Riffle Shuffling

riffle.py

import random

def _perfect_riffle():
    while True:
        yield 0
        yield 1

perfect_riffle = _perfect_riffle().__next__

def riffle(left, right, *, rand=random.random):
    result = []
    ileft,  cleft  = iter(left),  len(left)
    iright, cright = iter(right), len(right)
    while cleft + cright:
        if rand() < cleft / (cleft + cright):
            result.append(next(ileft))
            cleft -= 1
        else:
            result.append(next(iright))
            cright -= 1
    return result

def riffle_shuffle(seq, n=6, *, rand=random.random):
    half = len(seq) // 2
    for i in range(n):
        seq = riffle(seq[:half], seq[half:], rand=rand)
    return seq