Object-Oriented Python From scratch

OBJECT-ORIENTED PYTHON
From scratch
Leonardo Giordani
lgiordani.com
OOP

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I'm a software engineer, interested in operating
systems, versioning, Python and software architecture.
Coder since around 1988, Linux user since 1998,
Python lover since 1999.
Currently working with Python and C in the field of
satellite remote sensing.
about me
The Digital Cat
lgiordani.com
https://twitter.com/
tw_lgiordani
https://github.com/
lgiordani
https://plus.google.com/u/
LeonardoGiordani

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import users
import talk
assert all([a >= users.beginner for a in talk.attendees])
about_you.py

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ABOUT THIS TUTORIAL
This tutorial covers Python 3.
Almost all concepts are valid for Python 2.
Relevant differences will be highlighted.
Objects and types
Classes and members
Delegation
Polymorphism
Metaclasses
3

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https://github.com/lgiordani/oopy
Code available as IPython Notebooks

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PART 1
Objects and types

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Object-oriented Python from scratch – lgiordani.com - CC BY-SA 4.0
I-2 27
data = (13, 63, 5, 378, 58, 40)
def avg(d):
return sum(d)/len(d)
avg(data)
92.83333333333333
Plain old procedures
part1/01.py

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I-3 27
data = (13, 63, 5, 378, 58, 40)
def avg(d):
return sum(d)/len(d)
avg(data)
92.83333333333333
Very simple data:
Very simple data:
a sequence of numbers
a sequence of numbers
This returns new data
This returns new data
part1/01.py
Source data enters here
Source data enters here
Plain old procedures

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I-4 27
door1 = [1, 'closed']
def open_door(door):
door[1] = 'open'
open_door(door1)
door1
[1, 'open']
part1/02.py Procedures can modify data

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I-5 27
ldoor1 = [1, 'closed', 'unlocked']
door[1] = 'open'
def open_ldoor(door):
if door[2] == 'unlocked':
door[1] = 'open'
part1/03.py Things can get complicated

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I-6 27
Same “action”
Same “action”
You must use
You must use
the right function
the right function
ldoor1 = [1, 'closed', 'unlocked']
door[1] = 'open'
def open_ldoor(door):
if door[2] == 'unlocked':
door[1] = 'open'
open_door(door1)
door1
[1, 'open']
open_ldoor(ldoor1)
ldoor1
[1, 'open', 'unlocked']
part1/03.py Things can get complicated

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I-7 27
Behaves
Behaves
like a duck
like a duck
Behavioural meaning
The meaning of the word 'type'

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I-8 27
Dissection
Dissection
reveals
reveals
the
the truth
truth
Structural meaning
The meaning of the word 'type'

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I-9 27
The behavioural meaning is important

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I-10 27
Duck typing: make it behave like a duck

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I-11 27
Duck typing joke #1

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I-12 27
Duck typing joke #2

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I-13 27
The meaning of the word 'class'

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I-14 27
The meaning of the word 'instance'

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I-15 27
a = 6
a
6
type(a)

You already used classes

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Python
2.x
I-16 27
You already used classes
a = 6
a
6
type(a)

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I-17 27
The first class
part1/04.py
class Door:
def __init__(self, number, status):
self.number = number
self.status = status
def open(self):
self.status = 'open'
def close(self):
self.status = 'closed'

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Python
2.x
I-18 27
The first class
class Door(object):
def open(self):
def close(self):

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I-19 27
The first class
part1/04.py
class Door:
def open(self):
def close(self):
The 'class' keyword defines the class
The 'class' keyword defines the class

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I-20 27
The first class
part1/04.py
class Door:
def open(self):
def close(self):
Everything under the 'class'
Everything under the 'class'
keyword is part of the class
keyword is part of the class

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I-21 27
The first class
part1/04.py
class Door:
def open(self):
def close(self):
Method
Method

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I-22 27
The first class
part1/04.py
class Door:
def open(self):
def close(self):
Attributes
Attributes
Constructor (part of)
Constructor (part of)

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I-23 27
door1 = Door(1, 'closed')
The first class
class Door:
def open(self):
def close(self):

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I-24 27
type(door1)

The first class
class Door:
def open(self):
def close(self):

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I-25 27
type(door1)

door1.number
1
door1.status
'closed'
The first class
class Door:
def open(self):
def close(self):

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I-26 27
type(door1)

door1.number
1
door1.status
'closed'
door1.open()
The first class
class Door:
def open(self):
def close(self):

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I-27 27
type(door1)

door1.number
1
door1.status
'closed'
door1.open()
door1.number
1
door1.status
'open'
The first class
class Door:
def open(self):
def close(self):

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PART 2
Classes and members

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II-2 42
Everything is an object
a = 1
type(a)

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II-3 42
Everything is an object
a = 1
type(a)

type(int)

The type of an object is
The type of an object is
an object itself
an object itself

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II-4 42
Where is the class of an object?

