python adding 2 objects or object to a list

Question

I have a vector class which gives a vector with a list and I need to be able to add vectors or vectors to lists, tuples, and strings. If they are not of the same length, I need to throw a type error. Some examples of output:

Vector([6,8,2])+Vector([4,-3,2])
Vector([10, 5, 4])
>>> Vector([6,8,2])+[4,-3,2]
Vector([10, 5, 4])
>>> (6,8,2)+Vector([4,-3,2])
Vector([10, 5, 4])
>>> v=Vector(["f","b"])
>>> v+=("oo","oo")
>>> v
Vector(['foo', 'boo'])

I need to make a + function and a += function.

Which python methods do I use to override the + and += operations, also I need to make sure that my + operation works on an object and a sequence and the reverse

you can check the type of the other argument by with isinstance, e.g., isinstance(other, Vector) — Elmar Peise
– Elmar Peise, Commented Mar 1, 2016 at 23:28
I think the __iadd__ method is what you're looking for (for the += case). stackoverflow.com/questions/1047021/… — Garrett R
– Garrett R, Commented Mar 1, 2016 at 23:39
I made iadd work for 2 vectors and Im not sure why it doesn work for a vector and a sequence >>> v=Vector(["f","b"]) >>> v+=("oo","oo") Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unsupported operand type(s) for +=: 'instance' and 'tuple' >>> — Anatoliy Sokolov
– Anatoliy Sokolov, Commented Mar 1, 2016 at 23:49

ShadowRanger · Accepted Answer · 2016-03-02 16:42:00Z

1

To fully implement concatenation in arbitrary order, you need to implement three methods, __add__, __radd__ and __iadd__. You didn't implement __add__ at all (so Vector + sequence doesn't work, only sequence + Vector using __radd__) and you misspelled __iadd__ as __iadd (so Vector += anything doesn't work).

edited Mar 2, 2016 at 16:42

answered Mar 1, 2016 at 23:55

ShadowRanger

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3 Comments

Anatoliy Sokolov Over a year ago

Oh I didnt notice that. I kind of get it I put in add with the same exact implementation as radd and it worked.

ShadowRanger Over a year ago

@AnatoliySokolov: The basic rule is that the plain __xxx__ method for binary operators is called when the instance is on the left side of the operator, __rxxx__ is called when it is on the right side and the object on the left side's __xxx__ didn't exist (or returned NotImplemented), and __ixxx__ is called when using the augmented assignment operator. __ixxx__ is optional; for immutable types, Python uses __xxx__ when __ixxx__ is missing. You may want to look at the collections.abc interfaces

ShadowRanger Over a year ago

@AnatoliySokolov: You may also want to look at Implementing the arithmetic operations for tips on using an operator fallback based approach to avoiding lots of repetitious code; it's focused mathematical operations, but the same techniques apply to sequence operations, and the example given from fractions.Fraction can shorten code a lot.

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