Random number generation in programming has thousands of uses ranging from cryptography to game-development. Python supports random number generation with its built-in Random module.

In this tutorial, you’ll see how to generate different types of random numbers in Python. The process of randomly choosing an item from a Python list is also explained at the end of the tutorial along with a script that uses the Random module to randomly shuffle the order of items in a list.

The Random module in Python comes pre-installed with the default Python installation. To use the Random module, import it like we demonstrate below. After importing the random module, the following script calls the dir() method to print the names of all the methods and attributes of the Random module.

import random
print(dir(random))

Output:

['BPF', 'LOG4', 'NV_MAGICCONST', 'RECIP_BPF', 'Random', 'SG_MAGICCONST', 'SystemRandom', 'TWOPI', '_Sequence', '_Set', '__all__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', '_accumulate', '_acos', '_bisect', '_ceil', '_cos', '_e', '_exp', '_inst', '_log', '_os', '_pi', '_random', '_repeat', '_sha512', '_sin', '_sqrt', '_test', '_test_generator', '_urandom', '_warn', 'betavariate', 'choice', 'choices', 'expovariate', 'gammavariate', 'gauss', 'getrandbits', 'getstate', 'lognormvariate', 'normalvariate', 'paretovariate', 'randint', 'random', 'randrange', 'sample', 'seed', 'setstate', 'shuffle', 'triangular', 'uniform', 'vonmisesvariate', 'weibullvariate']

We’re going to dive deeper into some of these methods and see how they can be called to generate different styles of random numbers.

Generate Random Numbers Between 0 and 1

Requesting random numbers between 0 and 1 is really common. The random() method returns a random number between 0 (included) and 1 (not included). We can find out more about the random() function by passing it to the help() command in Python

help(random.random)

Output:

Help on built-in function random:

random() method of random.Random instance
    random() -> x in the interval [0, 1).

The output shows that the method returns a value in the interval [0, 1). Here, the round bracket after 1 signifies that 1 is not included in the random numbers generated by the random() method.

The following script generates 5 random numbers between 0 and 1.

for i in range(5):
    rand_num = random.random()
    print(rand_num)

Output:

0.5486183422034425
0.7305847722411378
0.5659219086519193
0.3578211188658361
0.16822115885603006

If we run the script above a second time, you’ll see 5 different numbers generated in the output.

for i in range(5):
    rand_num = random.random()
    print(rand_num)

Output:

0.11957209130893665
0.7142474917013404
0.18084006505984696
0.19900438096938133
0.04823446467719894

Generate Random Numbers Within a Range

You can also generate random numbers within a range of numbers using the Python Random module. To do so, use the uniform() method. You need to pass the lower limit and upper limit for the range as parameter values to the uniform() method. For instance, the following script generates five random numbers between 2 and 8.

for i in range(5):
    rand_num = random.uniform(2,8)
    print(rand_num)

Output:

6.104003335206205
5.281189213098911
4.620178058260187
4.9089864579109594
7.651300431562037

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Generate Random Numbers within a Normal Distribution

The random() and uniform() methods return a number selected randomly from a uniform distribution. To generate a random number within a normal or Gaussian distribution, use the normalvariate() method.

Let’s print the help for the normalvariate() method:

help(random.normalvariate)

Output:

Help on method normalvariate in module random:

normalvariate(mu, sigma) method of random.Random instance
    Normal distribution.

    mu is the mean, and sigma is the standard deviation.

To specify the normal distribution from which you want to generate a random number, you have to pass the mean value and the standard deviation value for the distribution, to the normalvariate() method. For example, the following script generates 5 random numbers from a normal distribution with a mean of 0 and a standard deviation of 7.

for i in range(5):
    rand_num = random.normalvariate(0,7)
    print(rand_num)

Output:

8.50769914598948
-4.052105282635025
-5.038572230389826
-2.8022205914911087
6.251332322729557

The 68-95-99.7 rule, or the empirical rule, can you help you remember that 68% of the values of a normal distribution will fall within 1 standard deviation of the mean, 95% will fall within 2 standard deviations and 99.7% will fall within 3 standard deviations.

