CODEWITHSUNDEEP
python
Niriel
Niriel

Reputation: 2705

How does python's random.Random.seed work?

I'm used to typing random.randrange. I'll do a from random import Random to spot the error from now on.

For a game involving procedural generation (nope, not a Minecraft clone :p) I'd like to keep several distinct pseudo-random number generators:

The rationale being that I want to be able to reproduce the first, so I don't want the second one to interfere.

I thought random.Random was made for that. However something is puzzling me:

import random
rnd = random.Random()
rnd.seed(0)
print [random.randrange(5) for i in range(10)]
rnd.seed(0)
print [random.randrange(5) for i in range(10)]

produces two different sequences. When I do rnd = random then things work as expected, but I do need several generators.

What am I missing?

Upvotes: 9

Views: 9850

Answers (2)

Johan Lundberg
Johan Lundberg

Reputation: 27068

It works almost exactly as you tried but the rnd.seed() applies to the rnd object

just use 

rnd = random.Random(0) # <<-- or set it here 
rnd.seed(7)
print [rnd.randrange(5) for i in range(10)]

or by setting the global seed, like this:

random.seed(7)
print [random.randrange(5) for i in range(10)]

Upvotes: 13

Rob Wouters
Rob Wouters

Reputation: 16327

Pass the seed to the constructor of Random:

>>> import random
>>> rnd = random.Random(0)
>>> [rnd.randint(0, 10) for i in range(10)]
[9, 8, 4, 2, 5, 4, 8, 3, 5, 6]
>>> rnd = random.Random(0)
>>> [rnd.randint(0, 10) for i in range(10)]
[9, 8, 4, 2, 5, 4, 8, 3, 5, 6]
>>> rnd = random.Random(1)
>>> [rnd.randint(0, 10) for i in range(10)]
[1, 9, 8, 2, 5, 4, 7, 8, 1, 0]

Upvotes: 4

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