Deep Inside Object-Oriented Python - PyCon Ireland 2014

Slides of the "Deep Inside Object-oriented Python" presentation held in Dublin at PyConIE 2014.

See the full series of articles on my blog http://lgiordani.com

Transcript

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

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2. 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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3. 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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4. 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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5. SOURCE
   CODE
   https://github.com/lgiordani/
   deep_inside_oop_python
   Code snippets used in this tutorial can be found at
   

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6. PART 1
   Objects and types
   

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   # 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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   # 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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   # 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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    # 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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    # 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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    The meaning of the word 'type'
    Behaves like a duck
    Behavioural meaning
    

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    The meaning of the word 'type'
    Dissection reveals
    the truth
    Structural meaning
    

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    The behavioural meaning is important
    Duck typing: making it behave like a duck
    'Type' is a noun,
    please.
    

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    Class: the generic concept
    Instance: a specific object of that type
    

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    Classes
    >>> a = 6
    >>> a
    6
    >>> type(a)
    
    >>> a.__class__
    
    >>>
    

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    Python
    2.x
    Python
    2.x
    Classes
    >>> a = 6
    >>> a
    6
    >>> type(a)
    
    >>> a.__class__
    
    >>>
    

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    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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    The first class
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    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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    Python
    2.x
    Python
    2.x
    The first class
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    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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    The first class
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    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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    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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    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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    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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    The first class
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    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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    The first class
    >>> door1 = Door(1, 'closed')
    >>> type(door1)
    
    >>>
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    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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    The first class
    >>> door1 = Door(1, 'closed')
    >>> type(door1)
    
    >>> door1.number
    1
    >>> door1.status
    'closed'
    >>>
    1
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    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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    The first class
    >>> door1 = Door(1, 'closed')
    >>> type(door1)
    
    >>> door1.number
    1
    >>> door1.status
    'closed'
    >>> door1.open()
    >>>
    1
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    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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    The first class
    >>> door1 = Door(1, 'closed')
    >>> type(door1)
    
    >>> door1.number
    1
    >>> door1.status
    'closed'
    >>> door1.open()
    >>> door1.number
    1
    >>> door1.status
    'open'
    >>>
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    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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30. PART 2
    Classes and members
    

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    Everything is an object
    >>> a = 1
    >>> type(a)
    
    >>>
    

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    Everything is an object
    >>> a = 1
    >>> type(a)
    
    >>> type(int)
    
    The type of an object is an object itself
    

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    Where is the class of an object?
    >>> door1 = Door(1, 'closed')
    >>> door2 = Door(1, 'closed')
    >>>
    

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    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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    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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    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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    Class attributes
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    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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    Class attributes
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    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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    Class attributes
    >>> door1 = Door(1, 'closed')
    >>> door2 = Door(2, 'closed')
    >>>
    1
    2
    class Door:
    colour = 'brown'
    [...]
    part2/01.py
    

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    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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    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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    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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    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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    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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    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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    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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    Door
    door1 door2
    __getattribute__()
    >>> door1.colour
    >>> Door.colour
    Let's dive into attribute resolution
    

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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    >>> door1 = Door(1, 'closed')
    >>> door2 = Door(2, 'closed')
    >>>
    Class methods
    

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

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    >>> 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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    >>> 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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    >>> 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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    >>> 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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74. PART 3
    Delegation
    

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    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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    Animal Cat
    Delegation
    Cat implements only 'new' or 'changed' features
    Cat delegates the remaining features to Animal
    

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    Composition: 'has'
    Car
    Engine
    Wheels
    turn_on()
    steer()
    get_color()
    

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    Inheritance: 'is'
    Animal
    Cat
    look()
    look()
    meow()
    

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    79 163
    

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    Inheritance
    1
    2
    class SecurityDoor(Door):
    pass
    >>> sdoor = SecurityDoor(1, 'closed')
    >>>
    part3/01.py
    

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    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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    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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    Inheritance
    1
    2
    class SecurityDoor(Door):
    pass
    >>> sdoor.__dict__
    {'number': 1, 'status': 'closed'}
    >>>
    part3/01.py
    

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    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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    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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    Inheritance
    1
    2
    class SecurityDoor(Door):
    pass
    >>> SecurityDoor.__bases__
    (,)
    >>>
    part3/01.py
    

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    Inheritance
    1
    2
    class SecurityDoor(Door):
    pass
    >>> SecurityDoor.__bases__
    (,)
    >>> sdoor.knock
    >
    >>>
    part3/01.py
    

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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    1
    2
    3
    4
    5
    6
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    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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    1
    2
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    7
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    11
    12
    13
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    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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     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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     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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     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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     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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     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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     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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     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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107. PART 4
     Polymorphism
     

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     References
     >>> a = 5
     >>> a
     5
     >>>
     

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     109 163
     References
     >>> a = 5
     >>> a
     5
     >>> type(a)
     
     >>> This was not declared!
     

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     110 163
     References
     >>> a = 5
     >>> a
     5
     >>> type(a)
     
     >>> hex(id(a))
     '0x83fe540'
     >>>
     

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     111 163
     References
     >>> a = 5
     >>> a
     5
     >>> type(a)
     
     >>> hex(id(a))
     '0x83fe540'
     >>> a = 'five'
     >>> a
     'five'
     >>>
     

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     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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     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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     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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     Every variable is a reference
     1
     2
     def echo(a):
     return a
     part4/01.py
     

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     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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     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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     118 163
     What is polymorphism?
     >>> 5 + 6
     11
     >>> 5.5 + 6.6
     12.1
     >>>
     

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     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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     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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     121 163
     >>> l = [1, 2, 3]
     >>> len(l)
     3
     >>>
     What is polymorphism?
     

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     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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     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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     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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     Polymorphism is based on delegation
     >>> [1,2,3].__add__([4,5,6])
     [1, 2, 3, 4, 5, 6]
     >>>
     

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     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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     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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     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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     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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     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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     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
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     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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     132 163
     Polymorphism in action
     >>> door = Door()
     >>> bool_door = BooleanDoor()
     >>> room = Room(door)
     >>> bool_room = Room(bool_door)
     >>>
     

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     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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     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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     135 163
     “Ask for permission” style
     1
     2
     3
     4
     if hasattr(someobj, 'open'):
     [...]
     else:
     [...]
     

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     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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     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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138. PART 5
     Metaclasses
     

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     139 163
     Everything is an object (again)
     >>> a = 5
     >>>
     

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     140 163
     Everything is an object (again)
     >>> a = 5
     >>> type(a)
     
     >>> a.__class__
     
     >>>
     

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     141 163
     Everything is an object (again)
     >>> a = 5
     >>> type(a)
     
     >>> a.__class__
     
     >>> a.__class__.__bases__
     (,)
     >>> object.__bases__
     ()
     >>>
     

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     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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     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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     144 163
     Where classes come from
     >>> type(a)
     
     >>>
     

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     145 163
     Where classes come from
     >>> type(a)
     
     >>> type(int)
     
     >>>
     

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     146 163
     Where classes come from
     >>> type(a)
     
     >>> type(int)
     
     >>> type(float)
     
     >>> type(dict)
     
     >>>
     

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147. 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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148. 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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149. 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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150. 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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151. 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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152. 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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153. 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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154. 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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155. 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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156. 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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157. 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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158. 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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159. 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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160. 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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161. 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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162. SOME
     LINKS
     The original posts
     http://lgiordani.com/blog/categories/python3/
     Some links about Python OOP
     http://goo.gl/UBdJDT
     

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163. 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
     

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