You’re more than welcome to test this rule yourself using the following script, which runs one million random numbers across a normal distribution with a mean of 0 and a standard deviation of 7. It then checks whether the results are within one, two, or three standard deviations and prints the percentage of results that fell within each bin.

import random
sig1=0
sig2=0
sig3=0
meanX=0
sigmaX=7
N=1000000
for i in range(N):
    rand_num = random.normalvariate(meanX,sigmaX)
    if rand_num>=-sigmaX and rand_num<=sigmaX:
        sig1=sig1+1
    if rand_num>=-2*sigmaX and rand_num<=2*sigmaX:
        sig2=sig2+1
    if rand_num>=-3*sigmaX and rand_num<=3*sigmaX:
        sig3=sig3+1
print("1 sigma: " + str(sig1/N*100) + "%")
print("2 sigma: " + str(sig2/N*100) + "%")
print("3 sigma: " + str(sig3/N*100) + "%")

Here’s the output I got:

1 sigma: 68.2921%
2 sigma: 95.4291%
3 sigma: 99.7231%

Generate Random Integers

In addition to generating decimal numbers, you can generate integers using the Random module. To do so, call the randint() method. Here’s what the help() command returns for the randint() method.

help(random.randint)

Output:

Help on method randint in module random:

randint(a, b) method of random.Random instance
    Return random integer in range [a, b], including both end points.

You need to pass the lower bound and the upper bound for the interval from which you want to select a random number. The following script randomly returns 5 integers between 1 and 15.

for i in range(5):
    rand_num = random.randint(1,15)
    print(rand_num)

Output:

2
5
12
7
9

You can also define a step size for generating random integers between an interval. To do so, you need to call the randrange() method and pass it three parameter values: the lower limit, the upper limit, and the step size. For instance, the randrange() method in the following script will return a random number between 1 and 15, with a step size of 3, which means that the random numbers generated are forced to be 1, 1 + 3 = 4, 4 + 3 = 7, and so on.

for i in range(5):
    rand_num = random.randrange(1,15,3)
    print(rand_num)

Output:

1
10
13
7
13

Generate a Set of Unique Random Numbers from a Collection

With the Random module, you can even generate a set of unique random numbers from any population sequence or set. The sample() method from the Random module does that for you. The help() command explains the functionality of the sample() method as follows:

help(random.sample)

Output:

Help on method sample in module random:

sample(population, k) method of random.Random instance
    Chooses k unique random elements from a population sequence or set.

The following script returns 5 random numbers between 0 and 9. Note that the range(10) function returns a sequence of items between 0 and 9. The print command just prints the output to your screen so you can see the resulting set.

print(random.sample(range(10), 5))

Output:

[8, 4, 2, 5, 6]

Let’s see another example. Here we have our pre-defined dummy list with some decimal numbers and integers, and the sample() method returns 3 random numbers from the list.

print(random.sample([0.45,0.82,0.65,20,25], 3))

Output:

[20, 0.82, 0.65]

It’s important to remember that the sample() method returns a new set of unique numbers from your existing set.

Select Random Items from a List

In addition to generating random numbers, the Random module also provides a method called choice() which randomly selects an item from a list. The following script prints the result of the help() command for the choice() function.

help(random.choice)

Output:

Help on method choice in module random:

choice(seq) method of random.Random instance
    Choose a random element from a non-empty sequence.

The following script randomly returns a string value from a list of 4 strings.

import random
colors = ["Red", "Green", "Blue", "Yellow"]
print(random.choice(colors))

The output shows that the string “yellow” is returned. If you execute the above script a couple more times, there’s a good chance different string values will be randomly chosen.

Output:

Yellow

Randomly Shuffle Items in a List

Finally, you can change the order of items in a python list by randomly shuffling them using the shuffle() method. The shuffle() method shuffles a list in place, meaning your original list order is altered. The following script prints the items in a list before and after shuffling them using the shuffle() method.

import random
colors = ["Red", "Green", "Blue", "Yellow"]
for c in colors:
    print(c)

print("=========")

random.shuffle(colors)
for c in colors:
    print(c)

Output:

Red
Green
Blue
Yellow
=========
Blue
Yellow
Red
Green

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