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II-5 42
hex(id(door1))
'0xb67e148c'
hex(id(door2))
'0xb67e144c'

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II-6 42
hex(id(door1))
'0xb67e148c'
hex(id(door2))
'0xb67e144c'
hex(id(door1.__class__))
'0xb685f56c'
hex(id(door2.__class__))
'0xb685f56c'
The class is not a mere concept!
The class is not a mere concept!
It is an object in the system.
It is an object in the system.

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II-7 42
Class attributes
part2/01.py
class Door:
colour = 'brown'
def open(self):
def close(self):

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II-8 42
part2/01.py
class Door:
colour = 'brown'
def open(self):
def close(self):
No 'self' here
No 'self' here
Class attributes

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II-9 42
Class attributes

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II-10 42
Door.colour
'brown'
door1.colour
'brown'
door2.colour
'brown'
Class attributes

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II-11 42
Door.colour = 'white'
Class attributes

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II-12 42
Door.colour
'white'
door1.colour
'white'
door2.colour
'white'
Class attributes

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II-13 42
Door.colour
'white'
door1.colour
'white'
door2.colour
'white'
hex(id(Door.colour))
'0xb67e1500'
hex(id(door1.colour))
'0xb67e1500'
hex(id(door2.colour))
'0xb67e1500'
Class attributes

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II-14 42
Door.__dict__
mappingproxy({'open': ,
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__module__': '__main__',
'__doc__': None,
'close': })
Class attributes

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II-15 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
{'number': 1, 'status': 'closed'}
Class attributes

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II-16 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
door1.__dict__['colour']
Traceback (most recent call last):
File "", line 1, in
KeyError: 'colour'
Class attributes

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II-17 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
File "", line 1, in
KeyError: 'colour'
door1.__class__.__dict__['colour']
'white'
door1.colour is Door.colour
True
Class attributes

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II-18 42
Door
door1 door2
__getattribute__()
__getattribute__()
Let's dive into attribute resolution
door1.colour
Door.colour

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II-19 42
door1.colour = 'white'
'white'
'brown'

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II-20 42
'white'
'brown'
door1.colour
'white'
Door.colour
'brown'

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II-21 42
'white'
'brown'
door1.colour
'white'
Door.colour
'brown'
Door.colour = 'red'

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II-22 42
'white'
'brown'
door1.colour
'white'
Door.colour
'brown'
Door.colour = 'red'
'white'
'red'

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II-23 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
True
What about methods?

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II-24 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
True
door1.open is Door.open
False
What about methods?

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II-25 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
True
False
Door.__dict__['open']

Door.open

What about methods?

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II-26 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
True
False

Door.open

door1.open
>
What about methods?

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Python
2.x
II-27 42
Door.__dict__
'colour': 'white',
'__dict__': ,
'__weakref__': ,
'__init__': ,
'__doc__': None,
'close': })
door1.__dict__
True
False

Door.open

door1.open
>
What about methods?

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II-28 42
Door.open()
File "", line 1, in
TypeError: open() missing 1 required positional argument: 'self'
From functions to bound methods

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Python
2.x
II-29 42
Door.open()
File "", line 1, in
TypeError: unbound method open() must be called with Door instance
as first argument (got nothing instead)

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II-30 42
Door.open()
File "", line 1, in
Door.open(door1)
door1.status
'open'

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II-31 42
Door.open()
File "", line 1, in
Door.open(door1)
door1.status
'open'
door1.__class__.__dict__['open']


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II-32 42
Door.open()
File "", line 1, in
Door.open(door1)
door1.status
'open'

dir(door1.__class__.__dict__['open'])
['__annotations__', '__call__', '__class__', '__closure__',
'__code__', '__defaults__', '__delattr__', '__dict__',
'__dir__', '__doc__', '__eq__', '__format__', '__ge__',
'__get__', '__getattribute__', '__globals__', '__gt__',
'__hash__', '__init__', '__kwdefaults__', '__le__', '__lt__',
'__module__', '__name__', '__ne__', '__new__', '__qualname__',
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__',
'__sizeof__', '__str__', '__subclasshook__']

