I would like to write a metaclass which would allow me to overload
names in the definition of its instances, like this
class Foo(object):
__metaclass__ = OverloadingClas s
att = 1
att = 3
def meth(self):
pass
def meth(self, arg):
return arg
I would then like the dictionary received by OverloadingClas s.__new__
to look something like this:
{'att': (1,3),
'meth: (<function meth at 0x4018e56c>, <function meth at 0x4018e80c>) }
IOW, each name bound in the class definition should have associated
with it, a tuple containing all the objects which were bound to that
name, rather merely keeping the most recent binding for any given
name.
I was wondering whether it would be possible to achieve this by
forcing Python to use some dicitonary proxy (which accumulates values,
rather that keeping just the last value to be associated with a key),
instead of dict, when executing the class definiton?
Is something like this at all possible in pure Python? or does in
require fiddling around in the guts of the parser?
Jul 18 '05
33  2529 
On 28 Sep 2004 09:47:51 +0200, Jacek Generowicz
<ja************ **@cern.ch> wrote: Carlos Ribeiro <ca********@gma il.com> writes:
On 27 Sep 2004 13:33:33 +0200, Jacek Generowicz
<ja************ **@cern.ch> wrote:
[...]
I was wondering whether it would be possible to achieve this by
forcing Python to use some dicitonary proxy (which accumulates
values, rather that keeping just the last value to be associated
with a key), instead of dict, when executing the class definiton?
[...]
No, you can't, and it's not just a parser issue. Python uses direct
C calls to the native dict type. It's hard coded,
I feared this would be the case.
p.s. In the particular case of the original poster, I'm wondering
what kind of application did he had in mind.
A standalone, lightweight SWIG-like tool, in this case. But in
general, I've had cause to wonder about declarative syntaxes in Python
every now and then.
I'm also exploring declarative alternatives for a lot of stuff in
Python. It started explicitly as an experiment, mainly because I could
not rationally explain why I did 'feel' that it was the right approach
for a class of applications: form definitions, reports, webpage
templates, etc. Now I think that I'm beginning to get a better
understanding that allows me to articulate better *why* should I
(ab)use Python for declarative programming, instead of using a data
driven approach with XML, or creating my own mini-declarative
language. In short, the argument goes like this:
Generic templating mechanisms start as simple variable substitution
engines, but as they start to be used, there's the need to add control
structures (if, for, etc); it's also needed to provide more ways for
the template to communicate with the main program, exchanging
variables and values. At this point, wouldn't be better to write all
templates in the main programming language of the system?
Thanks, to all who contributed ideas to the thread, particularly Alex,
Thomas and Lenard.
I learned a lot through this thread. As a matter of fact, I used your
problem as an exercise on decorators :-) And I think that, while still
exploring and making some (dumb) mistakes, I'm beginning to feel
comfortable with the more esoteric introspection features of Python.
After looking Lenard example, I've come to think about other
alternatives. There are a some interesting things that can still be
done, some even more esoteric than all stuff that we've done so far. A
generic solution for this problem would greatly simplify my own
search, and I'll keep looking for it.
--
Carlos Ribeiro
Consultoria em Projetos
blog: http://rascunhosrotos.blogspot.com
blog: http://pythonnotes.blogspot.com
mail: ca********@gmai l.com
mail: ca********@yaho o.com
Jacek Generowicz <ja************ **@cern.ch> wrote:
... No, you can't, and it's not just a parser issue. Python uses direct
C calls to the native dict type. It's hard coded,
I feared this would be the case.
It's not (not in 2.4 at least) -- the STORE_NAME is quite ready to find
a non-dict as the frame's f_locals. The problem is getting your object
to be used as the frame's f_locals in the first place -- hard but that
only affects a few spots in ceval.c.
Alex
Carlos Ribeiro <ca********@gma il.com> wrote:
... Generic templating mechanisms start as simple variable substitution
engines, but as they start to be used, there's the need to add control
structures (if, for, etc); it's also needed to provide more ways for
the template to communicate with the main program, exchanging
variables and values. At this point, wouldn't be better to write all
templates in the main programming language of the system?
At this point, your templating is not declarative -- it's imperative.
