object-oriented python from scratch

OBJECT-ORIENTED PYTHONFrom scratch

Leonardo Giordanilgiordani.comO

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.comhttps://twitter.com/

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lgiordanihttps://plus.google.com/u/

LeonardoGiordani

import usersimport talk

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

about_you.py

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

https://github.com/lgiordani/oopy

Code available as IPython Notebooks

PART 1

Objects and types

Object-oriented Python from scratch – lgiordani.com - CC BY-SA 4.0I-2 27

data = (13, 63, 5, 378, 58, 40)

def avg(d): return sum(d)/len(d)

avg(data)92.83333333333333

Plain old procedurespart1/01.py

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 numbersa sequence of numbers

This returns new dataThis returns new data

part1/01.py

Source data enters hereSource data enters here

Plain old procedures

door1 = [1, 'closed']door2 = [2, 'closed']

def open_door(door): door[1] = 'open'

open_door(door1)door1[1, 'open']

part1/02.py Procedures can modify data

ldoor1 = [1, 'closed', 'unlocked']

def open_ldoor(door): if door[2] == 'unlocked': door[1] = 'open'

part1/03.py Things can get complicated

Same “action”Same “action”

You must useYou must usethe right functionthe right function

ldoor1 = [1, 'closed', 'unlocked']

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

BehavesBehaveslike a ducklike a duck

Behavioural meaning

The meaning of the word 'type'

DissectionDissectionrevealsreveals

the the truthtruth

Structural meaning

The meaning of the word 'type'

The behavioural meaning is important

Duck typing: make it behave like a duck

Duck typing joke #1

Duck typing joke #2

The meaning of the word 'class'

The meaning of the word 'instance'

a = 6a6

type(a)<class 'int'>

You already used classes

Python

You already used classes

a = 6a6

type(a)<type 'int'>

The first classpart1/04.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'

Python

The first class

class Door(object): def __init__(self, number, status): self.number = number self.status = status

The 'class' keyword defines the classThe 'class' keyword defines the class

Everything under the 'class' Everything under the 'class' keyword is part of the classkeyword is part of the class

MethodMethod

AttributesAttributes

Constructor (part of)Constructor (part of)

door1 = Door(1, 'closed')

The first class

door1 = Door(1, 'closed')type(door1)<class '__main__.Door'>

The first class

door1 = Door(1, 'closed')type(door1)<class '__main__.Door'>door1.number1door1.status'closed'

The first class

door1 = Door(1, 'closed')type(door1)<class '__main__.Door'>door1.number1door1.status'closed'door1.open()

The first class

door1 = Door(1, 'closed')type(door1)<class '__main__.Door'>door1.number1door1.status'closed'door1.open()door1.number1door1.status'open'

The first class

PART 2

Classes and members

Object-oriented Python from scratch – lgiordani.com - CC BY-SA 4.0II-2 42

Everything is an object

a = 1type(a)<class 'int'>

Everything is an object

a = 1type(a)<class 'int'>type(int)<class 'type'> The type of an object is The type of an object is

an object itselfan object itself

Where is the class of an object?

door1 = Door(1, 'closed')door2 = Door(1, 'closed')

door1 = Door(1, 'closed')door2 = Door(1, 'closed')hex(id(door1))'0xb67e148c'hex(id(door2))'0xb67e144c'

door1 = Door(1, 'closed')door2 = Door(1, 'closed')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.

Class attributespart2/01.py

class Door: colour = 'brown'

def __init__(self, number, status): self.number = number self.status = status

part2/01.py

No 'self' hereNo 'self' here

Class attributes

door1 = Door(1, 'closed')door2 = Door(2, 'closed')Door.colour'brown'door1.colour'brown'door2.colour'brown'

Class attributes

Door.colour = 'white'

Class attributes

Door.colour = 'white'Door.colour'white'door1.colour'white'door2.colour'white'

Class attributes

Door.colour = 'white'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

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})

Class attributes

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}

Class attributes

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.__dict__['colour']Traceback (most recent call last): File "<stdin>", line 1, in <module>KeyError: 'colour'

Class attributes

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.__dict__['colour']Traceback (most recent call last): File "<stdin>", line 1, in <module>KeyError: 'colour'door1.__class__.__dict__['colour']'white'door1.colour is Door.colourTrue

