Question

I have the following code:

class VectorN(object):
    def __init__(self, param):
        if isinstance(param, int):
            self.dim = param
            self.data = [0.0] * param

        elif isinstance(param, tuple):
            self.dim = 3
            self.data = param
        #else:
            #raise TypeError("You must pass an int or sequence!")

    def __str__(self):
        return "<Vector" + str(self.dim) + ": " + str(self.data) + ">"

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


    def __setitem__(self, key, item): 
        self.data[key] = item

Now when i try to call the __setitem__ method using the following code,

w = VectorN((1.2, "3", 5))
w.setitem(0, 9.9)
print(z) 
print(w) 
print(z[0])
print(len(v))

it gives me:

AttributeError: ‘VectorN’ object has no attribute ‘setitem’

Answer 1

That’s because __setitem__ is a magic method. It’s a special function that allows you to create container objects.

Because it’s a magic method, you need not call it directly by its name- rather, built-in aspects of the Python language control its behavior. Note that it’s still just a normal method; you could call it by name, but then the syntax would be w.__setitem__(0, 9.9).

By defining __setitem__ you can instead set a value like so: w[0] = 9.9.

---

__setitem__, from A Guide to Python’s Magic Methods:

Defines behavior for when an item is assigned to, using the notation self[nkey] = value.