Like everything in Python, btw -- not ONE 'declarative' in sight (except
the 'global' statement, which is part of what makes it a wart;-).
There IS a case for purely declarative stuff _embedding_ Python code,
like strakt.com's "blam" (purely informal name, as Strakt's marketing
may lot like it, we just can't keep saying "Business Logic Module
Language" forever;-) does for (basically) ERD + actions/triggers. The
embedding makes the whole non-declarative, of course. But the
declarative part can still be way prettier than it would be if it wasn't
a separate language, e.g. it could use such keywords as 'entity',
'relation', 'attribute' and the like...
Alex
On Tue, 28 Sep 2004 14:37:31 +0200, Alex Martelli <al*****@yahoo. com> wrote: Carlos Ribeiro <ca********@gma il.com> wrote:
... Generic templating mechanisms start as simple variable substitution
engines, but as they start to be used, there's the need to add control
structures (if, for, etc); it's also needed to provide more ways for
the template to communicate with the main program, exchanging
variables and values. At this point, wouldn't be better to write all
templates in the main programming language of the system?
At this point, your templating is not declarative -- it's imperative.
Like everything in Python, btw -- not ONE 'declarative' in sight (except
the 'global' statement, which is part of what makes it a wart;-).
I knew I should have taken more time to write that paragraph :-) The
way I'm writing my code "reads" more like declarative code than
imperative. One can surely argue with my lack of academic rigour. I
think that I'm writing "declarativ e" code because I'm using class
declarations to create complex, hierarchic data structures. I want to
state __what it is__, not state __how it should be done__ step by
step.
Your comment also made me realize a point that should be highlighted.
Normal templates [1] are clearly imperative, and that's part of my
problem with them. But complex object-oriented structures, although
including code (in the form of methods and descriptors) are much more
dynamic than a simple template. Better than this -- normal templates
are inherently sequential and imperative in the way they're written.
Object oriented structures are much more flexible in this respect.
[1] I stress the term "normal templates" because I'm focusing on
standard, run-of-the-mill templating systems.
There IS a case for purely declarative stuff _embedding_ Python code,
like strakt.com's "blam" (purely informal name, as Strakt's marketing
may lot like it, we just can't keep saying "Business Logic Module
Language" forever;-) does for (basically) ERD + actions/triggers. The
embedding makes the whole non-declarative, of course. But the
declarative part can still be way prettier than it would be if it wasn't
a separate language, e.g. it could use such keywords as 'entity',
'relation', 'attribute' and the like...
In the end, you've raised another interesting point -- on the whole,
my current approach is not purely declarative. It's rather a mix of
imperative and declarative, but with a mostly declarative
infrastructure holding things together.
--
Carlos Ribeiro
Consultoria em Projetos
blog: http://rascunhosrotos.blogspot.com
blog: http://pythonnotes.blogspot.com
mail: ca********@gmai l.com
mail: ca********@yaho o.com
Jacek Generowicz <ja************ **@cern.ch> wrote in message news:<ty******* ******@pcepsft0 01.cern.ch>... I would like to write a metaclass which would allow me to overload
names in the definition of its instances, like this
class Foo(object):
__metaclass__ = OverloadingClas s
att = 1
att = 3
def meth(self):
pass
def meth(self, arg):
return arg
[snip] Is something like this at all possible in pure Python? or does in
require fiddling around in the guts of the parser?
Not exactly what you asked for, and a bit (litotes) ugly, but it does
allow convenient subgroups within a class. I used a similar trick
once when writing a little parser.
def alltuple(name,b ases,clsdict):
return tuple(clsdict.v alues())
class Foo(object):
class att:
__metaclass__ = alltuple
_1 = 1
_2 = 3
class meth:
__metaclass__ = alltuple
def _1(self):
pass
def _2(self,arg):
return arg
Making it nice and pretty left as an exercise.
--
CARL BANKS
On Tue, 28 Sep 2004 14:37:31 +0200, al*****@yahoo.c om (Alex Martelli) wrote:
[...]
At this point, your templating is not declarative -- it's imperative.
Like everything in Python, btw -- not ONE 'declarative' in sight (except
the 'global' statement, which is part of what makes it a wart;-).
