Python data modelling

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Mixing it all up with Python

We shall discuss
● Classes
● New style classes
● Inheritace heirarchy and MRO
● Mixins
● Meta Classes

Classes
● Blueprints for creating an object
● Objects have properties(attributes) and
actions(methods)
● Python Classes ar...

Class
class MyClass(BaseClass1, BaseClass2):
def __init__(self, attr1, attr2):
self.attr1 = attr1
self.attr2 = attr2
def d...

Stuff to discuss
● If there are no base classes you need not include
the braces after the class name
● object initializati...

New style classes
● Introduced in python 2.2
● Inherit from 'object'
● Yes 'object' is actually a class, which act as the
...

New style class
class MyClass(object):
def __init__(self, attr1, attr2):
super(MyClass, self).__init__()
self.attr1 = attr...

What is the benifit of new style
classes
● type class unification
● you can now subclass list, dict etc and create a
custo...

Old vs New
class X:
def __init__(self, value):
self.value = value
def __eq__(self, other):
return self.value == other.valu...

Beginners better not
● If you want to alter the object creation you are looking
for __new__()
● Unlike __init__, __new__ i...

example
class X(str):
def __new__(cls, arg1):
return str.__new__(cls, arg1.upper())
def __init__(self, my_text):
self.orig...

Class Customization
● You can customize your classes with
– __eq__, __ne__, __lt__, __gt__, __le__, __ge__,
__cmp__,
– __n...

MRO
● Defines where the interpreter looks, in the
inheritance chain, for a method when invoked
on an object
● It is the sa...

C3 algorithm
● Introduced in 2.3
● A MRO is bad when it breaks such fundamental properties
as
– local precedence ordering
...

A Bad MRO
class X(object):
pass
class Y(object):
pass
class A(X, Y):
pass
class B(Y, X):
pass
class C(A, B):
pass

C3 Algorithm
take the head of the first list, if this head is not in the
tail of any of the other lists, then add it to th...

Appy to our example
MRO(A) = AXYO
MRO(B) = BYXO
MRO(C) = C , AXYO, BYXO, AB
MRO(C) = C, A, XYO, BYXO, B
MRO(C) = C, A, B, ...

Mixins
● A form of inheritance where your bases implement a few
methods
– That can be mixed with out affecting the inherit...

Meta Classes
● It is the class of a class
● Allows creation of class at run time
● Possible because in python everything i...

$examples a = str('hello world') type(a) type(type(a)) str.__class__ new_str = type('new_string_type', (str,), {'creator': ...$

Mixing up
class dancer(object):
def dance(self):
print 'I am dancing'
class singer(object):
def sing(self):
print 'I am si...

Thank You
Ageeleshwar K
akilesh1597@gmail.com

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