random.py 文件源码

def _randbelow(self, n, int=int, maxsize=1<<BPF, type=type,
                   Method=_MethodType, BuiltinMethod=_BuiltinMethodType):
        "Return a random int in the range [0,n).  Raises ValueError if n==0."

        random = self.random
        getrandbits = self.getrandbits
        # Only call self.getrandbits if the original random() builtin method
        # has not been overridden or if a new getrandbits() was supplied.
        if type(random) is BuiltinMethod or type(getrandbits) is Method:
            k = n.bit_length()  # don't use (n-1) here because n can be 1
            r = getrandbits(k)          # 0 <= r < 2**k
            while r >= n:
                r = getrandbits(k)
            return r
        # There's an overridden random() method but no new getrandbits() method,
        # so we can only use random() from here.
        if n >= maxsize:
            _warn("Underlying random() generator does not supply \n"
                "enough bits to choose from a population range this large.\n"
                "To remove the range limitation, add a getrandbits() method.")
            return int(random() * n)
        rem = maxsize % n
        limit = (maxsize - rem) / maxsize   # int(limit * maxsize) % n == 0
        r = random()
        while r >= limit:
            r = random()
        return int(r*maxsize) % n

## -------------------- sequence methods  -------------------