Re: [Tutor] class arguments?
Forwarding to the list with my reply... On Fri, Jan 23, 2009 at 1:35 PM, spir wrote: > Le Fri, 23 Jan 2009 06:45:04 -0500, > Kent Johnson a écrit : > >> On Fri, Jan 23, 2009 at 6:04 AM, spir wrote: >> >> > Thank you Alan and sorry for not having been clear enough. The point >> > actually was class (definition) attributes. I thought at e.g. Guido's >> > views that lists were for homogeneous sequences as opposed to tuples >> > rather like records. And a way to ensure sich a homogeneity, in the sense >> > of items beeing of the same type or super type. >> > The straightforward path to ensure that, as I see it, is to add proper >> > argument to a class definition. >> >> A simple way to do this is with a class factory function, for example: >> >> def makeMonoList(typ, number): >> class MonoListSubtype(MonoList): >> item_type = type >> item_number = number >> return MonoListSubtype > > That's it! Stupid me!! [Just realize I have a kind of mental blocage that > prevents me *imagining* a class beeing defined inside a func. As for me a > class is a higher level kind of thing. Actually I also have problems with > defs insides defs. Maybe there should be more introduction to that in python > literature. Probably it may help and simplify a whole lot of models.] > Thank you again. >> then e.g. >> IntegerList = makeMonoList(int, 5) >> myIntegerList = IntegerList() >> >> This is similar in spirit to collections.namedtuple() in Python 2.6 >> though the implementation is different; namedtuple() actually creates >> and evaluates the text of the new class definition: >> http://docs.python.org/library/collections.html#namedtuple-factory-function-for-tuples-with-named-fields >> http://svn.python.org/view/python/trunk/Lib/collections.py?rev=68853&view=auto > > I have watched that some time ago -- as you pointed to it already. I take the > opportunity to ask why this contruct is so complicated. There is an > alternative in the cookbook (also pointed by Kent, if I remmember well) that > is only a few lines long. Something like: > > class Record(dict): >def __init__(self,**kwargs): >dict.__init__(self,kwargs) ># and/or >self.__dict__ = kwargs > > [There are several versons around, +/- based on the same principle] namedtuple() creates a new class that has exactly the desired attributes, so it is a bit more specific and typesafe - you have to have the correct number of points. The generated class subclasses tuple so it can be used as a dict key (if the items themselves can be). Class instances are lightweight because they don't have a __dict__ member. > Actually, the thing I like at least in the namedtuple recipe is that it > writes the class def as a string to be executed: Yeah, I know...with all the time we spend on the list telling people not to use eval()... > > I know there are several advantages: > * a docstring > * For large collections of records of the same (sub)type, as the list of > field is held by the class (instances record the actual data only), which > spares memory. But doesn't this lead to lower performance, as attribute > access by name requires adressing a class level attribute? The attributes are properties, so attribute access is like a method call, I suppose this is slower than direct field access but it is a common Python technique. > * attributes can be accessed by index, too > > Also, as this factory create kinds of records, meaning data constructs with > an identical structure, that would perfectly hold table records, why isn't it > simply called "record". > To sum up in a word: why so much *complication*? I guess you would have to search comp.lang.python or python-dev to find the reasons, I don't think there is a PEP for this (at least not referenced in the What's New). Kent > > Denis > -- > la vida e estranya > ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] class arguments?
On Fri, Jan 23, 2009 at 6:04 AM, spir wrote: > Thank you Alan and sorry for not having been clear enough. The point actually > was class (definition) attributes. I thought at e.g. Guido's views that lists > were for homogeneous sequences as opposed to tuples rather like records. And > a way to ensure sich a homogeneity, in the sense of items beeing of the same > type or super type. > The straightforward path to ensure that, as I see it, is to add proper > argument to a class definition. A simple way to do this is with a class factory function, for example: def makeMonoList(typ, number): class MonoListSubtype(MonoList): item_type = type item_number = number return MonoListSubtype then e.g. IntegerList = makeMonoList(int, 5) myIntegerList = IntegerList() This is similar in spirit to collections.namedtuple() in Python 2.6 though the implementation is different; namedtuple() actually creates and evaluates the text of the new class definition: http://docs.python.org/library/collections.html#namedtuple-factory-function-for-tuples-with-named-fields http://svn.python.org/view/python/trunk/Lib/collections.py?rev=68853&view=auto Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] class arguments?