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II-33 42
Door.open()
File "", line 1, in
Door.open(door1)
door1.status
'open'

...,'__get__',...]
door1.__class__.__dict__['open'].__get__


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II-34 42
Door.open()
File "", line 1, in
Door.open(door1)
door1.status
'open'

...,'__get__',...]

door1.__class__.__dict__['open'].__get__(door1)
>

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Python
2.x
II-35 42
Door.open()
File "", line 1, in
Door.open(door1)
door1.status
'open'

...,'__get__',...]

door1.__class__.__dict__['open'].__get__(door1)
>
door1.__class__.__dict__['open'].__get__(door1, Door)
>

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II-36 42
part2/02.py
class Door:
colour = 'brown'
@classmethod
def knock(cls):
print("Knock!")
def open(self):
def close(self):
Class methods

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II-37 42
door1.knock()
Knock!
Door.knock()
Knock!

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II-38 42
part2/03.py
class Door:
colour = 'brown'
@classmethod
def knock(cls):
print("Knock!")
@classmethod
def paint(cls, colour):
cls.colour = colour
def open(self):
def close(self):
Class methods

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II-39 42
Class methods

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II-40 42
Door.colour
'brown'
door1.colour
'brown'
door2.colour
'brown'
Class methods

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II-41 42
Door.paint('white')
Door.colour
'white'
door1.colour
'white'
door2.colour
'white'
Class methods

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II-42 42
Door.paint('white')
Door.colour
'white'
door1.colour
'white'
door2.colour
'white'
door1.paint('yellow')
Door.colour
'yellow'
door1.colour
'yellow'
door2.colour
'yellow'
Class methods

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PART 3
Delegation

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III-2 36
Specialization
Cat
Animal

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III-3 36
Cat has all the features of
Cat has all the features of
Animal, i.e. 'moves'
Animal, i.e. 'moves'
Specialization
Cat
Animal

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III-4 36
Cat can provide new features, i.e.
Cat can provide new features, i.e.
'has whiskers'
'has whiskers'
Specialization
Cat
Animal

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III-5 36
Cat performs some or all the tasks performed
Cat performs some or all the tasks performed
by Animal in a different way, i.e. 'moves silently'
by Animal in a different way, i.e. 'moves silently'
Specialization
Cat
Animal

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III-6 36
Cat implements only 'new' or
Cat implements only 'new' or
'changed' features
'changed' features
Cat delegates the remaining
Cat delegates the remaining
features to Animal
features to Animal
Delegation
Cat
Animal

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III-7 36
Composition: 'has'
Car
Engine turn_on()
Wheels steer()
get_color()

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III-8 36
Inheritance: 'is'
look()
look()
Cat
Animal
look()
mew()

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III-9 36

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III-10 36
Inheritance
part3/01.py
class SecurityDoor(Door):
pass

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III-11 36
Inheritance
pass
sdoor = SecurityDoor(1, 'closed')

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III-12 36
Inheritance
pass
SecurityDoor.colour is Door.colour
True
sdoor.colour is Door.colour
True

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III-13 36
Inheritance
pass
SecurityDoor.colour is Door.colour
True
sdoor.colour is Door.colour
True
sdoor.colour
sdoor.colour
SecurityDoor.colour
SecurityDoor.colour
Door.colour
Door.colour

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III-14 36
Inheritance
sdoor.__dict__

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III-15 36
Inheritance
sdoor.__dict__
sdoor.__class__.__dict__
mappingproxy({'__doc__': None, '__module__': '__main__'})
Door.__dict__
mappingproxy({'__dict__': ,
'colour': 'yellow',
'open': ,
'__init__': ,
'__doc__': None,
'close': ,
'knock': ,
'__weakref__': ,
'paint': })

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III-16 36
Inheritance
sdoor.__dict__
Door.__dict__
'colour': 'yellow',
'open': ,
'__init__': ,
'__doc__': None,
'close': ,
'knock': ,
'__weakref__': ,
'paint': })
SecurityDoor.__bases__
(,)

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III-17 36
Inheritance
sdoor.__dict__
Door.__dict__
'colour': 'yellow',
'open': ,
'__init__': ,
'__doc__': None,
'close': ,
'knock': ,
'__weakref__': ,
'paint': })
SecurityDoor.__bases__
(,)
sdoor.knock
>
sdoor.__class__.__bases__[0].__dict__['knock'].__get__(sdoor)
>

