刚学python，快帮我看看这个怎么回事

def fact(n):
    res = 1
    for i in range(1,n+1):
        res *= i/(2*i+1)
    return res

def pi():
    res = 1
    n = 1
    while fact(n)>=0.0005:
        res += fact(n)
        n += 1
    return res*2

print(pi())

3.140578169680336

增加精度：

def PI():
    res = 1
    n = 1
    while fact(n)>=10**-15:
        res += fact(n)
        n += 1
    return res*2

print(PI())

3.141592653589789