def EvalPoissonPmf(k, lam):
    """Computes the Poisson PMF.

    k: number of events
    lam: parameter lambda in events per unit time

    returns: float probability
    """
    # don't use the scipy function (yet).  for lam=0 it returns NaN;
    # should be 0.0
    # return scipy.stats.poisson.pmf(k, lam)

    return lam ** k * math.exp(-lam) / math.factorial(k)