Deep Inside Object-Oriented Python - PyCon Ireland 2014

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DEEP INSIDE OBJECT-ORIENTED PYTHON Leonardo Giordani lgiordani.com PyCon Ireland 2014

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ABOUT ME 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. https://github.com/ lgiordani https://twitter.com/ tw_lgiordani https://plus.google.com/u/ LeonardoGiordani The Digital Cat http://lgiordani.com

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ABOUT YOU To enjoy this tutorial and avoid falling asleep you should fit at least one of the following descriptions: * A Python beginner without any knowledge of OOP languages. * A Python beginner with knowledge of other OOP languages such as C++, Java, Ruby, etc. * A Python advanced user who has some knowledge about Python OOP but now wants to know what happens “behind the scenes”. Python experts: you shouldn't be here. You are going to be bored. No, seriously.

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ABOUT THIS TUTORIAL A beginners' introduction to the Python implementation of OOP concepts. Covers the following topics * Objects and types * Classes and members * Delegation * Polymorphism * Metaclasses This tutorial covers Python 3. Almost all concepts are valid for Python 2, when relevant differences will be highlighted. Source posts: http://lgiordani.com/blog/categories/python3/

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SOURCE CODE https://github.com/lgiordani/ deep_inside_oop_python Code snippets used in this tutorial can be found at

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PART 1 Objects and types

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 7 163 1 2 3 4 5 6 # This is some data data = (13, 63, 5, 378, 58, 40) # This is a procedure that computes the average def avg(d): return sum(d)/len(d) Plain old procedures >>> avg(data) 92.83333333333333 >>> part1/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 8 163 1 2 3 4 5 6 # This is some data data = (13, 63, 5, 378, 58, 40) # This is a procedure that computes the average def avg(d): return sum(d)/len(d) Very simple data: a sequence of numbers Original data enters here This returns new data Plain old procedures part1/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 9 163 1 2 3 4 5 6 7 # These are two numbered doors, initially closed door1 = [1, 'closed'] door2 = [2, 'closed'] # This procedure opens a door def open_door(door): door[1] = 'open' Procedures can modify data >>> open_door(door1) >>> door1 [1, 'open'] >>> part1/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 10 163 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 # These are two standard doors, initially closed door1 = [1, 'closed'] door2 = [2, 'closed'] # This is a lockable door, initially closed and unlocked ldoor1 = [1, 'closed', 'unlocked'] # This procedure opens a standard door def open_door(door): door[1] = 'open' # This procedure opens a lockable door def open_ldoor(door): if door[2] == 'unlocked': door[1] = 'open' Things can get complicated >>> open_door(door1) >>> door1 [1, 'open'] >>> open_ldoor(ldoor1) >>> ldoor1 [1, 'open', 'unlocked'] part1/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 11 163 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 # These are two standard doors, initially closed door1 = [1, 'closed'] door2 = [2, 'closed'] # This is a lockable door, initially closed and unlocked ldoor1 = [1, 'closed', 'unlocked'] # This procedure opens a standard door def open_door(door): door[1] = 'open' # This procedure opens a lockable door def open_ldoor(door): if door[2] == 'unlocked': door[1] = 'open' Things can get complicated >>> open_door(door1) >>> door1 [1, 'open'] >>> open_ldoor(ldoor1) >>> ldoor1 [1, 'open', 'unlocked'] Same “action” You must use the right function part1/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 12 163 The meaning of the word 'type' Behaves like a duck Behavioural meaning

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 13 163 The meaning of the word 'type' Dissection reveals the truth Structural meaning

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 14 163 The behavioural meaning is important Duck typing: making it behave like a duck 'Type' is a noun, please.

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 15 163 Class: the generic concept Instance: a specific object of that type

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 16 163 Classes >>> a = 6 >>> a 6 >>> type(a) >>> a.__class__ >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 17 163 Python 2.x Python 2.x Classes >>> a = 6 >>> a 6 >>> type(a) >>> a.__class__ >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 18 163 Classes >>> a = 6 >>> a 6 >>> type(a) >>> a.__class__ >>> What is the type of 'a'? The type of 'a' is the 'int' class It is written INSIDE the variable!