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III-18 36
Overriding
part3/02.py
colour = 'grey'
locked = True
def open(self):
if not self.locked:

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III-19 36
Overriding
part3/02.py
Overriding blocks implicit
Overriding blocks implicit
delegation
delegation
colour = 'grey'
locked = True
def open(self):
if not self.locked:
SecurityDoor.__dict__
mappingproxy({'__doc__': None,
'open': ,
'colour': 'grey',
'locked': True})

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III-20 36
Overriding
part3/03.py
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
Door.open(self)

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III-21 36
Overriding
part3/03.py
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
Door.open(self)
sdoor.status
'closed'

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III-22 36
Overriding
part3/03.py
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
Door.open(self)
sdoor.status
'closed'
sdoor.open()
sdoor.status
'closed'

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III-23 36
Overriding
part3/03.py
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
Door.open(self)
sdoor.status
'closed'
sdoor.open()
sdoor.status
'closed'
sdoor.locked = False
sdoor.open()
sdoor.status
'open'

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III-24 36
Avoid strong coupling
part3/04.py
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
super().open()

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Python
2.x
III-25 36
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
super(SecurityDoor, self).open(self)
Avoid strong coupling

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III-26 36
class SecurityDoor:
colour = 'grey'
locked = True
self.door = Door(number, status)
def open(self):
if self.locked:
return
self.door.open()
def close(self):
self.door.close()
Composition
part3/05.py

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III-27 36
class SecurityDoor:
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
self.door.open()
def close(self):
Self.door.close()
sdoor.status
File "", line 1, in
AttributeError: 'SecurityDorr' object has no attribute 'status'
Composition
part3/05.py

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III-28 36
class SecurityDoor:
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
self.door.open()
def close(self):
self.door.close()
def get_status(self):
return self.door.status
status = property(get_status)
sdoor.status
'closed'
Composition

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III-29 36
Python magic to the rescue
class SecurityDoor:
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
self.door.open()
def close(self):
self.door.close()
def __getattr__(self, attr):
return getattr(self.door, attr)
sdoor.status
'closed'
part3/06.py

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III-30 36
Python magic to the rescue
class SecurityDoor:
colour = 'grey'
locked = True
def open(self):
if self.locked:
return
self.door.open()
#def close(self):
# self.door.close()
part3/07.py

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III-31 36
class ComposedDoor:
Composed inheritance?
part3/08.py

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III-32 36
getattr()
l = [1,2,3]
dir(l)
['__add__', '__class__', '__contains__', '__delattr__',
'__delitem__', '__dir__', '__doc__', '__eq__', '__format__',
'__ge__', '__getattribute__', '__getitem__', '__gt__',
'__hash__', '__iadd__', '__imul__', '__init__', '__iter__',
'__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__reversed__',
'__rmul__', '__setattr__', '__setitem__', '__sizeof__',
'__str__', '__subclasshook__', 'append', 'clear', 'copy',
'count', 'extend', 'index', 'insert', 'pop', 'remove',
'reverse', 'sort']

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III-33 36
getattr()
l = [1,2,3]
dir(l)
..., 'append', 'clear', 'copy', ...]
l.append

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III-34 36
getattr()
l = [1,2,3]
dir(l)
l.append

a = l.append
a

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III-35 36
getattr()
l = [1,2,3]
dir(l)
l.append

a = l.append
a

b = getattr(l, 'append')
b

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III-36 36
getattr()
l = [1,2,3]
dir(l)
l.append

a = l.append
a

b = getattr(l, 'append')
b

a == b
True
a is b
False

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PART 4
Polymorphism

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IV-2 34
References
a = 5
a
5

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IV-3 34
References
a = 5
a
5
type(a)

hex(id(a))
'0x83fe540'

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IV-4 34
References
a = 5
a
5
type(a)

hex(id(a))
'0x83fe540'
a = 'five'
a
'five'

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IV-5 34
References
a = 5
a
5
type(a)

hex(id(a))
'0x83fe540'
a = 'five'
a
'five'
type(a)

hex(id(a))
'0xb70d6560'

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IV-6 34
References
a = 5
a
5
type(a)

hex(id(a))
'0x83fe540'
a = 'five'
a
'five'
type(a)

hex(id(a))
'0xb70d6560'
Strong
Strong type system: every
type system: every
variable has a type
variable has a type

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IV-7 34
References
a = 5
a
5
type(a)

hex(id(a))
'0x83fe540'
a = 'five'
a
'five'
type(a)

hex(id(a))
'0xb70d6560'
Dynamic
Dynamic type system: the type
type system: the type
changes with the content
changes with the content

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IV-8 34
Every variable is a reference
def echo(a):
return a
part4/01.py

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IV-9 34
Every variable is a reference
def echo(a):
return a
echo(5)
5
echo('five')
'five'
part4/01.py

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IV-10 34
5 + 6
11
What is polymorphism?

