<p>What is the difference between old style and new style classes in Python?  When should I use one or the other?</p>

From
New-style and classic classes
:
Up to Python 2.1, old-style classes were the only flavour available to the user.
The concept of (old-style) class is unrelated to the concept of type:
  if
```
x
```
is an instance of an old-style class, then
```
x.__class__
```
designates the class of
```
x
```
, but
```
type(x)
```
is always
```
<type
  'instance'>
```
.
This reflects the fact that all old-style instances, independently of
  their class, are implemented with a single built-in type, called
  instance.
New-style classes were introduced in Python 2.2 to unify the concepts of class and type
. 
  A new-style class is simply a user-defined type, no more, no less.
If x is an instance of a new-style class, then
```
type(x)
```
is typically
  the same as
```
x.__class__
```
(although this is not guaranteed – a
  new-style class instance is permitted to override the value returned
  for
```
x.__class__
```
).
The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model
.
It also has a number of immediate benefits, like the ability to
  subclass most built-in types, or the introduction of "descriptors",
  which enable computed properties.
For compatibility reasons, classes are still old-style by default
.
New-style classes are created by specifying another new-style class
  (i.e. a type) as a parent class, or the "top-level type" object if no
  other parent is needed.
The behaviour of new-style classes differs from that of old-style
  classes in a number of important details in addition to what type
  returns.
Some of these changes are fundamental to the new object model, like
  the way special methods are invoked. Others are "fixes" that could not
  be implemented before for compatibility concerns, like the method
  resolution order in case of multiple inheritance.
Python 3 only has new-style classes
.
No matter if you subclass from
```
object
```
or not, classes are new-style
  in Python 3.