Assigning values to custom class

Question

I want to create a custom array object. One of the functions I want to implement is the possibility to add two arrays of different lengths, more or less like so

[1, 2, 3] + [4, 5] = [5, 7, 3]

My custom class is defined like so

class MyArray:

    def __init__(self, values):
        self.values = values

    def __len__(self):
        return len(self.values)

    def __getitem__(self, key):
        """Makes MyArray sliceable"""
        if isinstance(key, slice):
            return self.values[key.start:key.stop:key.step]
        else:
            return self.values[key]

    def __add__(self, other):
        """"Make addition of two MyArrays of different lengths possible."""
        if len(self) >= len(other):
            a = self
            b = other
        else:
            a = other
            b = self
        a[:len(b)] += b[:]
        return MyArray(a)

Something is missing however. There is something about the implementation of the slicing I guess.

import numpy as np

a = MyArray(np.array([1,2,3]))
b = MyArray(np.array([4,5]))

Direct assignment fails

a[2] = 3
Traceback (most recent call last):

  File "<ipython-input-8-17351fe6de12>", line 1, in <module>
    a[2] = 3

TypeError: 'MyArray' object does not support item assignment

And, obviously since direct assignment fails, my __add__ method fails.

c = a + b

File "<ipython-input-1-2ce5425b4430>", line 28, in __add__
  a[:len(b)] += b[:]

TypeError: 'MyArray' object does not support item assignment

Answer 1

I also fixed some issues in your __getitem__ and __add__ functions (comments in code added) in addition to my comment , so will post as an answer:

class MyArray:
    def __init__(self, values):
        self.values = values

    def __len__(self):
        return len(self.values)

    def __getitem__(self, key):
        """Makes MyArray sliceable"""
        return self.values[key]  # you don't need to check for slice, it is supported by default

    def __add__(self, other):
        """"Make addition of two MyArrays of different lengths possible."""
        if len(self) >= len(other):
            a = self
            b = other
        else:
            a = other
            b = self
        a = MyArray(a.values)  # you need to make a copy before addition
        a[:len(b)] += b[:]
        return a

    def __setitem__(self, key, value):
        self.values[key] = value

import numpy as np

a = MyArray(np.array([1,2,3]))
b = MyArray(np.array([4,5]))

a[2] = 4
c = a + b
print(c.values)  # [5 7 4]