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.
Best Answer
There is no difference, under the hood it's all
varlena
(variable length array).Check this article from Depesz: http://www.depesz.com/index.php/2010/03/02/charx-vs-varcharx-vs-varchar-vs-text/
A couple of highlights:
The article does detailed testing to show that the performance of inserts and selects for all 4 data types are similar. It also takes a detailed look at alternate ways on constraining the length when needed. Function based constraints or domains provide the advantage of instant increase of the length constraint, and on the basis that decreasing a string length constraint is rare, depesz concludes that one of them is usually the best choice for a length limit.