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IV-11 34
5 + 6
11
5.5 + 6.6
12.1

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IV-12 34
5 + 6
11
5.5 + 6.6
12.1
"just a" + " string"
'just a string'

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IV-13 34
5 + 6
11
5.5 + 6.6
12.1
'just a string'
[1,2,3] + [4,5,6]
[1, 2, 3, 4, 5, 6]
(1,2,3) + (4,5,6)
(1, 2, 3, 4, 5, 6)

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IV-14 34
5 + 6
11
5.5 + 6.6
12.1
'just a string'
[1,2,3] + [4,5,6]
[1, 2, 3, 4, 5, 6]
(1,2,3) + (4,5,6)
(1, 2, 3, 4, 5, 6)
{'a':4, 'b':5} + {'c':7}
File "", line 1, in
TypeError: unsupported operand type(s) for +:
'dict' and 'dict'

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IV-15 34
s = "Just a sentence"
len(s)
15
l = [1, 2, 3]
len(l)
3
d = {'a': 1, 'b': 2}
len(d)
2

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IV-16 34
s = "Just a sentence"
len(s)
15
l = [1, 2, 3]
len(l)
3
d = {'a': 1, 'b': 2}
len(d)
2
i = 5
len(i)
File "", line 1, in
TypeError: object of type 'int' has no len()

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IV-17 34
s.__len__()
15
l.__len__()
3
d.__len__()
2

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IV-18 34
s.__len__()
15
l.__len__()
3
d.__len__()
2
i.__len__()
File "", line 1, in
AttributeError: 'int' object
has no attribute '__len__'

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IV-19 34
[1,2,3].__add__([4,5,6])
[1, 2, 3, 4, 5, 6]
Polymorphism is based on delegation

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IV-20 34
[1,2,3].__add__([4,5,6])
[1, 2, 3, 4, 5, 6]
dir([1,2,3])
['__add__', '__class__', '__contains__', ...]

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IV-21 34
[1,2,3].__add__([4,5,6])
[1, 2, 3, 4, 5, 6]
dir([1,2,3])
1 in [1,2,3]
True
[1,2,3].__contains__(1)
True

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IV-22 34
[1,2,3].__add__([4,5,6])
[1, 2, 3, 4, 5, 6]
dir([1,2,3])
1 in [1,2,3]
True
[1,2,3].__contains__(1)
True
6 in [1,2,3]
False
[1,2,3].__contains__(6)
False

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IV-23 34
def sum(a, b):
return a + b
part4/02.py

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IV-24 34
def sum(a, b):
return a + b
sum(5,6)
11
sum("Being ", "polymorphic")
'Being polymorphic'
sum([1,2,3], [4,5,6])
[1, 2, 3, 4, 5, 6]
part4/02.py

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IV-25 34
def sum(a, b):
return a + b
sum(5,6)
11
sum("Being ", "polymorphic")
'Being polymorphic'
sum([1,2,3], [4,5,6])
[1, 2, 3, 4, 5, 6]
sum([1,2,3], 8)
File "", line 1, in
File "", line 2, in sum
TypeError: can only concatenate list
(not "int") to list
part4/02.py

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IV-26 34
class Room:
def __init__(self, door):
self.door = door
def open(self):
self.door.open()
def close(self):
self.door.close()
def is_open(self):
return self.door.is_open()
Polymorphism in action
part4/03.py

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IV-27 34
class Door:
def __init__(self):
self.status = "closed"
def open(self):
self.status = "open"
def close(self):
self.status = "closed"
def is_open(self):
return self.status == "open"
part4/03.py

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IV-28 34
class BooleanDoor:
def __init__(self):
self.status = True
def open(self):
self.status = True
def close(self):
self.status = False
def is_open(self):
return self.status
part4/03.py