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 19 163 The first class 1 2 3 4 5 6 7 8 9 10 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' part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 20 163 Python 2.x Python 2.x The first class 1 2 3 4 5 6 7 8 9 10 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' part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 21 163 The first class 1 2 3 4 5 6 7 8 9 10 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 Everything under the 'class' keyword is part of the class part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 22 163 1 2 3 4 5 6 7 8 9 10 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 first class Functions inside a class are called 'methods', and must accept 'self' as the first parameter part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 23 163 1 2 3 4 5 6 7 8 9 10 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 first class Attributes Both attributes and methods are 'members' of the class part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 24 163 1 2 3 4 5 6 7 8 9 10 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 first class The __init__() special method, part of the constructor mechanism part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 25 163 The first class 1 2 3 4 5 6 7 8 9 10 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' >>> door1 = Door(1, 'closed') >>> part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 26 163 The first class >>> door1 = Door(1, 'closed') >>> type(door1) >>> 1 2 3 4 5 6 7 8 9 10 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' part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 27 163 The first class >>> door1 = Door(1, 'closed') >>> type(door1) >>> door1.number 1 >>> door1.status 'closed' >>> 1 2 3 4 5 6 7 8 9 10 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' part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 28 163 The first class >>> door1 = Door(1, 'closed') >>> type(door1) >>> door1.number 1 >>> door1.status 'closed' >>> door1.open() >>> 1 2 3 4 5 6 7 8 9 10 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' part1/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 29 163 The first class >>> door1 = Door(1, 'closed') >>> type(door1) >>> door1.number 1 >>> door1.status 'closed' >>> door1.open() >>> door1.number 1 >>> door1.status 'open' >>> 1 2 3 4 5 6 7 8 9 10 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' part1/04.py

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PART 2 Classes and members

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 31 163 Everything is an object >>> a = 1 >>> type(a) >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 32 163 Everything is an object >>> a = 1 >>> type(a) >>> type(int) The type of an object is an object itself

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 33 163 Where is the class of an object? >>> door1 = Door(1, 'closed') >>> door2 = Door(1, 'closed') >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 34 163 Where is the class of an object? >>> door1 = Door(1, 'closed') >>> door2 = Door(1, 'closed') >>> hex(id(door1)) '0xb67e148c' >>> hex(id(door2)) '0xb67e144c' >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 35 163 Where is the class of an object? >>> door1 = Door(1, 'closed') >>> door2 = Door(1, 'closed') >>> hex(id(door1)) '0xb67e148c' >>> hex(id(door2)) '0xb67e144c' >>> Your numbers will be different from these

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 36 163 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! It is an object in the system.

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 37 163 Class attributes 1 2 3 4 5 6 7 8 9 10 11 12 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' part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 38 163 Class attributes 1 2 3 4 5 6 7 8 9 10 11 12 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 part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 39 163 Class attributes >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 40 163 Class attributes >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 41 163 Class attributes >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> Door.colour = 'white' >>> Door.colour 'white' >>> door1.colour 'white' >>> door2.colour 'white' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 42 163 Class attributes 1 2 class Door: colour = 'brown' [...] >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> 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' >>> part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 43 163 Let's dive into attribute resolution >>> Door.__dict__ mappingproxy({'open': , 'colour': 'white', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 44 163 Let's dive into attribute resolution >>> Door.__dict__ mappingproxy({'open': , 'colour': 'white', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 45 163 Let's dive into attribute resolution >>> Door.__dict__ mappingproxy({'open': , 'colour': 'white', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> door1.__dict__['colour'] Traceback (most recent call last): File "", line 1, in KeyError: 'colour' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 46 163 Let's dive into attribute resolution >>> Door.__dict__ mappingproxy({'open': , 'colour': 'white', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> door1.__dict__['colour'] Traceback (most recent call last): File "", line 1, in KeyError: 'colour' >>> door1.__class__.__dict__['colour'] 'white' >>> door1.colour is Door.colour True 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 47 163 Door door1 door2 __getattribute__() >>> door1.colour >>> Door.colour Let's dive into attribute resolution

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 48 163 Let's dive into attribute resolution >>> door1 = Door(1, 'closed') >>> door1.colour = 'white' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py Remember to execute the class again (we manually changed the class 'colour' attribute)

