CODEWITHSUNDEEP
pythonclasspython-3.xmagic-methods
spencer
spencer

Reputation: 189

Python - self-referencing class (Vector addition using magic operators)

I'm having trouble understanding how the self-reference for this class works in this code:

class Vector2D:
  def __init__(self, x, y):
    self.x = x
    self.y = y
  def __add__(self, other):
    return Vector2D(self.x + other.x, self.y + other.y)

first = Vector2D(5, 7)
second = Vector2D(3, 9)
result = first + second
print(result.x)
print(result.y)

-- Just to check if I'm understanding how magic methods work, in result = first + second, the argument other refers to second right?

--Edit: Thanks, I guess that clears up my confusions regarding other. I still don't get how this line works though: return Vector2D(self.x + other.x, self.y + other.y) i.e. the class Vector2D being referenced inside it

Upvotes: 2

Views: 1457

Answers (3)

Martijn Pieters
Martijn Pieters

Reputation: 1123790

Yes, other is the right-hand-side expression result, second in your case.

From the object.__add__() documentation:

For instance, to evaluate the expression x + y, where x is an instance of a class that has an __add__() method, x.__add__(y) is called.

The expression Vector2D(self.x + other.x, self.y + other.y) creates a new instance of the class with new values for x and y, which here are constructed from the sum of the current instance x and y and the same attributes on the right-hand side instance.

A new instance is created because the normal semantics of + are to return a new instance, leaving the operands themselves untouched. Compare this to adding up two lists (['foo', 'bar'] + ['bar', 'baz']); there too a new list object is returned.

Upvotes: 1

paul-g
paul-g

Reputation: 3887

Yes, this is equivalent to:

result = first.__add__(second)

so:

  1. self is first
  2. other is second
  3. result is the new Vector2D

Upvotes: 3

DeepSpace
DeepSpace

Reputation: 81654

the argument other refers to second, right?

Correct. You can verify that with a print inside __add__:

class Vector2D:
  def __init__(self, x, y):
    self.x = x
    self.y = y
  def __add__(self, other):
      print(other.x, other.y)
      return Vector2D(self.x + other.x, self.y + other.y)

first = Vector2D(5, 7)
second = Vector2D(3, 9)
result = first + second

>> 3 9

Upvotes: 1

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