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IV-29 34
door = Door()
bool_door = BooleanDoor()

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IV-30 34
door = Door()
room = Room(door)
bool_room = Room(bool_door)

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IV-31 34
door = Door()
room = Room(door)
room.open()
room.is_open()
True
room.close()
room.is_open()
False

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IV-32 34
door = Door()
room = Room(door)
room.open()
room.is_open()
True
room.close()
room.is_open()
False
bool_room.open()
bool_room.is_open()
True
bool_room.close()
bool_room.is_open()
False

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IV-33 34
“Ask for permission” style
if hasattr(someobj, 'open'):
[...]
else:
[...]
It
It has
has the attribute
the attribute

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IV-34 34
“Ask for permission” style
try:
someobj.open()
[...]
except AttributeError:
[...]
It
It behaves
behaves like it has
like it has
the attribute
the attribute

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PART 5
Metaclasses

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V-2 20
Everything is an object (again)
a = 5
type(a)

a.__class__

a.__class__.__bases__
(,)
object.__bases__
()

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V-3 20
a = 5
type(a)

a.__class__

(,)
object.__bases__
()
'int'
'int' inherits from
inherits from
(its base is) 'object'
(its base is) 'object'
'a'
'a' is an
is an
instance of
instance of
(its type is) 'int'
(its type is) 'int'
object
int
a

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V-4 20
a = 5
type(a)

a.__class__

(,)
object.__bases__
()
Inheritance
Inheritance: 'int' finds
: 'int' finds
here attributes and
here attributes and
methods
methods
Instance
Instance: 'a'
: 'a'
description as a
description as a
class is here
class is here
object
int
a

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V-5 20
type(a)

type(int)

object
int
a

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V-6 20
type(a)

type(int)

type(float)

type(dict)

object
int
a

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V-7 20
type(a)

type(int)

type(float)

type(dict)

object
a
type
int int

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V-8 20
type(a)

type(int)

type(float)

type(dict)

object
a
type
int int
'int'
'int' inherits
inherits
from
from
(its base is)
(its base is)
'object'
'object'
'int'
'int' is an instance of
is an instance of
(its type is) 'type'
(its type is) 'type'

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V-9 20
type(a)

type(int)

type(float)

type(dict)

object
a
type
int int
'int' finds here
'int' finds here
attributes and
attributes and
methods
methods
'int' description as a
'int' description as a
class is here
class is here

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V-10 20
Food for thought
type(object)

type.__bases__
(,)

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V-11 20
Food for thought
type(object)

type.__bases__
(,)
object
type

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V-12 20
type(object)

type.__bases__
(,)
type(type)

object
type
Fatality

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Python
2.x
V-13 20
type(object)

type.__bases__
(,)
type(type)

object
type
Fatality

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V-14 20
Metaclasses
class MyType(type):
pass
class MySpecialClass(metaclass=MyType):
pass
part5/01.py

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Python
2.x
V-15 20
Metaclasses
class MyType(type):
pass
class MySpecialClass(object):
__metaclass__ = MyType

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V-16 20
Metaclasses
class MyType(type):
pass
pass
msp = MySpecialClass()
type(msp)

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V-17 20
Metaclasses
class MyType(type):
pass
pass
msp = MySpecialClass()
type(msp)

type(MySpecialClass)

MySpecialClass.__bases__
(,)

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V-18 20
Metaclasses in action: singleton
part5/02.py
class Singleton(type):
instance = None
def __call__(cls, *args, **kw):
if not cls.instance:
cls.instance = super(Singleton, cls).
__call__(*args, **kw)
return cls.instance
class ASingleton(metaclass=Singleton):
pass

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V-19 20
part5/02.py
instance = None
return cls.instance
pass
a = ASingleton()
b = ASingleton()

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V-20 20
part5/02.py
instance = None
return cls.instance
pass
a = ASingleton()
b = ASingleton()
a is b
True
hex(id(a))
'0xb68030ec'
hex(id(b))
'0xb68030ec'

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A lot of people
provided free information and code
Font aswesome icons
by Freepik
Python 3 OOP series
http://lgiordani.com/blog/categories/python3/
Some links about Python OOP
http://goo.gl/UBdJDT
CAST

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OBJECT-ORIENTED PYTHON
From scratch
Leonardo Giordani
lgiordani.com
OOP

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Object-Oriented Python From scratch

Object-Oriented Python From scratch

Leonardo Giordani

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