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 49 163 Let's dive into attribute resolution >>> door1 = Door(1, 'closed') >>> door1.colour = 'white' >>> door1.__dict__['colour'] 'white' >>> door1.__class__.__dict__['colour'] 'brown' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 50 163 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' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 51 163 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' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 52 163 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' >>> 1 2 class Door: colour = 'brown' [...] part2/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 53 163 What about methods? >>> door1 = Door(1, 'closed') >>> Door.__dict__ mappingproxy({'open': , 'colour': 'red', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> door1.colour is Door.colour True >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 54 163 What about methods? >>> door1 = Door(1, 'closed') >>> Door.__dict__ mappingproxy({'open': , 'colour': 'brown', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> door1.colour is Door.colour True >>> door1.open is Door.open False >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 55 163 What about methods? >>> door1 = Door(1, 'closed') >>> Door.__dict__ mappingproxy({'open': , 'colour': 'brown', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> door1.colour is Door.colour True >>> door1.open is Door.open False >>> Door.__dict__['open'] >>> Door.open >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 56 163 What about methods? >>> door1 = Door(1, 'closed') >>> Door.__dict__ mappingproxy({'open': , 'colour': 'brown', '__dict__': , '__weakref__': , '__init__': , '__module__': '__main__', '__doc__': None, 'close': }) >>> door1.__dict__ {'number': 1, 'status': 'closed'} >>> door1.colour is Door.colour True >>> door1.open is Door.open False >>> Door.__dict__['open'] >>> Door.open >>> door1.open > >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 57 163 Python 2.x Python 2.x What about methods? >>> door1 = Door(1, 'closed') >>> Door.__dict__ dict_proxy({'__module__': '__main__', 'colour': 'brown', '__weakref__': , '__dict__': , 'close': , 'open': , '__doc__': None, '__init__': }) >>> door1.__dict__ {'status': 'closed', 'number': 1} >>> door1.colour is Door.colour True >>> door1.open is Door.open False >>> Door.__dict__['open'] >>> Door.open >>> door1.open > >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 58 163 From functions to bound methods >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: open() missing 1 required positional argument: 'self' >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 59 163 Python 2.x Python 2.x >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: unbound method open() must be called with Door instance as first argument (got nothing instead) >>> From functions to bound methods

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 60 163 From functions to bound methods >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: open() missing 1 required positional argument: 'self' >>> Door.open(door1) >>> door1.status 'open' >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 61 163 From functions to bound methods >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: open() missing 1 required positional argument: 'self' >>> Door.open(door1) >>> door1.status 'open' >>> door1.__class__.__dict__['open'] >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 62 163 From functions to bound methods >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: open() missing 1 required positional argument: 'self' >>> Door.open(door1) >>> door1.status 'open' >>> door1.__class__.__dict__['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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 63 163 From functions to bound methods >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: open() missing 1 required positional argument: 'self' >>> Door.open(door1) >>> door1.status 'open' >>> door1.__class__.__dict__['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__'] >>> door1.__class__.__dict__['open'].__get__ >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 64 163 From functions to bound methods >>> Door.open() Traceback (most recent call last): File "", line 1, in TypeError: open() missing 1 required positional argument: 'self' >>> Door.open(door1) >>> door1.status 'open' >>> door1.__class__.__dict__['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__'] >>> door1.__class__.__dict__['open'].__get__ >>> door1.__class__.__dict__['open'].__get__(door1) > >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 65 163 Python 2.x Python 2.x From functions to bound methods >>> door1.__class__.__dict__['open'].__get__(door1) > >>> door1.__class__.__dict__['open'].__get__(door1, Door) > >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 66 163 Class methods 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 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' >>> door1 = Door(1, 'closed') >>> door1.knock() Knock! >>> Door.knock() Knock! >>> part2/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 67 163 Class methods 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 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' part2/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 68 163 >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Class methods

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 69 163 >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> Class methods

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 70 163 >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> Door.paint('white') >>> Class methods

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 71 163 >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> Door.paint('white') >>> Door.colour 'white' >>> door1.colour 'white' >>> door2.colour 'white' >>> Class methods

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 72 163 >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> Door.paint('white') >>> Door.colour 'white' >>> door1.colour 'white' >>> door2.colour 'white' >>> door1.paint('yellow') >>> Class methods