Class attributes

door1 door2

__getattribute__()__getattribute__()

Let's dive into attribute resolution

door1.colour

Door.colour

door1 = Door(1, 'closed')door1.colour = 'white'door1.__dict__['colour']'white'door1.__class__.__dict__['colour']'brown'

door1 = Door(1, 'closed')door1.colour = 'white'door1.__dict__['colour']'white'door1.__class__.__dict__['colour']'brown'door1.colour'white'Door.colour'brown'

door1 = Door(1, 'closed')door1.colour = 'white'door1.__dict__['colour']'white'door1.__class__.__dict__['colour']'brown'door1.colour'white'Door.colour'brown'Door.colour = 'red'

door1 = Door(1, 'closed')door1.colour = 'white'door1.__dict__['colour']'white'door1.__class__.__dict__['colour']'brown'door1.colour'white'Door.colour'brown'Door.colour = 'red'door1.__dict__['colour']'white'door1.__class__.__dict__['colour']'red'

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.colour is Door.colourTrue

What about methods?

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.colour is Door.colourTruedoor1.open is Door.openFalse

What about methods?

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.colour is Door.colourTruedoor1.open is Door.openFalseDoor.__dict__['open']<function Door.open at 0xb68604ac>Door.open<function Door.open at 0xb68604ac>

What about methods?

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.colour is Door.colourTruedoor1.open is Door.openFalseDoor.__dict__['open']<function Door.open at 0xb68604ac>Door.open<function Door.open at 0xb68604ac>door1.open<bound method Door.open of <__main__.Door object at 0xb67e162c>>

What about methods?

Python

Door.__dict__mappingproxy({'open': <function Door.open at 0xb68604ac>, 'colour': 'white', '__dict__': <attribute '__dict__' of 'Door' objects>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__init__': <function Door.__init__ at 0xb7062854>, '__module__': '__main__', '__doc__': None, 'close': <function Door.close at 0xb686041c>})door1.__dict__{'number': 1, 'status': 'closed'}door1.colour is Door.colourTruedoor1.open is Door.openFalseDoor.__dict__['open']<function Door.open at 0xb68604ac>Door.open<unbound method Door.open>door1.open<bound method Door.open of <__main__.Door object at 0xb67e162c>>

What about methods?

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'

From functions to bound methods

Python

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: unbound method open() must be called with Door instanceas first argument (got nothing instead)

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'Door.open(door1)door1.status'open'

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'Door.open(door1)door1.status'open'door1.__class__.__dict__['open']<function Door.open at 0xb68604ac>

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'Door.open(door1)door1.status'open'door1.__class__.__dict__['open']<function Door.open at 0xb68604ac>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__']

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'Door.open(door1)door1.status'open'door1.__class__.__dict__['open']<function Door.open at 0xb68604ac>dir(door1.__class__.__dict__['open'])['__annotations__', '__call__', '__class__', '__closure__', ...,'__get__',...]door1.__class__.__dict__['open'].__get__<method-wrapper '__get__' of function object at 0xb68604ac>

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'Door.open(door1)door1.status'open'door1.__class__.__dict__['open']<function Door.open at 0xb68604ac>dir(door1.__class__.__dict__['open'])['__annotations__', '__call__', '__class__', '__closure__', ...,'__get__',...]door1.__class__.__dict__['open'].__get__<method-wrapper '__get__' of function object at 0xb68604ac>door1.__class__.__dict__['open'].__get__(door1)<bound method Door.open of <__main__.Door object at 0xb67e162c>>

Python

Door.open()Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: open() missing 1 required positional argument: 'self'Door.open(door1)door1.status'open'door1.__class__.__dict__['open']<function Door.open at 0xb68604ac>dir(door1.__class__.__dict__['open'])['__annotations__', '__call__', '__class__', '__closure__', ...,'__get__',...]door1.__class__.__dict__['open'].__get__<method-wrapper '__get__' of function object at 0xb68604ac>door1.__class__.__dict__['open'].__get__(door1)<bound method ?.open of <__main__.Door instance at 0xb6977aac>>door1.__class__.__dict__['open'].__get__(door1, Door)<bound method Door.open of <__main__.Door object at 0xb73f956c>>

part2/02.py

@classmethod def knock(cls): print("Knock!")

Class methods

door1 = Door(1, 'closed')door1.knock()Knock!Door.knock()Knock!

part2/03.py

@classmethod def knock(cls): print("Knock!")

