random.seed(): What does it do?

Question

I am a bit confused on what random.seed() does in Python. For example, why does the below trials do what they do (consistently)?

>>> import random >>> random.seed(9001) >>> random.randint(1, 10) 1 >>> random.randint(1, 10) 3 >>> random.randint(1, 10) 6 >>> random.randint(1, 10) 6 >>> random.randint(1, 10) 7 

I couldn't find good documentation on this.

Answer 1

All the other answers don't seem to explain the use of random.seed(). Here is a simple example (source):

import random random.seed( 3 ) print "Random number with seed 3 : ", random.random() #will generate a random number  #if you want to use the same random number once again in your program random.seed( 3 ) random.random()   # same random number as before 

Answer 2

Pseudo-random number generators work by performing some operation on a value. Generally this value is the previous number generated by the generator. However, the first time you use the generator, there is no previous value.

Seeding a pseudo-random number generator gives it its first "previous" value. Each seed value will correspond to a sequence of generated values for a given random number generator. That is, if you provide the same seed twice, you get the same sequence of numbers twice.

Generally, you want to seed your random number generator with some value that will change each execution of the program. For instance, the current time is a frequently-used seed. The reason why this doesn't happen automatically is so that if you want, you can provide a specific seed to get a known sequence of numbers.