Having said that i should also note that even though

Having said that, I should also note that even though you might not code diamond
patterns in classes you write yourself, because the implied
object
superclass is above
every class in 3.0, every case of multiple inheritance exhibits the diamond pattern today.
That is, in new-style classes,
object
automatically plays the role that the class
A
does in
the example we just considered. Hence the new-style search rule not only modifies
logical semantics, but also optimizes performance by avoiding visiting the same class
more than once.
Just as important, the implied
object
superclass in the new-style model provides default
methods for a variety of built-in operations, including the
__str__
and
__repr__
display
format methods. Run a
dir(object)
to see which methods are provided. Without the
new-style search order, in multiple inheritance cases the defaults in
object
would al-
ways override redefinitions in user-coded classes, unless they were always made in the
leftmost superclass. In other words, the new-style class model itself makes using the
new-style search order more critical!
New-Style Class Changes | 789
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For a more visual example of the implied
object
superclass in 3.0, and other examples
of diamond patterns created by it, see the
ListTree
class’s output in the lister.py example
in the preceding chapter, as well as the classtree.py tree walker example in Chapter 28 .
New-Style Class Extensions
Beyond the changes described in the prior section (which, frankly, may be too academic
and obscure to matter to many readers of this book), new-style classes provide a handful
of more advanced class tools that have more direct and practical application. The fol-
lowing sections provide an overview of each of these additional features, available for
new-style class in Python 2.6 and all classes in Python 3.0.
Instance Slots
By assigning a sequence of string attribute names to a special
__slots__
class attribute,
it is possible for a new-style class to both limit the set of legal attributes that instances
of the class will have and optimize memory and speed performance.
This special attribute is typically set by assigning a sequence of string names to the
variable
__slots__
at the top level of a
class
statement: only those names in the
__slots__
list can be assigned as instance attributes. However, like all names in Python,
instance attribute names must still be assigned before they can be referenced, even if
they’re listed in
__slots__
. For example:
>>> class limiter(object):
... __slots__ = ['age', 'name', 'job']
...
>>> x = limiter()
>>> x.age # Must assign before use
AttributeError: age
>>> x.age = 40
>>> x.age
40
>>> x.ape = 1000 # Illegal: not in __slots__
AttributeError: 'limiter' object has no attribute 'ape'
Slots are something of a break with Python’s dynamic nature, which dictates that any
name may be created by assignment. However, this feature is envisioned as both a way
to catch “typo” errors like this (assignments to illegal attribute names not in
__slots__
are detected), as well as an optimization mechanism. Allocating a namespace