@classmethod def paint(cls, colour): cls.colour = colour

Class methods

door1 = Door(1, 'closed')door2 = Door(2, 'closed')Door.colour'brown'door1.colour'brown'door2.colour'brown'

Class methods

Door.paint('white')Door.colour'white'door1.colour'white'door2.colour'white'

Class methods

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

PART 3

Delegation

Object-oriented Python from scratch – lgiordani.com - CC BY-SA 4.0III-2 36

Specialization

CatAnimal

Cat has all the features of Cat has all the features of Animal, i.e. 'moves'Animal, i.e. 'moves'

Specialization

CatAnimal

Cat can provide new features, i.e. Cat can provide new features, i.e. 'has whiskers''has whiskers'

Specialization

CatAnimal

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

CatAnimal

Cat implements only 'new' or Cat implements only 'new' or 'changed' features'changed' features

Cat delegates the remaining Cat delegates the remaining features to Animalfeatures to Animal

Delegation

CatAnimal

Composition: 'has'

Engine turn_on()

Wheels steer()

get_color()

Inheritance: 'is'

look()look()

Animal

look()

Inheritancepart3/01.py

class SecurityDoor(Door): pass

Inheritance

sdoor = SecurityDoor(1, 'closed')

Inheritance

sdoor = SecurityDoor(1, 'closed')SecurityDoor.colour is Door.colourTruesdoor.colour is Door.colourTrue

Inheritance

sdoor = SecurityDoor(1, 'closed')SecurityDoor.colour is Door.colourTruesdoor.colour is Door.colourTrue

sdoor.coloursdoor.colour

SecurityDoor.colourSecurityDoor.colour

Door.colourDoor.colour

Inheritancesdoor.__dict__{'number': 1, 'status': 'closed'}

Inheritancesdoor.__dict__{'number': 1, 'status': 'closed'}sdoor.__class__.__dict__mappingproxy({'__doc__': None, '__module__': '__main__'})Door.__dict__mappingproxy({'__dict__': <attribute '__dict__' of 'Door' objects>, 'colour': 'yellow', 'open': <function Door.open at 0xb687e224>, '__init__': <function Door.__init__ at 0xb687e14c>, '__doc__': None, 'close': <function Door.close at 0xb687e1dc>, 'knock': <classmethod object at 0xb67ff6ac>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__module__': '__main__', 'paint': <classmethod object at 0xb67ff6ec>})

Inheritancesdoor.__dict__{'number': 1, 'status': 'closed'}sdoor.__class__.__dict__mappingproxy({'__doc__': None, '__module__': '__main__'})Door.__dict__mappingproxy({'__dict__': <attribute '__dict__' of 'Door' objects>, 'colour': 'yellow', 'open': <function Door.open at 0xb687e224>, '__init__': <function Door.__init__ at 0xb687e14c>, '__doc__': None, 'close': <function Door.close at 0xb687e1dc>, 'knock': <classmethod object at 0xb67ff6ac>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__module__': '__main__', 'paint': <classmethod object at 0xb67ff6ec>})SecurityDoor.__bases__(<class '__main__.Door'>,)

Inheritancesdoor.__dict__{'number': 1, 'status': 'closed'}sdoor.__class__.__dict__mappingproxy({'__doc__': None, '__module__': '__main__'})Door.__dict__mappingproxy({'__dict__': <attribute '__dict__' of 'Door' objects>, 'colour': 'yellow', 'open': <function Door.open at 0xb687e224>, '__init__': <function Door.__init__ at 0xb687e14c>, '__doc__': None, 'close': <function Door.close at 0xb687e1dc>, 'knock': <classmethod object at 0xb67ff6ac>, '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__module__': '__main__', 'paint': <classmethod object at 0xb67ff6ec>})SecurityDoor.__bases__(<class '__main__.Door'>,)sdoor.knock<bound method type.knock of <class '__main__.SecurityDoor'>>sdoor.__class__.__bases__[0].__dict__['knock'].__get__(sdoor)<bound method type.knock of <class '__main__.SecurityDoor'>>

Overridingpart3/02.py

class SecurityDoor(Door): colour = 'grey' locked = True

def open(self): if not self.locked: self.status = 'open'

Overriding blocks implicit Overriding blocks implicit delegationdelegation

def open(self): if not self.locked: self.status = 'open'

SecurityDoor.__dict__mappingproxy({'__doc__': None, '__module__': '__main__', 'open': <function SecurityDoor.open at 0xb6fcf89c>, 'colour': 'grey', 'locked': True})

def open(self): if self.locked: return Door.open(self)

sdoor = SecurityDoor(1, 'closed')sdoor.status'closed'

sdoor = SecurityDoor(1, 'closed')sdoor.status'closed'sdoor.open()sdoor.status'closed'

sdoor = SecurityDoor(1, 'closed')sdoor.status'closed'sdoor.open()sdoor.status'closed'sdoor.locked = Falsesdoor.open()sdoor.status'open'

