questions (& answers) about object, type, builtin types, class, metaclass and __getattribute__
I'm learning a bit of python internals lately and I'm trying to figure
out the
relationship between type, objects, class, callables and
__getattribute__ resolution.
While understanding Python mechanics/concepts, I'm trying to figure
how it
translates in CPython. This post is Python centric. Questions about
the
implementation of this concepts might be the subject of a future post
[1].
I will proceed this way: I will write statements about each subject.
Don't hesitate
to pick one and provide more information or better phrasing, or
explaining why it's
not true.
Be aware that I'm considering only new-style class.
A) type vs object
-
1) object is the base object, it has no bases : len(object.__bases__)
== 0
2) every object in python inherit object :
any_object_except_object.__bases__[-1] is object
3) object's type is type : object.__class__ is type
4) type parent object is object : type.__bases__ == (object,)
B) type vs metaclass
1) type is the first metaclass ?
2) type is its own metaclass : type(type) is type ?
3) object's metaclass is type ?
4) other metaclasses *MUST* inherit type ?
5) type(any_object) == last_metaclass_..., which is, most of the time,
type ?
C) type vs class
1) Type is the metaclass of most classes
2) The class statement::
class MyClass(object):
attribute = 1
def method(self):
pass
translates to::
MyClass = type('MyClass', (object,), {'attribute': 1, 'method':
def method: pass })
3) Instantiation of any class ``MyClass(*args, **kwargs)`` translates
to::
type(MyClass).__call__(MyClass, *args, **kwargs)
This is due to __getattribute__ algorithm (see E)
4) It's in type.__call__ that happens calls to __new__ and __init__
5) 3) => classes are instance of type
6) Since type.__call__ is used to instantiate instance of instance of
type
(rephrased: __call__ is used to instantiate classes) where is the
code which
is executed when we write ``type(myobject)`` or ``type('MyClass',
bases, attributes)``
__getattribute__ resolution algorithm (see E) tells me that it
should be type.__call__
but type.__call__ is already used to class instatiation.
C') class vs class instances aka. objects
-
1) A class type is a metaclass : issubclass(type(MyClass), type),
MyClass.__class__ == type(MyClass)
2) An object type is a class : most of the time
isinstance(type(my_object), type)
generally issubclass(type(type(my_object)), type)
D) builtin types
1) builtin types are their own metaclass ?
2) why function builtin type can not be subclassed ?
3) how does builtin function type relate to callable objects ?
4) int(1) is the same as int.__call__(1), since type(int) is type,
shouldn't int(1)
translates to type.__call__(int, 1) ?
E) __getattribute__
---
1) ``my_object.attribute`` always translates to
``my_object.__getattribute__('attribute')``
2) Is the following algorithm describing __getattribute__ correct [2],
beware that I've
added a comment:
a) If attrname is a special (i.e. Python-provided) attribute for
objectname,
return it. # what does it mean to be Python-provided ?
b ) Check objectname.__class__.__dict__ for attrname. If it exists
and is a
data-descriptor, return the descriptor result. Search all bases of
objectname.__class__
for the same case.
c) Check objectname.__dict__ for attrname, and return if found.
d) If it is a class and a descriptor exists in it or its bases,
return the
descriptor result.
d) Check objectname.__class__.__dict__ for attrname. If it exists
and is a
non-data descriptor, return the descriptor result. If it
exists, and is
not a descriptor, just return it. If it exists and is a data
descriptor,
we shouldn't be here because we would have returned at point 2.
Search all
bases of objectname.__class__ for same case.
e) Raise AttributeError
Thanks in advance,
Regards,
Amirouche
[1] or maybe you can point me to the right direction before I ask
stupid questions.
I'm a bit familiar with Jython code, which seems to be easier than
PyPy and CPython
to read even if there is some magic due to the fact that Jython use
some JVM API
that hides somewhat how it works.
[2] this is a rewritten version of
http://www.cafepy.com/article/python_attributes_and_methods/python_attributes_and_methods.html#attribute-search-summary
--
http://mail.python.org/mailman/listinfo/python-list
Re: questions (& answers) about object, type, builtin types, class, metaclass and __getattribute__
On Aug 22, 1:57 pm, Steven D'Aprano wrote: > The relationship between type and object is somewhat special, and needs to > be bootstrapped by the CPython virtual machine. Since you are talking about CPython, I'm wondering how it is bootstraped since you can easly reference PyType in PyObject that part is not hard. > > 2) type is its own metaclass : type(type) is type ? > > Yes. Another bit of bootstrapping that the compiler does. self reference is easy same as referencing PyType from PyObject and PyObject from PyType. > > 5) type(any_object) == last_metaclass_..., which is, most of the time, > > type ? > > I'm not sure what you mean by "last_metaclass". But no. The type of an > object is its class: see this code for example proove my point: class meta_a(type): def __new__(cls, *args, **kwargs): return type.__new__(cls, *args, **kwargs) # see (¤) class meta_b(meta_a): def __new___(cls, *args, **kwargs): return meta_a.__new__(cls, *args, **kwargs) # same as above class ClassWithTypeMetaA(object): __metaclass__ = meta_a class ClassWithTypeMetaB(object): __metaclass__ = meta_b type(ClassWithTypeMetaA) == meta_a type(ClassWithTypeMetaB) == meta_b [¤] super call doesn't work here, anyone can say why ? Regards, Amirouche -- http://mail.python.org/mailman/listinfo/python-list
Re: questions (& answers) about object, type, builtin types, class, metaclass and __getattribute__
On Aug 22, 5:41 pm, Stephen Hansen wrote:
> > 3) object's type is type : object.__class__ is type
> > 4) type parent object is object : type.__bases__ == (object,)
>
> Saying "type" and "parent" and the like for new-style classes is
> something of a misnomer. For "type" and "object", these things aren't
> constructed like this.
