Why __radd__ does not work

Question

Hi

Trying to understand how it works __radd__. I have a code

>>> class X(object):
    def __init__(self, x):
        self.x = x
    def __radd__(self, other):
        return X(self.x + other.x)


>>> a = X(5)
>>> b = X(10)
>>> a + b

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    a + b
TypeError: unsupported operand type(s) for +: 'X' and 'X'
>>> b + a

Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    b + a
TypeError: unsupported operand type(s) for +: 'X' and 'X'

Why is this not working? What am I doing wrong here?

Answer 1

Python docs for statements

"These functions are only called if the left operand does not support the corresponding operation, and the operands are of different types."

See also Footnote 2

Since the operands are of the same type, you need to define __add__.

Answer 2

These functions are called only if the left operand does not support the corresponding operation, and the operands are of different types.

I am using Python3, so please ignore the grammatical differences:

>>> class X:
    def __init__(self,v):
        self.v=v
    def __radd__(self,other):
        return X(self.v+other.v)
>>> class Y:
    def __init__(self,v):
    self.v=v

>>> x=X(2)
>>> y=Y(3)
>>> x+y
Traceback (most recent call last):
  File "<pyshell#120>", line 1, in <module>
    x+y
TypeError: unsupported operand type(s) for +: 'X' and 'Y'
>>> y+x
<__main__.X object at 0x020AD3B0>
>>> z=y+x
>>> z.v
5

Answer 3

, . ,

class X:
  def __init__(self, num):
    self.num = num

---

class Y:
  def __init__(self, num):
    self.num = num

  def __radd__(self, other_obj):
    return Y(self.num+other_obj.num)

  def __str__(self):
    return str(self.num)

---

>>> x = X(2)
>>> y = Y(3)
>>> print(x+y)
5
>>>
>>> print(y+x)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-60-9d7469decd6e> in <module>()
----> 1 print(y+x)

TypeError: unsupported operand type(s) for +: 'Y' and 'X'

Answer 4

class X (object):

 def __init__(self, x):

     self.x = x

 def __radd__(self, other):

     return self.x + other

a = X (5)

b = X (10)

stamp of sum ([a, b])