the super(type, instance) calls which I've been discussing. I've never needed, and don't understand, the two type version of super(), so I can't comment on it. -- Steven

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HCL Jobs database
by David July 31, 2010

July 31, 2010

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Google job leads
by Charles Jo July 31, 2010

July 31, 2010

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