Avoid strong couplingpart3/04.py

def open(self): if self.locked: return super().open()

Python

def open(self): if self.locked: return super(SecurityDoor, self).open(self)

Avoid strong coupling

class SecurityDoor: colour = 'grey' locked = True

def __init__(self, number, status): self.door = Door(number, status)

def open(self): if self.locked: return self.door.open()

def close(self): self.door.close()

Compositionpart3/05.py

def close(self): Self.door.close()

sdoor = SecurityDoor(1, 'closed')sdoor.statusTraceback (most recent call last): File "<stdin>", line 1, in <module>AttributeError: 'SecurityDorr' object has no attribute 'status'

Compositionpart3/05.py

def get_status(self): return self.door.status status = property(get_status)

Composition

Python magic to the rescue

def __getattr__(self, attr): return getattr(self.door, attr)

part3/06.py

Python magic to the rescue

#def close(self): # self.door.close()

part3/07.py

class ComposedDoor: def __init__(self, number, status): self.door = Door(number, status)

Composed inheritance?part3/08.py

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']

getattr()

l = [1,2,3]dir(l)['__add__', '__class__', '__contains__', '__delattr__', ..., 'append', 'clear', 'copy', ...]l.append<built-in method append of list object at 0xb70a2c2c>

getattr()

l = [1,2,3]dir(l)['__add__', '__class__', '__contains__', '__delattr__', ..., 'append', 'clear', 'copy', ...]l.append<built-in method append of list object at 0xb70a2c2c>a = l.appenda<built-in method append of list object at 0xb70a2c2c>

getattr()

l = [1,2,3]dir(l)['__add__', '__class__', '__contains__', '__delattr__', ..., 'append', 'clear', 'copy', ...]l.append<built-in method append of list object at 0xb70a2c2c>a = l.appenda<built-in method append of list object at 0xb70a2c2c>b = getattr(l, 'append')b<built-in method append of list object at 0xb70a2c2c>

getattr()

l = [1,2,3]dir(l)['__add__', '__class__', '__contains__', '__delattr__', ..., 'append', 'clear', 'copy', ...]l.append<built-in method append of list object at 0xb70a2c2c>a = l.appenda<built-in method append of list object at 0xb70a2c2c>b = getattr(l, 'append')b<built-in method append of list object at 0xb70a2c2c>a == bTruea is bFalse

PART 4

Polymorphism

Object-oriented Python from scratch – lgiordani.com - CC BY-SA 4.0IV-2 34

References

a = 5a5

References

a = 5a5type(a)<class 'int'>hex(id(a))'0x83fe540'

References

a = 5a5type(a)<class 'int'>hex(id(a))'0x83fe540'a = 'five'a'five'

References

a = 5a5type(a)<class 'int'>hex(id(a))'0x83fe540'a = 'five'a'five'type(a)<class 'str'>hex(id(a))'0xb70d6560'

References

StrongStrong type system: every type system: every variable has a typevariable has a type

References

DynamicDynamic type system: the type type system: the type changes with the contentchanges with the content

Every variable is a reference

def echo(a): return a

part4/01.py

Every variable is a reference

def echo(a): return a

echo(5)5echo('five')'five'

part4/01.py

5 + 611

What is polymorphism?

5 + 6115.5 + 6.612.1

5 + 6115.5 + 6.612.1"just a" + " string"'just a string'

5 + 6115.5 + 6.612.1"just a" + " string"'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)

5 + 6115.5 + 6.612.1"just a" + " string"'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}Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: unsupported operand type(s) for +: 'dict' and 'dict'

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

s = "Just a sentence"len(s)15l = [1, 2, 3]len(l)3d = {'a': 1, 'b': 2}len(d)2i = 5len(i)Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: object of type 'int' has no len()

s.__len__()15l.__len__()3d.__len__()2

s.__len__()15l.__len__()3d.__len__()2i.__len__()Traceback (most recent call last): File "<stdin>", line 1, in <module>AttributeError: 'int' objecthas no attribute '__len__'

[1,2,3].__add__([4,5,6])[1, 2, 3, 4, 5, 6]

Polymorphism is based on delegation

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

[1,2,3].__add__([4,5,6])[1, 2, 3, 4, 5, 6]dir([1,2,3])['__add__', '__class__', '__contains__', ...]1 in [1,2,3]True[1,2,3].__contains__(1)True