Le Thu, 22 Jan 2009 23:29:59 -, "Alan Gauld" a écrit : > > "Alan Gauld" wrote > > >> is there a way to give arguments to a class definition? > > I see that Kent interpreted your question differently to me. > If you do mean that you want to dynamically define class > attributes rather than instance attributes then __init__() > won't work. But I'd be interested to understand why and > how you would want to do that? And in particular how > you would use them after creating them? Thank you Alan and sorry for not having been clear enough. The point actually was class (definition) attributes. I thought at e.g. Guido's views that lists were for homogeneous sequences as opposed to tuples rather like records. And a way to ensure sich a homogeneity, in the sense of items beeing of the same type or super type. The straightforward path to ensure that, as I see it, is to add proper argument to a class definition. But I couldn't find a way to do this. In pseudo-code, it would look like that: class MonoList(list, item_type): typ = item_type def __init__(self,items): self._check_types(items) list.__init__(self,items) def _check_types(self,items): for item in items: if not isinstance(item,MonoList.typ): message = "blah!" raise TypeError(message) def __setitem__(self,index,item): if not isinstance(item,MonoList.typ): message = "blah!" raise TypeError(message) list.__setitem__(self,index,item) def __add__(self,other): self._check_types(other) list.__add__(self,other) ... Well, I realize now that it is a bit more complicated. MonoList itself should be an intermediate base class between list and and subclasses that each allow only a single item type. Otherwise all monolist-s have the same item type ;-) Just exploring around... denis > ___ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > -- la vida e estranya ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] class arguments?
"Alan Gauld" wrote is there a way to give arguments to a class definition? I see that Kent interpreted your question differently to me. If you do mean that you want to dynamically define class attributes rather than instance attributes then __init__() won't work. But I'd be interested to understand why and how you would want to do that? And in particular how you would use them after creating them? Yes thats what the __init__ method is for. class MonoList: def __init__(self, lst, typ, num): self.item_type = typ self.number = num etc... myList = MonoList([1,2,3], int, 3)# or whatever... Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] class arguments?
On Thu, Jan 22, 2009 at 5:18 PM, Kent Johnson wrote:
> On Thu, Jan 22, 2009 at 4:51 PM, spir wrote:
>> Hello,
>>
>> is there a way to give arguments to a class definition? Eg
>>
>> class MonoList(list, typ, number):
>>item_type = typ
>>item_number = number
>
> Use the type() function (which is the constructor for the type 'type')
> to dynamically create classes (i.e. types), e.g.
>
> In [1]: typ = int
>
> In [2]: number = 3
>
> In [3]: MonoList = type("MonoList", (list,), dict(item_type=typ,
> item_number=number))
>
> In [4]: ml = MonoList()
>
> In [5]: ml.item_type
> Out[5]:
>
> In [6]: ml.item_number
> Out[6]: 3
>
> In [7]: isinstance(ml, list)
> Out[7]: True
And to show that these are class attributes:
In [4]: MonoList.item_number
Out[4]: 3
In [5]: MonoList.item_type
Out[5]:
Kent
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Re: [Tutor] class arguments?
On Thu, Jan 22, 2009 at 4:51 PM, spir wrote:
> Hello,
>
> is there a way to give arguments to a class definition? Eg
>
> class MonoList(list, typ, number):
>item_type = typ
>item_number = number
Use the type() function (which is the constructor for the type 'type')
to dynamically create classes (i.e. types), e.g.
In [1]: typ = int
In [2]: number = 3
In [3]: MonoList = type("MonoList", (list,), dict(item_type=typ,
item_number=number))
In [4]: ml = MonoList()
In [5]: ml.item_type
Out[5]:
In [6]: ml.item_number
Out[6]: 3
In [7]: isinstance(ml, list)
Out[7]: True
http://docs.python.org/library/functions.html#type
Kent
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Re: [Tutor] class arguments?
"spir" wrote is there a way to give arguments to a class definition? Eg class MonoList(list, typ, number): item_type = typ item_number = number Yes thats what the __init__ method is for. class MonoList: def __init__(self, lst, typ, num): self.item_type = typ self.number = num etc... myList = MonoList([1,2,3], int, 3)# or whatever... HTH, -- Alan Gauld Author of the Learn to Program web site http://www.freenetpages.co.uk/hp/alan.gauld ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] class arguments?
Hello, is there a way to give arguments to a class definition? Eg class MonoList(list, typ, number): item_type = typ item_number = number [I guess you understand what I try to do...] denis -- la vida e estranya ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
