#!/usr/bin/env python3

"""
A demonstration of classes and their special methods in Python.
"""

class Vec:
 """A simple vector class.

 Instances of the Vec class can be constructed from numbers

 >>> a = Vec(1, 2, 3)
 >>> b = Vec(3, 2, 1)

 added
 >>> a + b
 Vec(4, 4, 4)

 subtracted
 >>> a - b
 Vec(-2, 0, 2)

 and multiplied by a scalar on the left
 >>> 3.0 * a
 Vec(3.0, 6.0, 9.0)

 or on the right
 >>> a * 3.0
 Vec(3.0, 6.0, 9.0)
 """
 def __init__(self, *v):
 self.v = list(v)

 @classmethod
 def fromlist(cls, v):
 if not isinstance(v, list):
 raise TypeError
 inst = cls()
 inst.v = v
 return inst

 def __repr__(self):
 args = ', '.join(repr(x) for x in self.v)
 return 'Vec({})'.format(args)

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

 def __getitem__(self, i):
 return self.v[i]

 def __add__(self, other):
 # Element-wise addition
 v = [x + y for x, y in zip(self.v, other.v)]
 return Vec.fromlist(v)

 def __sub__(self, other):
 # Element-wise subtraction
 v = [x - y for x, y in zip(self.v, other.v)]
 return Vec.fromlist(v)

 def __mul__(self, scalar):
 # Multiply by scalar
 v = [x * scalar for x in self.v]
 return Vec.fromlist(v)

 __rmul__ = __mul__


def test():
 import doctest
 doctest.testmod()

test()