{Algo} base64补全

根据base64的原理，它占用的字符数一定是4的整数。不够4字符的，用=来填充。请写一个算法。
.net code

class TestString
{
    public string Fill(string str)
    {
        return string.Format("{0}{1}", str, "=".Multi((uint)(4 - str.Length % 4)%4));
    }
}

static class StringExtends
{
    public static string Multi(this string self, uint n)
    {
        if (n == 0)
            return string.Empty;

        if (n>255)
            throw new ArgumentException("n is too big");

        if (n == 1)
            return self;

        if (n == 2)
            return self + self;

        if (n == 3)
            return self + self + self;

        if (n == 4)
            return self + self + self + self;

        var sb = new StringBuilder();
        for (int i=0;i<n;++i)
        {
            sb.Append(self);
        }

        return sb.ToString();
    }
}

python code

def fill(base64):
    return base64 + "="*((4-len(base64)%4)%4)