[1,2,3].__add__([4,5,6])[1, 2, 3, 4, 5, 6]dir([1,2,3])['__add__', '__class__', '__contains__', ...]1 in [1,2,3]True[1,2,3].__contains__(1)True6 in [1,2,3]False[1,2,3].__contains__(6)False

def sum(a, b): return a + b

Polymorphism is based on delegationpart4/02.py

sum(5,6)11sum("Being ", "polymorphic")'Being polymorphic'sum([1,2,3], [4,5,6])[1, 2, 3, 4, 5, 6]

sum(5,6)11sum("Being ", "polymorphic")'Being polymorphic'sum([1,2,3], [4,5,6])[1, 2, 3, 4, 5, 6]sum([1,2,3], 8)Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in sumTypeError: can only concatenate list (not "int") to list

class Room: def __init__(self, door): self.door = door

def open(self): self.door.open()

def is_open(self): return self.door.is_open()

Polymorphism in actionpart4/03.py

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"

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

door = Door()bool_door = BooleanDoor()

Polymorphism in action

door = Door()bool_door = BooleanDoor()room = Room(door)bool_room = Room(bool_door)

door = Door()bool_door = BooleanDoor()room = Room(door)bool_room = Room(bool_door)room.open()room.is_open()Trueroom.close()room.is_open()False

door = Door()bool_door = BooleanDoor()room = Room(door)bool_room = Room(bool_door)room.open()room.is_open()Trueroom.close()room.is_open()Falsebool_room.open()bool_room.is_open()Truebool_room.close()bool_room.is_open()False

“Ask for permission” style

if hasattr(someobj, 'open'): [...]else: [...]

It It hashas the attribute the attribute

“Ask for permission” style

try: someobj.open() [...]except AttributeError: [...]

It It behavesbehaves like it has like it has the attributethe attribute

PART 5

Metaclasses

Object-oriented Python from scratch – lgiordani.com - CC BY-SA 4.0V-2 20

Everything is an object (again)

a = 5type(a)<class 'int'>a.__class__<class 'int'>a.__class__.__bases__(<class 'object'>,)object.__bases__()

'int' 'int' inherits frominherits from(its base is) 'object'(its base is) 'object'

'a' 'a' is an is an instance ofinstance of

(its type is) 'int' (its type is) 'int'

object

InheritanceInheritance: 'int' finds : 'int' finds here attributes and here attributes and

methodsmethods

InstanceInstance: 'a' : 'a' description as a description as a class is hereclass is here

object

type(a)<class 'int'>type(int)<class 'type'>

object

type(a)<class 'int'>type(int)<class 'type'>type(float)<class 'type'>type(dict)<class 'type'>

object

int int

object

int int'int' 'int' inherits inherits

fromfrom(its base is) (its base is)

'object''object'

'int' 'int' is an instance ofis an instance of(its type is) 'type' (its type is) 'type'

object

int int

'int' finds here 'int' finds here attributes and attributes and

methodsmethods

'int' description as a 'int' description as a class is hereclass is here

Food for thought

type(object)<class 'type'>type.__bases__(<class 'object'>,)

Food for thought

type(object)<class 'type'>type.__bases__(<class 'object'>,)

object

type(object)<class 'type'>type.__bases__(<class 'object'>,)type(type)<class 'type'>

object

Fatality

Python

type(object)<class 'type'>type.__bases__(<class 'object'>,)type(type)<type 'type'>

object

Fatality

Metaclasses

class MyType(type): pass

class MySpecialClass(metaclass=MyType): pass

part5/01.py

Python

Metaclasses

class MySpecialClass(object):__metaclass__ = MyType

Metaclasses

msp = MySpecialClass()type(msp)<class '__main__.MySpecialClass'>

Metaclasses

msp = MySpecialClass()type(msp)<class '__main__.MySpecialClass'>type(MySpecialClass)<class '__main__.MyType'>MySpecialClass.__bases__(<class 'object'>,)

Metaclasses in action: singletonpart5/02.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

a = ASingleton()b = ASingleton()

a = ASingleton()b = ASingleton()a is bTruehex(id(a))'0xb68030ec'hex(id(b))'0xb68030ec'

A lot of peopleprovided free information and code

Font aswesome iconsby Freepik

Python 3 OOP serieshttp://lgiordani.com/blog/categories/python3/

Some links about Python OOPhttp://goo.gl/UBdJDT

OBJECT-ORIENTED PYTHONFrom scratch

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