>
> What you have here is technically true if you go poke at it in the
> interpreter, but it doesn't really /mean/ anything because its not how
> these objects came to be and is circular and a bit confusing. These
> fundamental objects are created special.
The code snippet is here to illustrate how it is visible in the
interpreter. But
you are right.
> > 2) type is its own metaclass : type(type) is type ?
>
> Only in a purely theoretical way. It doesn't actually mean anything;
> moreover, type(something) is NOT how you determine somethings metaclass.
> Its how you determine somethings type.
see the answer to Steven D'Aprano. type(class_object) ==
a_meta_class_object
> The two concepts may be very distinct. Lots of things don't have
> metaclasses.
All object in new style class have a metaclass at least type.
> > 3) object's metaclass is type ?
>
> Again, only theoretically.
and again the famous "bootstrapping" make it like type created object.
>From the
outside world of the Python implementation object looks like a type
instance.
> > 5) type(any_object) == last_metaclass_..., which is, most of the time,
> > type ?
>
> Not necessarily at all. In fact, there is no way I'm aware of to
> determine if a metaclass was involved in a classes construction unless
> said metaclass wants to provide such a mechanism.
>
> Metaclasses are kind of a hack. They are a way to hook into the class
> construction that's normally done and do something, anything you want,
> (even hijack the whole procedure and NOT construct a class at all, but
> play a song if you want) before its all finished.
>
> For example, this is a metaclass I've used:
>
> PageTypes = {}
>
> class _PageRegistration(type):
> def __new__(cls, name, bases, dct):
> klass = type.__new__(cls, name, bases, dct)
> typename = name[:-9].lower()
> if not typename:
> typename = None
>
> PageTypes[typename] = klass
> klass.Type = typename
>
> return klass
>
> class QueuePage(sc.SizedPanel):
> __metaclass__ = _PageRegistration
>
> Note, the fact that my _PageRegistration metaclass inherits is itself a
> class which inherits from type is just one convenient way to write
> metaclasses. It could as simply have been just a function.
>
> Metaclasses are somewhat poorly named in that they are really, "creation
> hooks".
It the same issue in django, views are only function, until you need
complex
behavior and you want a "namespace" to put everything in it. IMO
that's why class
based views exists for complex cases. That said being able to declare
a metaclass
only as a functions is neat.
> > C) type vs class
> >
>
> > 1) Type is the metaclass of most classes
>
> Yes and no. Yes, in that most classes are created using the default
> mechanism inside CPython. The class body is executed in a scope, the
> resulting dictionary is bound to a new class object, bases and the like
> are set, and such.
>
> No in that it really just, IIUC, skips the whole "metaclass" part of the
> process because this 'default mechanism' doesn't need to call out into
> other code to do its job. At least, I think-- May be wrong here,
> metaclasses are something of a dark voodoo and I'm not 100% entirely
> familiar with the internal workings of CPython.
>
> But functionally, a metaclass is the chunk of code responsible for the
> actual physical construction of the class object.
For me it takes some variables, namely ``bases``, ``class_dict`` and
``configuration class_name`` and do something with it, probably
creating a
class_object which behaviour is parametred with the context. I did not
know
Python before new-style class, so probably for most people explainning
that
metaclasses are a creation hook is easier for them...
> > 4) It's in type.__call__ that happens calls to __new__ and __init__
>
> Again, "translates to" is suggesting "this is what happens when you do
> X", which I don't know if is strictly true. CPython inside may be
> optimizing this whole process.
> Especially when it comes to "magic
> methods", __x__ and the like -- CPython rarely uses __get*_ for those.
> It just calls the methods directly on the class object.
IIUC the code of Jython tells me what I've written. If the first part
of the algorithm
is "lookup for special methods" (what you seem to say) then we both
agree that we
agree, isn't it ?
Moreover I'm not looking in this part to understand how CPython works
internally,
but how Python works. Since I'm most proeffencient in Python I
translate it to
Python.
*Translates* means "it's a shortcut for".
> > 5) 3) => c