Hm ;-) Is a module source a declaration of (imperative) intent, passive until imported?
ISTM we are getting into shades of semantics. Interesting though ;-)
For a language that plays well both ways, I would try scheme or lisp, I think.
Regards,
Bengt Richter
On 28 Sep 2004 21:07:27 GMT, Bengt Richter <bo**@oz.net> wrote: On Tue, 28 Sep 2004 14:37:31 +0200, al*****@yahoo.c om (Alex Martelli) wrote:
[...]
At this point, your templating is not declarative -- it's imperative.
Like everything in Python, btw -- not ONE 'declarative' in sight (except
the 'global' statement, which is part of what makes it a wart;-).
Hm ;-) Is a module source a declaration of (imperative) intent, passive until imported?
ISTM we are getting into shades of semantics. Interesting though ;-)
For a language that plays well both ways, I would try scheme or lisp, I think.
I was just about to reply to Alex, but managed to stop my fingers.
It's indeed a fine line, and I'm not enough of an academicist to
discuss it with all detail it deserves. We could go on weeks debating
it here (and I'm afraid we do). Broadly speaking, my take is as
follows:
Class definitions are executed (imperative), but are normally used to
store definitions (that's declarative, in a broad sense). I think
that's exactly what has attracted me to this kind of 'hack'. The
ability to write intelligent, complex, hierarchic data structures
seamlessly intermingled with code. Templating languages or XML fall
short in this respect. If you really want to *integrate* them both --
and I'm not talking about simply reading static resource files here --
either you have a cross beast that is data based but has some
imperative statements interspersed with a clumsy syntax, or you have
source code filled with unneeded clutter to manage the data
manipulation part, in a rather obstrusive way to the logic of the
system.
(XML based systems use complex parsers to work with the data. It's not
possible, in most cases, to include enough intelligence in the data
stream itself for it to instruct the parser to do something
"different" -- unless you care to define your own language to do it,
and that's clumsy, at best, given XML "great" readability).
--
Carlos Ribeiro
Consultoria em Projetos
blog: http://rascunhosrotos.blogspot.com
blog: http://pythonnotes.blogspot.com
mail: ca********@gmai l.com
mail: ca********@yaho o.com
Bengt Richter <bo**@oz.net> wrote: On Tue, 28 Sep 2004 14:37:31 +0200, al*****@yahoo.c om (Alex Martelli) wrote:
[...]
At this point, your templating is not declarative -- it's imperative.
Like everything in Python, btw -- not ONE 'declarative' in sight (except
the 'global' statement, which is part of what makes it a wart;-).
Hm ;-) Is a module source a declaration of (imperative) intent, passive
until imported?
Every piece of code is 'passive unless executed', but that doesn't mean
every language is declarative.
ISTM we are getting into shades of semantics. Interesting though ;-)
Not all that much (to me), since redefining a word so that it applies to
every possible language is basically robbing that word of any meaning.
For a language that plays well both ways, I would try scheme or lisp, I think.
Hard to argue with this (or Dylan for syntax-sugar reasons, maybe). One
alternative might be to explore pure functional languages, which can be
seeing as remapping imperativeness into declarativeness .
Alex
Carlos Ribeiro <ca********@gma il.com> writes: On 28 Sep 2004 21:07:27 GMT, Bengt Richter <bo**@oz.net> wrote:
[...]
For a language that plays well both ways, I would try scheme or
lisp, I think.
[...]
The ability to write intelligent, complex, hierarchic data
structures seamlessly intermingled with code.
Yes, this is one of the great advantages of Lisp ... and has been for
about four decades. al*****@yahoo.c om (Alex Martelli) writes: Jacek Generowicz <ja************ **@cern.ch> wrote:
... No, you can't, and it's not just a parser issue. Python uses
direct C calls to the native dict type. It's hard coded,
I feared this would be the case.
It's not (not in 2.4 at least)
For what definition of "hard coded" ?
-- the STORE_NAME is quite ready to find a non-dict as the frame's
f_locals. The problem is getting your object to be used as the
frame's f_locals in the first place -- hard but that only affects a
few spots in ceval.c.
