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Python Data Model🔗

Python Data Model is an API that we use to make our objects play well with the most idiomatic language features
Its more like a description of Python as a framework which formalizes the interfaces of the building blocks of the language itself like sequences, functions, iterators, coroutines etc
Example : obj[key] this syntax is supported by a special dunder method __getitem__.
These functions are meant to be called by python interpretor. for e.g. len(obj) is used to get length not obj.__len__().

NOTE: sometimes python interpretor takes shortcut when dealing with builtin types like list, str, bytearray using PyVarObject which has ob_size field holding number of items in collections.

for i in x: invokes iter(x) which in turn calls x.__iter__() if its available
Users should not call these unless they are doing Metaprogramming.

Important uses of special methods:🔗

Emulating Numeric Types

Implementing Vector Class

import math

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

  # string representation of class
  # !r calls standard representation of attributes to be displayed
  # it shows difference b/w Vector(1, 2) and Vector('1', '2')
  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))

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

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

String Representation of objects

__str__ is called by str() built-in and implicitly used by print functions.

NOTE: __repr__ is fallback for __str__. But as a practice implement former always. https://fpy.li/1-5

Boolean Value of an Object

Although bool type is supported in python but accepts any object in a Boolean context. To find whether x is truthy or falsy, python applies bool(x) which return True or False.

By default instances of user-defined classes are truthy, unless __bool__ or __len__ is implemented.

Implementing Collection

All of these classes are ABCs (abstract base classes).

Each of the top ABCs has a single special method

Iterable to support for, unpacking and other forms of iteration
Sized to support the len built-in function
Container to support the in operator

There is no need for a class to inherit from any of these ABCs, any class that implements __len__ satisfies Sized interface.