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 73 163 >>> door1 = Door(1, 'closed') >>> door2 = Door(2, 'closed') >>> Door.colour 'brown' >>> door1.colour 'brown' >>> door2.colour 'brown' >>> 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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 75 163 Cat Specialization Cat can provide new features i.e. 'has whiskers' Cat performs some or all the tasks performed by Animal in a different way i.e. 'moves silently' Animal Cat has all the features of Animal i.e. 'moves'

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 76 163 Animal Cat Delegation Cat implements only 'new' or 'changed' features Cat delegates the remaining features to Animal

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 77 163 Composition: 'has' Car Engine Wheels turn_on() steer() get_color()

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 78 163 Inheritance: 'is' Animal Cat look() look() meow()

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 79 163

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 80 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> sdoor = SecurityDoor(1, 'closed') >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 81 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> sdoor = SecurityDoor(1, 'closed') >>> SecurityDoor.colour is Door.colour True >>> sdoor.colour is Door.colour True part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 82 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> sdoor = SecurityDoor(1, 'closed') >>> SecurityDoor.colour is Door.colour True >>> sdoor.colour is Door.colour True sdoor.colour SecurityDoor.colour Door.colour part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 83 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> sdoor.__dict__ {'number': 1, 'status': 'closed'} >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 84 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> sdoor.__dict__ {'number': 1, 'status': 'closed'} >>> sdoor.__class__.__dict__ mappingproxy({'__doc__': None, '__module__': '__main__'}) >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 85 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> sdoor.__dict__ {'number': 1, 'status': 'closed'} >>> sdoor.__class__.__dict__ mappingproxy({'__doc__': None, '__module__': '__main__'}) >>> Door.__dict__ mappingproxy({'__dict__': , 'colour': 'yellow', 'open': , '__init__': , '__doc__': None, 'close': , 'knock': , '__weakref__': , '__module__': '__main__', 'paint': }) >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 86 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> SecurityDoor.__bases__ (,) >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 87 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> SecurityDoor.__bases__ (,) >>> sdoor.knock > >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 88 163 Inheritance 1 2 class SecurityDoor(Door): pass >>> SecurityDoor.__bases__ (,) >>> sdoor.knock > >>> sdoor.__class__.__bases__[0].__dict__['knock'].__get__(sdoor) > >>> part3/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 89 163 Overriding 1 2 3 4 5 6 7 class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if not self.locked: self.status = 'open' part3/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 90 163 Overriding 1 2 3 4 5 6 7 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': , 'colour': 'grey', 'locked': True}) >>> Overriding blocks implicit delegation part3/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 91 163 Calling the parent implementation 1 2 3 4 5 6 7 8 class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return Door.open(self) >>> sdoor = SecurityDoor(1, 'closed') >>> part3/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 92 163 Calling the parent implementation 1 2 3 4 5 6 7 8 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' >>> part3/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 93 163 Calling the parent implementation 1 2 3 4 5 6 7 8 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' >>> part3/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 94 163 Avoid strong coupling 1 2 3 4 5 6 7 8 class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return super().open() >>> sdoor = SecurityDoor(1, 'closed') >>> sdoor.status 'closed' >>> sdoor.open() >>> sdoor.status 'closed' >>> sdoor.locked = False >>> sdoor.open() >>> sdoor.status 'open' >>> part3/04.py super() to the rescue!

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 95 163 Python 2.x Python 2.x Avoid strong coupling 1 2 3 4 5 6 7 8 class SecurityDoor(Door): colour = 'grey' locked = True def open(self): if self.locked: return super(SecurityDoor, self).open(self) part3/04.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 96 163 1 2 3 4 5 6 7 8 9 10 11 12 13 14 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

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 97 163 1 2 3 4 5 6 7 8 9 10 11 12 13 14 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 No way to get the status part3/05.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 98 163 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 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) Python magic to the rescue part3/06.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 99 163 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 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) Python magic to the rescue part3/07.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 100 163 Composed inheritance? 1 2 3 4 5 6 class ComposedDoor: def __init__(self, number, status): self.door = Door(number, status) def __getattr__(self, attr): return getattr(self.door, attr) part3/08.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 101 163 Composed inheritance? 1 2 3 4 5 6 class ComposedDoor: def __init__(self, number, status): self.door = Door(number, status) def __getattr__(self, attr): return getattr(self.door, attr) getattr() != __getattr__() part3/08.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 102 163 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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 103 163 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'] >>> l.append >>> a = l.append >>> a >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 104 163 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'] >>> l.append >>> a = l.append >>> a >>> b = getattr(l, 'append') >>> b >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 105 163 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'] >>> l.append >>> a = l.append >>> a >>> b = getattr(l, 'append') >>> b >>> a == b True >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 106 163 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'] >>> 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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 108 163 References >>> a = 5 >>> a 5 >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 109 163 References >>> a = 5 >>> a 5 >>> type(a) >>> This was not declared!

