How to randomly select an item from a list?
How to randomly select an item from a list?
Question
Assume I have the following list:
foo = ['a', 'b', 'c', 'd', 'e']
What is the simplest way to retrieve an item at random from this list?
Accepted Answer
Use random.choice()
import random
foo = ['a', 'b', 'c', 'd', 'e']
print(random.choice(foo))
For cryptographically secure random choices (e.g. for generating a passphrase from a wordlist) use secrets.choice()
import secrets
foo = ['battery', 'correct', 'horse', 'staple']
print(secrets.choice(foo))
secrets is new in Python 3.6, on older versions of Python you can use the random.SystemRandom class:
import random
secure_random = random.SystemRandom()
print(secure_random.choice(foo))
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If you want to randomly select more than one item from a list, or select an item from a set, I'd recommend using random.sample instead.
import random
group_of_items = {1, 2, 3, 4} # a sequence or set will work here.
num_to_select = 2 # set the number to select here.
list_of_random_items = random.sample(group_of_items, num_to_select)
first_random_item = list_of_random_items[0]
second_random_item = list_of_random_items[1]
If you're only pulling a single item from a list though, choice is less clunky, as using sample would have the syntax random.sample(some_list, 1)[0] instead of random.choice(some_list).
Unfortunately though, choice only works for a single output from sequences (such as lists or tuples). Though random.choice(tuple(some_set)) may be an option for getting a single item from a set.
EDIT: Using Secrets
As many have pointed out, if you require more secure pseudorandom samples, you should use the secrets module:
import secrets # imports secure module.
secure_random = secrets.SystemRandom() # creates a secure random object.
group_of_items = {1, 2, 3, 4} # a sequence or set will work here.
num_to_select = 2 # set the number to select here.
list_of_random_items = secure_random.sample(group_of_items, num_to_select)
first_random_item = list_of_random_items[0]
second_random_item = list_of_random_items[1]
EDIT: Pythonic One-Liner
If you want a more pythonic one-liner for selecting multiple items, you can use unpacking.
import random
first_random_item, second_random_item = random.sample(group_of_items, 2)
If you also need the index, use random.randrange
from random import randrange
random_index = randrange(len(foo))
print(foo[random_index])
As of Python 3.6 you can use the secrets module, which is preferable to the random module for cryptography or security uses.
To print a random element from a list:
import secrets
foo = ['a', 'b', 'c', 'd', 'e']
print(secrets.choice(foo))
To print a random index:
print(secrets.randbelow(len(foo)))
For details, see PEP 506.
I propose a script for removing randomly picked up items off a list until it is empty:
Maintain a set and remove randomly picked up element (with choice) until list is empty.
s=set(range(1,6))
import random
while len(s)>0:
s.remove(random.choice(list(s)))
print(s)
Three runs give three different answers:
>>>
set([1, 3, 4, 5])
set([3, 4, 5])
set([3, 4])
set([4])
set([])
>>>
set([1, 2, 3, 5])
set([2, 3, 5])
set([2, 3])
set([2])
set([])
>>>
set([1, 2, 3, 5])
set([1, 2, 3])
set([1, 2])
set([1])
set([])
foo = ['a', 'b', 'c', 'd', 'e']
number_of_samples = 1
In python 2:
random_items = random.sample(population=foo, k=number_of_samples)
In python 3:
random_items = random.choices(population=foo, k=number_of_samples)