NumPy random seed produces different random numbers

Question

I run the following code:

 np.random.RandomState(3)
 idx1 = np.random.choice(range(20),(5,))
 idx2 = np.random.choice(range(20),(5,))  
 np.random.RandomState(3)
 idx1S = np.random.choice(range(20),(5,))
 idx2S = np.random.choice(range(20),(5,))       

The output I get is the following:

idx1:  array([ 2, 19, 19,  9,  4])  
idx1S: array([ 2, 19, 19,  9,  4])  

idx2:  array([ 9,  2,  7, 10,  6]) 
idx2S: array([ 5, 16,  9, 11, 15]) 

idx1 and idx1S match, but idx2 and idx2S do not match. I expect that once I seed the random number generator and repeat the same sequence of commands - it should produce the same sequence of random numbers. Is this not true? Or is there something else that I am missing?

Answer 1

You're confusing RandomState with seed. Your first line constructs an object which you can then use as your random source. For example, we make

>>> rnd = np.random.RandomState(3)
>>> rnd
<mtrand.RandomState object at 0xb17e18cc>

and then

>>> rnd.choice(range(20), (5,))
array([10,  3,  8,  0, 19])
>>> rnd.choice(range(20), (5,))
array([10, 11,  9, 10,  6])
>>> rnd = np.random.RandomState(3)
>>> rnd.choice(range(20), (5,))
array([10,  3,  8,  0, 19])
>>> rnd.choice(range(20), (5,))
array([10, 11,  9, 10,  6])

[I don't understand why your idx1 and idx1S agree-- but you didn't actually post a self-contained transcript, so I suspect user error.]

If you want to affect the global state, use seed:

>>> np.random.seed(3)
>>> np.random.choice(range(20),(5,))
array([10,  3,  8,  0, 19])
>>> np.random.choice(range(20),(5,))
array([10, 11,  9, 10,  6])
>>> np.random.seed(3)
>>> np.random.choice(range(20),(5,))
array([10,  3,  8,  0, 19])
>>> np.random.choice(range(20),(5,))
array([10, 11,  9, 10,  6])

Using a specific RandomState object may seem less convenient at first, but it makes a lot of things easier when you want different entropy streams you can tune.