"""
Arithmetic progression class

# BEGIN ARITPROG_CLASS_DEMO

    >>> ap = ArithmeticProgression(0, 1, 3)
    >>> list(ap)
    [0, 1, 2]
    >>> ap = ArithmeticProgression(1, .5, 3)
    >>> list(ap)
    [1.0, 1.5, 2.0, 2.5]
    >>> ap = ArithmeticProgression(0, 1/3, 1)
    >>> list(ap)
    [0.0, 0.3333333333333333, 0.6666666666666666]
    >>> from fractions import Fraction
    >>> ap = ArithmeticProgression(0, Fraction(1, 3), 1)
    >>> list(ap)
    [Fraction(0, 1), Fraction(1, 3), Fraction(2, 3)]
    >>> from decimal import Decimal
    >>> ap = ArithmeticProgression(0, Decimal('.1'), .3)
    >>> list(ap)
    [Decimal('0.0'), Decimal('0.1'), Decimal('0.2')]

# END ARITPROG_CLASS_DEMO
"""


# BEGIN ARITPROG_CLASS
class ArithmeticProgression:

    def __init__(self, begin, step, end=None):  # <1>
        self.begin = begin
        self.step = step
        self.end = end  # None -> "infinite" series

    def __iter__(self):
        result = type(self.begin + self.step)(self.begin)  # <2>
        forever = self.end is None  # <3>
        index = 0
        while forever or result < self.end:  # <4>
            yield result  # <5>
            index += 1
            result = self.begin + self.step * index  # <6>
# END ARITPROG_CLASS