Object-Oriented Python From scratch

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

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

import users import 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 3

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 4.0
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

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

I-4 27 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

I-5 27 door1 = [1, 'closed'] door2 = [2, 'closed'] ldoor1 = [1, 'closed', 'unlocked'] def open_door(door): door[1] = 'open' def open_ldoor(door): if door[2] == 'unlocked': door[1] = 'open' part1/03.py Things can get complicated

I-6 27 Same “action” Same “action” You must use You must use the right function the right function door1 = [1, 'closed'] door2 = [2, 'closed'] ldoor1 = [1, 'closed', 'unlocked'] def open_door(door): 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

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

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

I-9 27 The behavioural meaning is important

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

I-11 27 Duck typing joke #1

I-12 27 Duck typing joke #2

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

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

I-15 27 a = 6 a 6 type(a) <class 'int'> You already used classes

Python 2.x Object-oriented Python from scratch – lgiordani.com - CC
BY-SA 4.0 I-16 27 You already used classes a = 6 a 6 type(a) <type 'int'>

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'

BY-SA 4.0 I-18 27 The first class class Door(object): def __init__(self, number, status): self.number = number self.status = status def open(self): self.status = 'open' def close(self): self.status = 'closed'

I-19 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' The 'class' keyword defines the class The 'class' keyword defines the class

I-20 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' Everything under the 'class' Everything under the 'class' keyword is part of the class keyword is part of the class

I-21 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' Method Method

I-22 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' Attributes Attributes Constructor (part of) Constructor (part of)

I-23 27 door1 = Door(1, 'closed') The first class 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'

I-24 27 door1 = Door(1, 'closed') type(door1) <class '__main__.Door'> The first class 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'

I-25 27 door1 = Door(1, 'closed') type(door1) <class '__main__.Door'> door1.number 1 door1.status 'closed' The first class 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'

I-26 27 door1 = Door(1, 'closed') type(door1) <class '__main__.Door'> door1.number 1 door1.status 'closed' door1.open() The first class 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'

I-27 27 door1 = Door(1, 'closed') type(door1) <class '__main__.Door'> door1.number 1 door1.status 'closed' door1.open() door1.number 1 door1.status 'open' The first class 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'

PART 2 Classes and members

II-2 42 Everything is an object a = 1 type(a) <class 'int'>

II-3 42 Everything is an object a = 1 type(a) <class 'int'> type(int) <class 'type'> The type of an object is The type of an object is an object itself an object itself

II-4 42 Where is the class of an object? door1 = Door(1, 'closed') door2 = Door(1, 'closed')

II-5 42 Where is the class of an object? door1 = Door(1, 'closed') door2 = Door(1, 'closed') hex(id(door1)) '0xb67e148c' hex(id(door2)) '0xb67e144c'

II-6 42 Where is the class of an object? 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.

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

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

II-9 42 door1 = Door(1, 'closed') door2 = Door(2, 'closed') Class attributes

II-10 42 door1 = Door(1, 'closed') door2 = Door(2, 'closed') Door.colour 'brown' door1.colour 'brown' door2.colour 'brown' Class attributes

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

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

II-13 42 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

II-14 42 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

II-15 42 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

II-16 42 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

II-17 42 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.colour True Class attributes

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

II-19 42 Let's dive into attribute resolution door1 = Door(1, 'closed') door1.colour = 'white' door1.__dict__['colour'] 'white' door1.__class__.__dict__['colour'] 'brown'

II-20 42 Let's dive into attribute resolution door1 = Door(1, 'closed') door1.colour = 'white' door1.__dict__['colour'] 'white' door1.__class__.__dict__['colour'] 'brown' door1.colour 'white' Door.colour 'brown'

II-21 42 Let's dive into attribute resolution 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'

II-22 42 Let's dive into attribute resolution 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'

II-23 42 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.colour True What about methods?

II-24 42 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.colour True door1.open is Door.open False What about methods?

II-25 42 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.colour True door1.open is Door.open False Door.__dict__['open'] <function Door.open at 0xb68604ac> Door.open <function Door.open at 0xb68604ac> What about methods?

II-26 42 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.colour True door1.open is Door.open False Door.__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?

BY-SA 4.0 II-27 42 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.colour True door1.open is Door.open False Door.__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?