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 110 163 References >>> a = 5 >>> a 5 >>> type(a) >>> hex(id(a)) '0x83fe540' >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 111 163 References >>> a = 5 >>> a 5 >>> type(a) >>> hex(id(a)) '0x83fe540' >>> a = 'five' >>> a 'five' >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 112 163 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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 113 163 References >>> a = 5 >>> a 5 >>> type(a) >>> hex(id(a)) '0x83fe540' >>> a = 'five' >>> a 'five' >>> type(a) >>> hex(id(a)) '0xb70d6560' >>> A strong type system: every variable has a type

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 114 163 References >>> a = 5 >>> a 5 >>> type(a) >>> hex(id(a)) '0x83fe540' >>> a = 'five' >>> a 'five' >>> type(a) >>> hex(id(a)) '0xb70d6560' >>> A strong type system: every variable has a type A dynamic type system: the type changes with the content

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 115 163 Every variable is a reference 1 2 def echo(a): return a part4/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 116 163 Every variable is a reference 1 2 def echo(a): return a >>> echo(5) 5 >>> echo('five') 'five' >>> part4/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 117 163 Every variable is a reference 1 2 def echo(a): return a >>> echo(5) 5 >>> echo('five') 'five' >>> Both are references part4/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 118 163 What is polymorphism? >>> 5 + 6 11 >>> 5.5 + 6.6 12.1 >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 119 163 >>> 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?

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 120 163 >>> 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 "", line 1, in TypeError: unsupported operand type(s) for +: 'dict' and 'dict' >>> What is polymorphism?

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 121 163 >>> l = [1, 2, 3] >>> len(l) 3 >>> What is polymorphism?

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 122 163 >>> l = [1, 2, 3] >>> len(l) 3 >>> s = "Just a sentence" >>> len(s) 15 >>> d = {'a': 1, 'b': 2} >>> len(d) 2 >>> What is polymorphism?

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 123 163 >>> l = [1, 2, 3] >>> len(l) 3 >>> s = "Just a sentence" >>> len(s) 15 >>> d = {'a': 1, 'b': 2} >>> len(d) 2 >>> i = 5 >>> len(i) Traceback (most recent call last): File "", line 1, in TypeError: object of type 'int' has no len() >>> What is polymorphism?

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 124 163 >>> l = [1, 2, 3] >>> len(l) 3 >>> s = "Just a sentence" >>> len(s) 15 >>> d = {'a': 1, 'b': 2} >>> len(d) 2 >>> i = 5 >>> len(i) Traceback (most recent call last): File "", line 1, in TypeError: object of type 'int' has no len() >>> l.__len__() 3 >>> s.__len__() 15 >>> d.__len__() 2 >>> i.__len__() Traceback (most recent call last): File "", line 1, in AttributeError: 'int' object has no attribute '__len__' >>> What is polymorphism?

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 125 163 Polymorphism is based on delegation >>> [1,2,3].__add__([4,5,6]) [1, 2, 3, 4, 5, 6] >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 126 163 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__', '__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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 127 163 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__', '__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'] >>> 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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 128 163 Polymorphism is based on delegation 1 2 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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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 129 163 Polymorphism is based on delegation 1 2 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 "", line 1, in File "", line 2, in sum TypeError: can only concatenate list (not "int") to list >>> part4/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 130 163 Polymorphism in action 1 2 3 4 5 6 7 8 9 10 11 12 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() part4/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 131 163 Polymorphism in action 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 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 part4/03.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 132 163 Polymorphism in action >>> door = Door() >>> bool_door = BooleanDoor() >>> room = Room(door) >>> bool_room = Room(bool_door) >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 133 163 Polymorphism in action >>> 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 >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 134 163 Polymorphism in action >>> 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 >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 135 163 “Ask for permission” style 1 2 3 4 if hasattr(someobj, 'open'): [...] else: [...]

