def __init__(self, gamma, init_value=None):
self._value = init_value<EOL>self._gamma = gamma<EOL>
Keep a running estimate of a quantity. This is a bit like mean
        but more sensitive to recent changes.

        Parameters
        ----------
        gamma: float
            Must be between 0 and 1, where 0 is the most sensitive to recent
            changes.
        init_value: float or None
            Initial value of the estimate. If None, it will be set on the first update.