What exactly is the function of random.seed() in python?

+2 votes

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. Thanks in advance!

Oct 12, 2018 in Python by charlie_brown
• 7,710 points
10,862 views

5 answers to this question.

+2 votes
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.
answered Oct 12, 2018 by ariaholic
• 7,320 points
+3 votes

The seed method is used to initialize the pseudorandom number generator in Python.

The random module uses the seed value as a base to generate a random number. if seed value is not present it takes system current time. if you provide same seed value before generating random data it will produce the same data. refer  https://pynative.com/python-random-seed/ for more details

Example:

import random

random.seed( 30 )
print ("first number  - ", random.randint(25,50))  

random.seed( 30 )
print ("Second number- ", random.randint(25,50))

Output:

first number - 42
Second  number - 42

answered Oct 29, 2018 by Rahul

Hey @Rahul.. I get the same output when I run your code from a file but when I run the same code in the python console, I get different output. Can you please tell me why is that so?

@kumar,
You need to run random.seed(30) again to set the seed back to its previous value. Generally, the seed is the previous value generated by the generator. So, when you ran random.randint(25,50) second time, your seed was 42 and not 30. 

If you run random.seed(30) again, 42, 50,...will be the sequence of numbers generated.

Suppose I generate 10 values, will all these values be stored? If yes, where are these values stored? Thanks
No. the last value it generated will be stored because that value will work as the seed for next number
import random

for i in range(2):
    random.seed(10)
    for i in range(20):
        print(random.random())
    print()

Use the above code to see a repeated output for yourself

ohh okayyy! Thanks, the example makes it clearer.
0 votes
We dont really know
answered Mar 15 by anonymous
0 votes

Check this link, but basically the seed is a reference to your random number and when you call it it will generate the same random number

https://pynative.com/python-random-seed/

answered Apr 16 by Yordan Ramshev
0 votes
Random sequence generation starts with reference to the provided seed. So if same seed provided twice same sequence is obtained as the same algorithm works in background.
answered May 3 by Abhijit Choudhary
Is it like an XOR function? If you use it twice you get the same output again.

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