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

Re: PEP 354: Enumerations in Python
by Roy Smith March 1, 2006

March 1, 2006

>> Should I amend the PEP to propose "either in the builtins >> or in the collections module"? Or should I propose two >> PEPs and let them compete? I see the issue of whether it's a built-in or part of the standard library as being a detail. My personal opinion is that they're important enough to be built in, but having to write "import enum" (or whatever) won't kill me. If making it a builtin becomes a point of contention, drop it and concentrate on the more key issues of how enums will behave.

1 0

Re: newbie question
by D March 1, 2006

March 1, 2006

Yep, that should work. Just keep in mind that if python.exe is not in your path, you will need to either specify the entire path to it (i.e. 'C:\python24\python C:\path\to\script\myscript.py'), or cd into its directory first (i.e. if you want to just run 'python C:\path\to\script\myscript.py'). Doug

2 1

why descriptors? (WAS: Make staticmethod objects callable?)
by Steven Bethard March 1, 2006

March 1, 2006

Steven Bethard wrote: > (For anyone else out there reading who doesn't already know this, > Steven D'Aprano's comments are easily explained by noting that the > __get__ method of staticmethod objects returns functions, and classes > always call the __get__ methods of descriptors when those descriptors > are class attributes: Steven D'Aprano wrote: > Why all the indirection to implement something which is, conceptually, > the same as an ordinary function? While I wasn't around when descriptors and new-style classes were introduced, my guess is that it's mainly because what you *usually* want when defining a function in a class is for that function to be an instance method. That is, the following code:: class C(object): def foo(self): pass c = C() c.foo() should be much more common than:: class C(object): def foo(): pass C.foo() because the whole point of creating a class is to allow you to create instances. But if ``C.foo`` and ``c.foo`` are just regular functions, then how will ``c.foo()`` get the ``self`` argument? Certainly a normal ``foo()`` shouldn't be inserting a magical ``self`` argument. So *some* indirection has to happen when a function is used in a class. Python's solution to this problem is to introduce descriptors, which are the "something" that classes have to do. All classes invoke __get__ whenever any of their attributes are accessed. With a normal function object, invoking __get__ turns it into an instance method: >>> class C(object): ... pass ... >>> def foo(self): ... pass ... >>> foo <function foo at 0x00E69530> >>> foo.__get__(C(), C) <bound method C.foo of <__main__.C object at 0x00E738F0>> >>> class C(object): ... def foo(self): ... pass ... >>> C().foo <bound method C.foo of <__main__.C object at 0x00E59C50>> As a result, if you want to have a callable as a class attribute and you don't want that callable to give you an instance method when you access it, you can't use a regular Python function. Personally, I think that's pretty reasonable since 99% of the time, I *do* want an instance method[1]. STeVe [1] The other 1% of the time, I pretty much always want a classmethod. I'm still convinced that staticmethods are basically silly when I can just declare a module level function. ;)

1 0

Decimal(1) > 0.0 == False
by Konstantin Veretennicov March 1, 2006

March 1, 2006

Hello, I'm having hard time trying to justify this decimal quirk to my coworker (he calls it a bug): ActivePython 2.4.1 Build 247 (ActiveState Corp.) based on Python 2.4.1 (#65, Jun 20 2005, 17:01:55) [MSC v.1310 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> import decimal >>> 2.0 > decimal.Decimal(1) > 0.0 False Why False? It's so counter-intuitive. I understand that it may not be possible to do precise comparison of decimal and float, but an exception would be so much better than silent False! - kv

1 0

Re: Proper class initialization
by Christoph Zwerschke March 1, 2006

March 1, 2006

Jack Diederich wrote: > ... __metaclass__ = MyMeta Thanks. I was not aware of the __metaclass__ attribute. Still a bit complicated and as you said, difficult to read, as the other workarounds already proposed. Anyway, this is probably not needed so often. -- Christoph

1 0

Make staticmethod objects callable?
by Nicolas Fleury March 1, 2006

March 1, 2006

Hi everyone, I was wondering if it would make sense to make staticmethod objects callable, so that the following code would work: class A: @staticmethod def foo(): pass bar = foo() I understand staticmethod objects don't need to implement __call__ for their other use cases, but would it still make sense to implement __call__ for that specific use case? Would it be error-prone in any way? Thx and regards, Nicolas