II-28 42 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

BY-SA 4.0 II-29 42 Door.open() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unbound method open() must be called with Door instance as first argument (got nothing instead) From functions to bound methods

II-30 42 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' From functions to bound methods

II-31 42 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> From functions to bound methods

II-32 42 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__'] From functions to bound methods

II-33 42 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> From functions to bound methods

II-34 42 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>> From functions to bound methods

BY-SA 4.0 II-35 42 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>> From functions to bound methods

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

II-37 42 door1 = Door(1, 'closed') door1.knock() Knock! Door.knock() Knock! From functions to bound methods

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

II-39 42 door1 = Door(1, 'closed') door2 = Door(2, 'closed') Class methods

II-40 42 door1 = Door(1, 'closed') door2 = Door(2, 'closed') Door.colour 'brown' door1.colour 'brown' door2.colour 'brown' Class methods

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

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

PART 3 Delegation

III-2 36 Specialization Cat Animal

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

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

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

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

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

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

III-9 36

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

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

III-12 36 Inheritance class SecurityDoor(Door): pass sdoor = SecurityDoor(1, 'closed') SecurityDoor.colour is Door.colour True sdoor.colour is Door.colour True

III-13 36 Inheritance class SecurityDoor(Door): pass sdoor = SecurityDoor(1, 'closed') SecurityDoor.colour is Door.colour True sdoor.colour is Door.colour True sdoor.colour sdoor.colour SecurityDoor.colour SecurityDoor.colour Door.colour Door.colour

III-14 36 Inheritance sdoor.__dict__ {'number': 1, 'status': 'closed'}

III-15 36 Inheritance sdoor.__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>})

III-16 36 Inheritance sdoor.__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'>,)

III-17 36 Inheritance sdoor.__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'>>

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

III-19 36 Overriding part3/02.py Overriding blocks implicit Overriding blocks implicit delegation delegation class SecurityDoor(Door): colour = 'grey' locked = True 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})

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

III-21 36 Overriding part3/03.py class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return Door.open(self) sdoor = SecurityDoor(1, 'closed') sdoor.status 'closed'

III-22 36 Overriding part3/03.py class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return Door.open(self) sdoor = SecurityDoor(1, 'closed') sdoor.status 'closed' sdoor.open() sdoor.status 'closed'

III-23 36 Overriding part3/03.py class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return Door.open(self) sdoor = SecurityDoor(1, 'closed') sdoor.status 'closed' sdoor.open() sdoor.status 'closed' sdoor.locked = False sdoor.open() sdoor.status 'open'

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

BY-SA 4.0 III-25 36 class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return super(SecurityDoor, self).open(self) Avoid strong coupling

III-26 36 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() Composition part3/05.py

III-27 36 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() sdoor = SecurityDoor(1, 'closed') sdoor.status Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'SecurityDorr' object has no attribute 'status' Composition part3/05.py

III-28 36 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() def get_status(self): return self.door.status status = property(get_status) sdoor = SecurityDoor(1, 'closed') sdoor.status 'closed' Composition

III-29 36 Python magic to the rescue 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() def __getattr__(self, attr): return getattr(self.door, attr) sdoor = SecurityDoor(1, 'closed') sdoor.status 'closed' part3/06.py

III-30 36 Python magic to the rescue 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() def __getattr__(self, attr): return getattr(self.door, attr) part3/07.py

III-31 36 class ComposedDoor: def __init__(self, number, status): self.door = Door(number, status) def __getattr__(self, attr): return getattr(self.door, attr) Composed inheritance? part3/08.py

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']

III-33 36 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>

III-34 36 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.append a <built-in method append of list object at 0xb70a2c2c>

III-35 36 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.append a <built-in method append of list object at 0xb70a2c2c> b = getattr(l, 'append') b <built-in method append of list object at 0xb70a2c2c>

III-36 36 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.append a <built-in method append of list object at 0xb70a2c2c> b = getattr(l, 'append') b <built-in method append of list object at 0xb70a2c2c> a == b True a is b False

PART 4 Polymorphism

IV-2 34 References a = 5 a 5

IV-3 34 References a = 5 a 5 type(a) <class 'int'> hex(id(a)) '0x83fe540'

IV-4 34 References a = 5 a 5 type(a) <class 'int'> hex(id(a)) '0x83fe540' a = 'five' a 'five'

IV-5 34 References a = 5 a 5 type(a) <class 'int'> hex(id(a)) '0x83fe540' a = 'five' a 'five' type(a) <class 'str'> hex(id(a)) '0xb70d6560'

IV-6 34 References a = 5 a 5 type(a) <class 'int'> hex(id(a)) '0x83fe540' a = 'five' a 'five' type(a) <class 'str'> hex(id(a)) '0xb70d6560' Strong Strong type system: every type system: every variable has a type variable has a type

IV-7 34 References a = 5 a 5 type(a) <class 'int'> hex(id(a)) '0x83fe540' a = 'five' a 'five' type(a) <class 'str'> hex(id(a)) '0xb70d6560' Dynamic Dynamic type system: the type type system: the type changes with the content changes with the content

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

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

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

IV-11 34 5 + 6 11 5.5 + 6.6 12.1 What is polymorphism?

