Dunder methods

Usually implemented when we want objects to support and interact with fundamental elements of the language, as:

• Collections
• Attribute access
• Iteration (including async for)
• Operator overloading
• invoking functions and methods
• representation and string formatting
• async programing using await
• creation and destruction of objects
• context management using with and async with

Simple deck of cards:

import collections

Card = collections.namedtuple('Card', ['rank', 'suit'])

class FrenchDeck:
  ranks = [str(n) for n in range(2, 11)] + list('JQKA')
  suits = 'spadeds diamonds clubs hearts'.split()
  
  def __init__(self):
    self._cards = [Card(rank, suit) for suit in self.suits for rank in self.ranks]
    
  def __len__(self):
    return len(self._cards)
  
  def __getitem__(self, position):
    return self._cards[position]

We use namedtuple to build a simple class representing individual cards. We use it because they're just a group of attributes, without methods (as a DB register). As we use __getitem__ and __len__ we can reuse python stuff, as slicing, random, reversed and everything.

Iteration is many times implicit. If a collection do not have a __contains__ method, the in operator realizes a sequential search. In the case above, the operator works because FrenchDeck is iterable.

Example: vector implementation (very simplistic)

import math

class Vector:

    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def __repr__(self):
        return f'Vector({self.x!r}, {self.y!r})'

    def __abs__(self):
        return math.hypot(self.x, self.y)

    def __bool__(self):
        return bool(abs(self))

    def __add__(self, other):
        x = self.x + other.x
        y = self.y + other.y
        return Vector(x, y)

    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)

Note that the "correct" behavior for a method like __add__ is return a new vector, (create a new object) and not touch in its operands. Note too that in this example we break the commutative property of scale multiplication (we will fix with __rmul__ later.

String representation

The special method __repr__ is called by reprto get an object representation as a string. Without a personalized __repr__, python console would show an instance as Vector object at 0x1231 .

repr is also called with %r positional marker and in '{my_obj!r}'

On the other hand, __str__ is called by the str() method and used implicitly by print. If no __str__ is implemented, __repr__ is called.

Boolean types

Class instances defined by the user are considered to be true, unless __bool__ or __len__ is implemented. Basically, bool(x) calls bool dunder and if not implemented, calls len dunder and return false if len is 0.

Collection's API

It documents interfaces of essencial collection types in the language.

List of dunder methods excluding operators: https://pythonfluente.com/#special_names_tbl Dunder methods for operators: https://pythonfluente.com/#special_operators_tbl

(we can use matrix1 @ matrix2 to perform matmul.

A collection of Sequences

the default library offers a good selection of sequence types, implemented in C:

Container sequences They can store different types of itens, including nested containers and objects of all kind. Some examples are: list tuple collections.deque.

Plane sequences Store items of some simple type, but not other collections or object references. Ex: str bytes array.array

A container sequence keeps references for objects that it contains, that can be of any type, and a plane sequence stores the value of its content in its own memory space, not as distinct python objects.

Note: All python object in memory has a metadata header. For example, float has a field of value and two metadata fields: ob_refcnt: reference counting, ob_type pointer to obj type, ob_fval double of C keeping the float val.

We can also group sequence as mutable or immutable.

Expression generators

>>> symbols = '$¢£¥€¤'
>>> tuple(ord(symbol) for symbol in symbols)  (1)
(36, 162, 163, 165, 8364, 164)
>>> import array
>>> array.array('I', (ord(symbol) for symbol in symbols))  (2)
array('I', [36, 162, 163, 165, 8364, 164])