Assume I have the following list:
foo = [\'a\', \'b\', \'c\', \'d\', \'e\']
What is the simplest way to retrieve an item at random from thi
if you need the index just use:
import random
foo = ['a', 'b', 'c', 'd', 'e']
print int(random.random() * len(foo))
print foo[int(random.random() * len(foo))]
random.choice does the same:)
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.
This may already be an answer but you could use random.shuffle
. Example:
import random
foo = ['a', 'b', 'c', 'd', 'e']
random.shuffle(foo)
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)
How to randomly select an item from a list?
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?
If you want close to truly random, then I suggest secrets.choice
from the standard library (New in Python 3.6.):
>>> from secrets import choice # Python 3 only
>>> choice(list('abcde'))
'c'
The above is equivalent to my former recommendation, using a SystemRandom
object from the random
module with the choice
method - available earlier in Python 2:
>>> import random # Python 2 compatible
>>> sr = random.SystemRandom()
>>> foo = list('abcde')
>>> foo
['a', 'b', 'c', 'd', 'e']
And now:
>>> sr.choice(foo)
'd'
>>> sr.choice(foo)
'e'
>>> sr.choice(foo)
'a'
>>> sr.choice(foo)
'b'
>>> sr.choice(foo)
'a'
>>> sr.choice(foo)
'c'
>>> sr.choice(foo)
'c'
If you want a deterministic pseudorandom selection, use the choice
function (which is actually a bound method on a Random
object):
>>> random.choice
<bound method Random.choice of <random.Random object at 0x800c1034>>
It seems random, but it's actually not, which we can see if we reseed it repeatedly:
>>> random.seed(42); random.choice(foo), random.choice(foo), random.choice(foo)
('d', 'a', 'b')
>>> random.seed(42); random.choice(foo), random.choice(foo), random.choice(foo)
('d', 'a', 'b')
>>> random.seed(42); random.choice(foo), random.choice(foo), random.choice(foo)
('d', 'a', 'b')
>>> random.seed(42); random.choice(foo), random.choice(foo), random.choice(foo)
('d', 'a', 'b')
>>> random.seed(42); random.choice(foo), random.choice(foo), random.choice(foo)
('d', 'a', 'b')
This is not about whether random.choice is truly random or not. If you fix the seed, you will get the reproducible results -- and that's what seed is designed for. You can pass a seed to SystemRandom, too.
sr = random.SystemRandom(42)
Well, yes you can pass it a "seed" argument, but you'll see that the SystemRandom object simply ignores it:
def seed(self, *args, **kwds):
"Stub method. Not used for a system random number generator."
return None