Exceptions.py

import math

class TriangleError(Exception):
    """docstring for TriangleError
    error.args - single element tuple which conteins the msg passed to 
    the constructor"""

    def __init__(self, text, sides):
        super().__init__(text)
        self.sides = tuple(sides)

    @property
    def sides(self):
        return self._sides

    def __str__():
        return "'{}' for sides {}".format(self.args[0], self._sides)

    def __repr__():
        "TriangleError({!r}, {!r}".format(self.args[0], self._sides)

def triagle_areas(a, b, c):
    sides = sorted((a, b, c))
    if sides [2] > sides[0] + sides[1]:
        raise TriangleError("Illegal troangle", sides)
    p = (a + b + c) / 2
    return math.sqrt(p * (p - a) * (p - b) * (p - c))

class A:
  def __init__(self, some)
    self._b = some

  @property
  def b(self):
    return self._b

  @b.setter
  def b_setter(self, val):
    self._b = val

A = A(‘123’)
print(a.b)
a.b = ‘123’