Hi,
I've been looking at writing parameterized metaclasses and here are the
two solutions I've come to:
(my goal was to build a way to automatically add a hash function that
would take into account a selected list of object attributes)
1. all-in-one metametaclass:
class Autohash2(type) :
"""
Metametaclass that instantiates into a metaclass creating
a hash function based on the attributes passed on instantiation.
"""
def __new__(cls, hash_attributes ):
def class_new(cls, name, bases, d):
print "New class", name
l = hash_attributes
_hash = hash
_tuple = tuple
c = _hash(_tuple([_hash(k) for k in l]))
def object_hash(obj ):
g = obj.__getattrib ute__
return _hash(_tuple([_hash(g(k)) for k in l]))
d['__hash__'] = object_hash
return super(Autohash2 , cls).__new__(cl s, name, bases, d)
name = '__private'
bases = (type,)
d = {'__new__': class_new}
print "New metaclass", name
return type.__new__(cl s, name, bases, d)
2. with the metametaclass property slightly abstracted away:
class Metametaclass(t ype):
def __new__(cls, name, bases, dict_):
d = { '__new__': dict_['class_new'] }
def meta_new(cls, *args, **kargs):
print "New metaclass"
name = '__private'
bases = (type,)
return super(Metametac lass, cls).__new__(cl s, name, bases, d)
dict_['__new__'] = meta_new
print "New metametaclass", name
return type.__new__(cl s, name, bases, dict_)
class Autohash(type):
__metaclass__ = Metametaclass
def __init__(cls, hash_attributes ):
cls.hash_attrib utes = hash_attributes
def class_new(cls, name, bases, d):
print "New class", name
l = cls.hash_attrib utes
_hash = hash
_tuple = tuple
c = _hash(_tuple([_hash(k) for k in l]))
def object_hash(obj ):
g = obj.__getattrib ute__
return _hash(_tuple([_hash(g(k)) for k in l]))
d['__hash__'] = object_hash
return super(Autohash, cls).__new__(cl s, name, bases, d)
Both of those metametaclasses can be used (successfully!) in the
following way:
class Address(object) :
__metaclass__ = Autohash3(('hos t', 'port'))
# <snip rest of class definition>
a = Address()
a.host = 'localhost'
a.port = 5555
b = copy.copy(a)
hash(a) == hash(b)
I was wondering if there is some simpler way of building parameterized
metaclasses ?
Regards
Antoine. 1 2276
> I was wondering if there is some simpler way of building
parameterized metaclasses ?
Why not just a function returning metaclasses?
def metaFactory(*ar gs):
dic = <something possibly depending on args>
return type("somemetac lass", (type,), dic)
Alternatively, a metaclass classmethod returning a metaclass, so that
you can use something like
__metaclass__ = MyMetaclass.wit h(*args) # classmethod returning a
metaclass
Michele Simionato This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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: 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: Jacek Generowicz |
last post by:
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__ = OverloadingClass
att = 1
att = 3
|
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: ironfroggy |
last post by:
Hoping this isn't seeming too confusing, but I need to create a
metaclass and a class using that metaclass, such that one of the bases
of the metaclass is the class created with that metaclass. I can't
figure out a way to do this, even after trying to add the class as a
base after the classes have been created.
Is there some way I can get this to work properly?
| |
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: lbolognini |
last post by:
Hi all,
I dare risk my brain exploding by reaching for the understanding of
metaclasses.
At first i thought i almost got them, even if vaguely back in a corner
of my mind, my understanding was that, as classes' class a metaclass
would be able to return a different class based on input...
.... until i thought of factory functions and, Python considering
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can effortlessly switch the default language on Windows 10 without reinstalling. I'll walk you through it.
First, let's disable language synchronization. With a Microsoft account, language settings sync across devices. To prevent any complications,...
|
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed.
This is as boiled down as I can make it.
Here is my compilation command:
g++-12 -std=c++20 -Wnarrowing bit_field.cpp
Here is the code in...
| |
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: tracyyun |
last post by:
Dear forum friends,
With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each protocol has its own unique characteristics and advantages, but as a user who is planning to build a smart home system, I am a bit confused by the choice of these technologies. I'm particularly interested in Zigbee because I've heard it does some...
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules.
He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms.
Adolph will...
|
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: muto222 |
last post by:
How can i add a mobile payment intergratation into php mysql website.
| |