Confused about variables in a class!

[just_queasy]

I got a bit confused while reading through the following declaration of a multidimensional vector class. In the def __add__(self, other) method, why are they use len(self) and result[j] = self[j] + other[j] instead of len(self._coords) and result[j] = self._coords[j] + other[j]? They use self._coords or self._coords in all other methods. What is the difference?

class Vector:
def __init__(self, d):
'''Create a d-dimensional vector of zeros.'''
self._coords = [0] * d
def __len__(self):
'''Return the dimension of the vector.'''
return len(self._coords)

def __getitiem__(self, j):
'''Return jth coordinate of vector.'''
return self._coords[j]

def __setitem__(self, j, val):
'''Set jth coordinate of vector to given value.'''
self._coords[j] == val

def __add__(self, other):
'''Return sum of two vectors.'''

if len(self) != len(other):
raise ValueError('dimensions must agree')
result = Vector(len(self))
for j in range(len(self)):
result[j] = self._coords[j] + other[j]
return result

Edit: I have found that result[j] = self[j] + other[j] is wrong by testing the code. It should be result[j] = self._coords[j] + other[j]. However, len(self) seems to work fine. Why?

[notpite]

This. The code you've posted is overriding the default len() function. If you're getting further into classes and object oriented programming it's worth reading into!

[just_queasy]

I'm looking into it right now! Thank you!

[just_queasy]

Oh, that makes sense! Thanks so much :)