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 136 163 “Ask for forgiveness” style 1 2 3 4 if hasattr(someobj, 'open'): [...] else: [...] 1 2 3 4 5 try: someobj.open() [...] except AttributeError: [...]

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 137 163 “Ask for forgiveness” style 1 2 3 4 if hasattr(someobj, 'open'): [...] else: [...] 1 2 3 4 5 try: someobj.open() [...] except AttributeError: [...] It 'has' the attribute It behaves like it has the attribute

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PART 5 Metaclasses

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 139 163 Everything is an object (again) >>> a = 5 >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 140 163 Everything is an object (again) >>> a = 5 >>> type(a) >>> a.__class__ >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 141 163 Everything is an object (again) >>> a = 5 >>> type(a) >>> a.__class__ >>> a.__class__.__bases__ (,) >>> object.__bases__ () >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 142 163 Everything is an object (again) a int object >>> a = 5 >>> type(a) >>> a.__class__ >>> a.__class__.__bases__ (,) >>> object.__bases__ () >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 143 163 a int object >>> a = 5 >>> type(a) >>> a.__class__ >>> a.__class__.__bases__ (,) >>> object.__bases__ () >>> Everything is an object (again) 'int' inherits from (its base is) 'object' 'a' is an instance of (its type is) 'int'

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 144 163 Where classes come from >>> type(a) >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 145 163 Where classes come from >>> type(a) >>> type(int) >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 146 163 Where classes come from >>> type(a) >>> type(int) >>> type(float) >>> type(dict) >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 147 163 >>> type(a) >>> type(int) >>> type(float) >>> type(dict) >>> a object Everything is an object (again) 'int' inherits from (its base is) 'object' 'int' is an instance of (its type is) 'type' type int

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 148 163 >>> type(a) >>> type(int) >>> type(float) >>> type(dict) >>> a int object Everything is an object (again) 'int' finds here attributes and methods 'int' description as a class is here type

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 149 163 Food for thought object type >>> type(object) >>> type.__bases__ (,) >>>

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 150 163 Fatality >>> type(type) object type

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 151 163 Python 2.x Python 2.x >>> type(type) Fatality

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 152 163 Metaclasses 1 2 3 4 5 class MyType(type): pass class MySpecialClass(metaclass=MyType): pass part5/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 153 163 Python 2.x Python 2.x 1 2 3 4 5 class MyType(type): pass class MySpecialClass(object): __metaclass__ = MyType part5/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 154 163 Metaclasses >>> msp = MySpecialClass() >>> 1 2 3 4 5 class MyType(type): pass class MySpecialClass(metaclass=MyType): pass part5/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 155 163 Metaclasses >>> msp = MySpecialClass() >>> type(msp) >>> 1 2 3 4 5 class MyType(type): pass class MySpecialClass(metaclass=MyType): pass part5/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 156 163 Metaclasses >>> msp = MySpecialClass() >>> type(msp) >>> type(MySpecialClass) >>> 1 2 3 4 5 class MyType(type): pass class MySpecialClass(metaclass=MyType): pass part5/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 157 163 Metaclasses >>> msp = MySpecialClass() >>> type(msp) >>> type(MySpecialClass) >>> MySpecialClass.__bases__ (,) >>> 1 2 3 4 5 class MyType(type): pass class MySpecialClass(metaclass=MyType): pass part5/01.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 158 163 1 2 3 4 5 6 7 8 9 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 Metaclasses in action: singleton part5/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 159 163 Metaclasses in action: singleton 1 2 3 4 5 6 7 8 9 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() >>> part5/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 160 163 Metaclasses in action: singleton 1 2 3 4 5 6 7 8 9 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 >>> part5/02.py

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Deep inside Object-oriented Python – PyConIE 2014 – lgiordani.com - CC BY-SA 4.0 161 163 Metaclasses in action: singleton 1 2 3 4 5 6 7 8 9 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' >>> part5/02.py

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SOME LINKS The original posts http://lgiordani.com/blog/categories/python3/ Some links about Python OOP http://goo.gl/UBdJDT

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THANK YOU! Any comment, correction or contribution is warmly welcome. Feel free to contact me. https://github.com/ lgiordani https://twitter.com/ tw_lgiordani https://plus.google.com/u/ LeonardoGiordani The Digital Cat http://lgiordani.com