So, from the perspective of trying to code it in pure Python, it _is_
hard coded, IIUC.
(Unfortunately, I cannot afford the luxury of playing with the Python
implementation itself; I must deliver code which works with a
bog-standard Python 2.3.4. I'd love to have the time to play with
ceval.c on my own account ... but that is another luxury I cannot
afford :-( ) This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
by: Jp Calderone |
last post by:
Due to some bizarre constraints placed on me, I've written the following
metaclass:
import types
def remove(t, o):
return tuple()
class BizarreMetaclass(type):
def __new__(klass, name, bases, attrs):
|
by: Fernando Rodriguez |
last post by:
Hi,
I'm having trouble with a metaclass suposed to check the method signature of
its classes.
Here's the metaclass:
class MetaChecker(type):
def __new__(cls, name, bases, attribs):
for name, value in attribs.iteritems():
|
by: Robin Becker |
last post by:
A colleague wanted to initialize his class __new__ and tried code resembling this
#######################1
class Metaclass (type):
def __init__(cls, name, bases, *args, **kwargs):
super(Metaclass, cls).__init__(cls, name, bases, *args, **kwargs)
print 'cls=',cls, cls.__new
cls.__new__ = staticmethod(cls.__new)
def __new(self,cls,*args):
|
by: Paul Morrow |
last post by:
One of the beautiful things about Python is its clear, minimal syntax.
So we must resist adding new syntax to the language, especially where
there is a reasonable alternative.
I believe that Stefen Eischet's suggestion for automatically determining
a method's type (class/instance/static) from the name of its first
formal parameter is a reasonable alternative to any/all of the decorator
syntax proposals.
Just as the Python system...
|
by: Irmen de Jong |
last post by:
Hi,
I've developed the Metaclass below, because I needed a way
to make a bunch of classes thread-safe.
I didn't want to change every method of the class by adding
lock.aqcuire()..lock.release() around the existing code.
So I made a metaclass that essentially replaces every method
of a class with a 'wrapper' method, that does the locking,
invocation, unlocking.
Is this the right approach? It seems to work fine. But I have
| | |
by: zipher |
last post by:
After searching through comp.lang.python and the web regarding
metaclasses, I could not find an example for customing classes using
metaclass parameters.
I want to be able to create a class at runtime by calling some
function or 'meta-constructor' which returns a customized class and
sets a class attribute according a given parameter.
Ideally, I'd be able to do something like:
|
by: Pedro Werneck |
last post by:
Hi
I have a class A, with metaclass M_A, and class B, subclass of A, with
metaclass M_B, subclass of M_A.
A class C, subclass of B must have M_B or a subclass of it as metaclass,
but what if I need to 'disable' the code in M_B on C ? The correct way
to do that seems to be with a M_C metaclass, subclass of M_B,
implementing but not calling parent class methods, or calling 'type'
methods.
|
by: Christian Eder |
last post by:
Hi,
I think I have discovered a problem in context of
metaclasses and multiple inheritance in python 2.4,
which I could finally reduce to a simple example:
Look at following code:
class M_A (type) :
|
by: Pedro Werneck |
last post by:
Hi all
I noticed something strange here while explaining decorators to someone.
Not any real use code, but I think it's worth mentioning.
When I access a class attribute, on a class with a custom metaclass with
a __getattribute__ method, the method is used when acessing some
attribute directly with the class object, but not when you do it from
the instance.
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However, people are often confused as to whether an ONU can Work As a Router. In this blog post, we’ll explore What is ONU, What Is Router, ONU & Router’s main usage, and What is the difference between ONU and Router. Let’s take a closer look !
Part I. Meaning of...
|
by: jinu1996 |
last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth.
The Art of Business Website Design
Your website is...
| | |
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one.
At the time of converting from word file to html my equations which are in the word document file was convert into image.
Globals.ThisAddIn.Application.ActiveDocument.Select();...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols.
I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
|
by: adsilva |
last post by:
A Windows Forms form does not have the event Unload, like VB6. What one acts like?
|
by: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
|
by: muto222 |
last post by:
How can i add a mobile payment intergratation into php mysql website.
| | |
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...
| |