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

IV-13 34 5 + 6 11 5.5 + 6.6 12.1 "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) What is polymorphism?

IV-14 34 5 + 6 11 5.5 + 6.6 12.1 "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' What is polymorphism?

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 What is polymorphism?

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) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: object of type 'int' has no len() What is polymorphism?

IV-17 34 s.__len__() 15 l.__len__() 3 d.__len__() 2 What is polymorphism?

IV-18 34 s.__len__() 15 l.__len__() 3 d.__len__() 2 i.__len__() Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'int' object has no attribute '__len__' What is polymorphism?

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

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

IV-21 34 [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 Polymorphism is based on delegation

IV-22 34 [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 6 in [1,2,3] False [1,2,3].__contains__(6) False Polymorphism is based on delegation

IV-23 34 def sum(a, b): return a + b Polymorphism is based on delegation part4/02.py

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] Polymorphism is based on delegation part4/02.py

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) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in sum TypeError: can only concatenate list (not "int") to list Polymorphism is based on delegation part4/02.py

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

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" Polymorphism in action part4/03.py

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 Polymorphism in action part4/03.py

IV-29 34 door = Door() bool_door = BooleanDoor() Polymorphism in action

IV-30 34 door = Door() bool_door = BooleanDoor() room = Room(door) bool_room = Room(bool_door) Polymorphism in action

IV-31 34 door = Door() bool_door = BooleanDoor() room = Room(door) bool_room = Room(bool_door) room.open() room.is_open() True room.close() room.is_open() False Polymorphism in action

IV-32 34 door = Door() bool_door = BooleanDoor() room = Room(door) bool_room = Room(bool_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 Polymorphism in action

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

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

PART 5 Metaclasses

V-2 20 Everything is an object (again) a = 5 type(a) <class 'int'> a.__class__ <class 'int'> a.__class__.__bases__ (<class 'object'>,) object.__bases__ ()

V-3 20 Everything is an object (again) a = 5 type(a) <class 'int'> a.__class__ <class 'int'> a.__class__.__bases__ (<class 'object'>,) 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

V-4 20 Everything is an object (again) a = 5 type(a) <class 'int'> a.__class__ <class 'int'> a.__class__.__bases__ (<class 'object'>,) 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

V-5 20 Everything is an object (again) type(a) <class 'int'> type(int) <class 'type'> object int a

V-6 20 Everything is an object (again) type(a) <class 'int'> type(int) <class 'type'> type(float) <class 'type'> type(dict) <class 'type'> object int a

V-7 20 Everything is an object (again) type(a) <class 'int'> type(int) <class 'type'> type(float) <class 'type'> type(dict) <class 'type'> object a type int int

V-8 20 Everything is an object (again) type(a) <class 'int'> type(int) <class 'type'> type(float) <class 'type'> type(dict) <class 'type'> 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'

V-9 20 Everything is an object (again) type(a) <class 'int'> type(int) <class 'type'> type(float) <class 'type'> type(dict) <class 'type'> 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

V-10 20 Food for thought type(object) <class 'type'> type.__bases__ (<class 'object'>,)

V-11 20 Food for thought type(object) <class 'type'> type.__bases__ (<class 'object'>,) object type

V-12 20 type(object) <class 'type'> type.__bases__ (<class 'object'>,) type(type) <class 'type'> object type Fatality

BY-SA 4.0 V-13 20 type(object) <class 'type'> type.__bases__ (<class 'object'>,) type(type) <type 'type'> object type Fatality

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

BY-SA 4.0 V-15 20 Metaclasses class MyType(type): pass class MySpecialClass(object): __metaclass__ = MyType

V-16 20 Metaclasses class MyType(type): pass class MySpecialClass(metaclass=MyType): pass msp = MySpecialClass() type(msp) <class '__main__.MySpecialClass'>

V-17 20 Metaclasses class MyType(type): pass class MySpecialClass(metaclass=MyType): pass msp = MySpecialClass() type(msp) <class '__main__.MySpecialClass'> type(MySpecialClass) <class '__main__.MyType'> MySpecialClass.__bases__ (<class 'object'>,)

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

V-19 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 a = ASingleton() b = ASingleton()

V-20 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 a = ASingleton() b = ASingleton() a is b True hex(id(a)) '0xb68030ec' hex(id(b)) '0xb68030ec'

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

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