Létez? wrote:
> I use Python 2.4.4. Please read the code below:
>
> -----------------------------------------------------------
> from new import classobj
>
> def mymeta(name,bases,clsdict):
> print 'meta: %s'%name
> return classobj(name,bases,clsdict)
mymeta is not a class.
> class A(object):
> __metaclass__=mymeta
Throw in
print "A.__class__", A.__class__
here and read the sentence from the manual again: The __class__ is 'type',
so no print... side effects are to be expected below.
> class B(A):
> pass
>
> -----------------------------------------------------------
>
> This should print
>
> meta: A
> meta: B
>
> when classes A and B are created. But only meta: B is missing,
> since mymeta() is not called when class B is created.
>
> Related python documentation states that mymeta() must be called when B is
> created, since metaclasses are looked up in bases classes if not found in
> the dictionary of the class itself.
>
>>From Python 2.4.4's manual: "Otherwise, if there is at least one base
>>class,
> its metaclass is used (this looks for a __class__ attribute first and if
> not found, uses its type)."
One way to get the output you expect:
>>> class AType(type):
... def __init__(cls, name, bases, clsdict):
... print "meta:", name
...
>>> class A:
... __metaclass__ = AType
...
meta: A
>>> class B(A): pass
...
meta: B
I tried with classobj first, but that didn't work:
>>> from new import classobj
>>> class AType(classobj): pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
type 'classobj' is not an acceptable base type
Peter
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