In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses.
Questions tagged [metaclass]
1179 questions
7262
votes
25 answers
What are metaclasses in Python?
What are metaclasses? What are they used for?
Bite code
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Creating a singleton in Python
This question is not for the discussion of whether or not the singleton design pattern is desirable, is an anti-pattern, or for any religious wars, but to discuss how this pattern is best implemented in Python in such a way that is most pythonic. In…
theheadofabroom
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1002
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170
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What are some (concrete) use-cases for metaclasses?
I have a friend who likes to use metaclasses, and regularly offers them as a solution.
I am of the mind that you almost never need to use metaclasses. Why? because I figure if you are doing something like that to a class, you should probably be…
Ali Afshar
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120
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Understanding __init_subclass__
I finally upgraded my python version and I was discovering the new features added. Among other things, I was scratching my head around the new __init_subclass__ method. From the docs:
This method is called whenever the containing class is…
EsotericVoid
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Understanding metaclass and inheritance in Python
I have some confusion regarding meta-classes.
With inheritance
class AttributeInitType(object):
def __init__(self,**kwargs):
for name, value in kwargs.items():
setattr(self, name, value)
class Car(AttributeInitType):
def…
Nikhil Rupanawar
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87
votes
6 answers
Resolving metaclass conflicts
I need to create a class that uses a different base class depending on some condition. With some classes I get the infamous:
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all…
Yves Dorfsman
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Using the __call__ method of a metaclass instead of __new__?
When discussing metaclasses, the docs state:
You can of course also override other class methods (or add new
methods); for example defining a custom __call__() method in the
metaclass allows custom behavior when the class is called, e.g. not
always…
Eli Bendersky
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65
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python subclasscheck & subclasshook
The methods __subclasscheck__ and __subclasshook__ are used to determine if a class is regarded as subclass of another. However, their documentation is very limited, even in advanced Python books. How are they meant to be used and what is their…
blue_note
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Python Metaclass : Understanding the 'with_metaclass()'
I want to ask what the with_metaclass() call means in the definition of a class.
E.g.:
class Foo(with_metaclass(Cls1, Cls2)):
Is it a special case where a class inherits from a metaclass?
Is the new class a metaclass, too?
Zakos
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Is there any reason to choose __new__ over __init__ when defining a metaclass?
I've always set up metaclasses something like this:
class SomeMetaClass(type):
def __new__(cls, name, bases, dict):
#do stuff here
But I just came across a metaclass that was defined like this:
class SomeMetaClass(type):
def…
Jason Baker
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Using abc.ABCMeta in a way it is compatible both with Python 2.7 and Python 3.5
I'd like to create a class which has abc.ABCMeta as a metaclass and is compatible both with Python 2.7 and Python 3.5. Until now, I only succeeded doing this either on 2.7 or on 3.5 - but never on both versions simultaneously. Could someone give me…
Tatiana Al-Chueyr
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59
votes
6 answers
How to auto register a class when it's defined
I want to have an instance of class registered when the class is defined. Ideally the code below would do the trick.
registry = {}
def register( cls ):
registry[cls.__name__] = cls() #problem here
return cls
@register
class MyClass( Base…
deft_code
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Metaclass multiple inheritance inconsistency
Why is this:
class MyType(type):
def __init__(cls, name, bases, attrs):
print 'created', cls
class MyMixin:
__metaclass__ = MyType
class MyList(list, MyMixin): pass
okay, and works as expected:
created
Matt Anderson
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Error when calling the metaclass bases: function() argument 1 must be code, not str
I tried to subclass threading.Condition earlier today but it didn't work out. Here is the output of the Python interpreter when I try to subclass the threading.Condition class:
>>> import threading
>>> class ThisWontWork(threading.Condition):
... …
David Underhill
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