7 9

Cross compile generation of .pyc from .py files...
by venkatbo＠yahoo.com March 1, 2006

March 1, 2006

Hi all, We've managed to cross-compile (from i686 targeting ppc) python (2.4.2) binaries and extension modules. However we cannot figure out how to cross-compile the .py (of distribution) files and generate the .pyc files for the target ppc from an i686 system. Is it possible to cross compile .pyc files from .py files? Some of the errors we get when we run 'python -v' on the target system include: .... # /usr/lib/python2.4/site.pyc has bad magic import site # from /usr/lib/python2.4/site.py # /usr/lib/python2.4/types.pyc has bad magic import types # from /usr/lib/python2.4/types.py ..... Thanks, /venkat

4 4

Threading - will threads run in parallel?
by SolaFide March 1, 2006

March 1, 2006

I have 2 threads that I want to run at the same time: for instance: from threading import Thread class test(Thread): def run(self): while True: get(asdf) class test2(Thread): def run(self): while True: get(jkl) ack=test() jkl=test2() ack.start() jkl.start() (get() is a function which waits for a ping on a specific port, thus stopping the program for a while.) Will these run together, or do I have to find some other way of running in parallel? Also, is it possible to split off a program for the terminal that started it? As in I type in python test.py and test.py runs in the background and lets me do other things in that terminal?

5 5

Re: Accessing 'mangled' class attrbutes
by Gerard Flanagan March 1, 2006

March 1, 2006

Steve Juranich wrote: > Gerard Flanagan wrote: > > > I tried that Steve but it didn't work, and i don't think I can do what > > I want in any case. There is no method '__write' in the base class, it > > is only declared as an instance attribute in the constructor, like so: > > > > def __init__(self, file, encoding="us-ascii"): > > ... > > self.__write = file.write > > ... > > > > I tried putting '__write = None' at the class level (in the base class > > XMLWriter) but then, although '_XMLWriter__write' appears in > > 'dir(HtmlWriter)', I get 'NoneType is not callable'. > > > > I also tried 'def __write(self, text) : pass ' in the base class, but > > then the code runs but doesn't write the text I want - and anyway, if > > I'm going to change the base class, then i may as well just add the > > 'write_raw' method to the base directly! > > > > It's just some toy code at any rate, and I've learnt something new! > > Thanks for your reply. > > > > Gerard > > > > Make sure you're calling the super's constructor before you try and access > the mangled member. Then (I forgot this part), you can just call the > mangled member from `self'. Example follows. > > <foo.py> > class A(object): > def __init__(self): > self.__foo = lambda x, y : x + y > > class B(A): > def __init__(self, x, y): > # Make sure you're calling the super's constructor first. > super(B, self).__init__() > self.sum = self._A__foo(x, y) > </foo.py> > > >>> import foo > >>> b = foo.B(3, 4) > >>> b.sum > 7 > >>> > > -- > Steve Juranich > Tucson, AZ > USA It's all becoming clear! Yes, calling the base constructor was all I needed to do: class HtmlWriter(elementtree.SimpleXMLWriter.XMLWriter, object): def __init__(self, file): super( HtmlWriter, self).__init__(file) def write_raw(self, text): self.flush() self._XMLWriter__write(text) -works a charm. Appreciate your help Steve, thanks again. Gerard

1 0

calling a class method
by John Salerno March 1, 2006

March 1, 2006

I'm reading about class methods now and I have a question about calling them. I know it is wrong of me to assume it needs to work in a parallel manner to an instance method, but I'm curious why it doesn't. Here's the book's example: class Multi: def imeth(self, x): print self, x def cmeth(cls, x): print cls, x cmeth = classmethod(cmeth) Now, when you call the instance method through the class, you say: obj = Multi() Multi.imeth(obj, 3) But when you call the class method through an instance, you say: obj.cmeth(3) Why don't you have to explicitly pass the class name as the first argument when you don't qualify the call with the class name? the cmeth function has two arguments, so I would think you'd get an exception (as you would with Multi.imeth(3)). But do class methods automatically extract the class name from the qualifying instance and implicitly send it to the method? Thanks.

2 2