random.random()返回范围为[0，1）的 均匀分布的 伪随机浮点数。该数字小于p[0,1）范围内给定概率的给定数字p。从而：
def flip(p):
return 'H' if random.random() < p else 'T'
一些实验：
>>> N = 100
>>> flips = [flip(0.2) for i in xrange(N)]
>>> float(flips.count('H'))/N
0.17999999999999999 # Approximately 20% of the coins are heads
>>> N = 10000
>>> flips = [flip(0.2) for i in xrange(N)]
>>> float(flips.count('H'))/N
0.20549999999999999 # Better approximation
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