poisson.py

class poisson_:
    
    def __init__(self, λ):
        self.λ = λ
        
        ε = 0.01
        
        # [α , β] is the range of n's for which the pmf value is above ε
        self.α = 0
        state = 1
        self.vals = {}
        summer = 0
        
        while(1):
            if state == 1:
                temp = poisson.pmf(self.α, self.λ) 
                if(temp <= ε):
                    self.α+=1
                else:
                    self.vals[self.α] = temp
                    summer += temp
                    self.β = self.α+1
                    state = 2
            elif state == 2:
                temp = poisson.pmf(self.β, self.λ)
                if(temp > ε):
                    self.vals[self.β] = temp
                    summer += temp
                    self.β+=1
                else:
                    break    
        
        # normalizing the pmf, values of n outside of [α, β] have pmf = 0
        
        added_val = (1-summer)/(self.β-self.α)
        for key in self.vals:
            self.vals[key] += added_val
        
            
    def f(self, n):
        try:
            Ret_value = self.vals[n]
        except(KeyError):
            Ret_value = 0
        finally:
            return Ret_value