The type function returns inconsistent value depending on class hierarchy.

>>> class X:
...     pass
... 
>>> x = X()
>>> type(x)
<type 'instance'>
>>> class Y(object):
...     pass
... 
>>> x = Y()
>>> type(x)
<class '__main__.Y'>
>>> 
>>> class Z(X):
...     pass
...  
>>> x = Z()
>>> type(x)
<type 'instance'>
>>> class K(Y):
...     pass
...   
>>> x = K()
>>> type(x)
<class '__main__.K'>

All should reply 'instance'. As long as I have read on documentation the
only way to change the type function return is by writing a
__metaclass__ (PEP3115).

The relevant documentation for Python 2.6 is there.
http://docs.python.org/reference/datamodel.html#new-style-and-classic-classes

The PEP 3115 is implemented for Python >= 3.0.

There's nothing wrong.

`instance` means it is an instance of an old-style class. Old-style
classes are classes which don't have `object` in their inheritance
hierarchy.
On the other hand, for instance new-style classes type() returns the
actual class.
Bottom line: this is by design. Of course in an ideal world (or in
Python 3) there are only new-style classes, but we had to maintain
compatibility, and that's why there are two slightly different object
models cohabiting in Python 2.x.