Metaclasses are not called in subclasses. What did I wrong?

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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)

class A(object):
__metaclass__=mymeta

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)."

Thanks for your help.

Viktor

 

[Peter Otten]

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.

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:
.... def __init__(cls, name, bases, clsdict):
.... print "meta:", name
........ __metaclass__ = AType
....
meta: A....
meta: B

I tried with classobj first, but that didn't work:
....
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

 

[Georg Brandl]

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)

class A(object):
__metaclass__=mymeta

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.

its metaclass is used (this looks for a __class__ attribute first and if not
found, uses its type)."

Peter already explained that when __metaclass__ is not directly defined in the
class body, it's not used as the metaclass, but the class of the first base
class is.

This is an inconsistency IMHO, since Python normally respects inherited
attributes just like those defined in the class itself.

But well, you can live with it

Georg