How do i randomly select a string from a list in python?

How to randomly select an item from a list?

Assume I have the following list:

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• How to randomly select an item from a list?
• Pick a random element: random.choice()
• Random sample without replacement: random.sample()
• Random sample with replacement: random.choices()
• Initialize the random number generator: random.seed()
• How do you randomly select an item from a list in Python?
• How do you generate a random string in Python?
• How do I randomly select multiple elements from a list in Python?
• How do you randomly select N items from a 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
>

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')

A comment:

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

In Python, you can randomly sample elements from a list with choice(), sample(), and choices() of the random module. These functions can also be applied to a string and tuple.

choice() returns one random element, and sample() and choices() return a list of multiple random elements. sample() is used for random sampling without replacement, and choices() is used for random sampling with replacement.

• random — Generate pseudo-random numbers — Python 3.8.1 documentation

This article describes the following contents.

• Pick a random element: random.choice()
• Random sample without replacement: random.sample()
• Random sample with replacement: random.choices()
• Initialize the random number generator: random.seed()

If you want to select elements from a list by conditions, see the following article.

• Extract, replace, convert elements of a list in Python

If you want to shuffle the entire list, or get a list of random numbers, see the following article.

• Shuffle a list, string, tuple in Python (random.shuffle, sample)
• Generate random int/float in Python (random, randrange, randint, etc.)

Pick a random element: random.choice()

random.choice() returns a random element from a list.

• random.choice() — Generate pseudo-random numbers — Python 3.8.1 documentation

import random

l = [0, 1, 2, 3, 4]

print(random.choice(l))
# 1

Tuples and strings are processed similarly. In the case of a string, one character is returned.

print(random.choice(('xxx', 'yyy', 'zzz')))
# yyy

print(random.choice('abcde'))
# b

An error is raised if a list, tuple, or string is empty.

# print(random.choice([]))
# IndexError: Cannot choose from an empty sequence

Random sample without replacement: random.sample()

random.sample() randomly samples multiple elements from a list without replacement.

Pass a list to the first argument and the number of elements you want to get to the second argument. A list is returned.

• random.sample — Generate pseudo-random numbers — Python 3.8.1 documentation

import random

l = [0, 1, 2, 3, 4]

print(random.sample(l, 3))
# [1, 3, 2]

print(type(random.sample(l, 3)))
# 

If the second argument is set to 1, a list with one element is returned. If set to 0, an empty list is returned. If set to the value that exceeds the number of elements of the list, an error is raised.

print(random.sample(l, 1))
# [0]

print(random.sample(l, 0))
# []

# print(random.sample(l, 10))
# ValueError: Sample larger than population or is negative

Even if you pass a tuple or a string to the first argument, the list is returned.

print(random.sample(('xxx', 'yyy', 'zzz'), 2))
# ['xxx', 'yyy']

print(random.sample('abcde', 2))
# ['a', 'e']

To convert a list to a tuple or string, use tuple(), join().

• Convert list and tuple to each other in Python
• Concatenate strings in Python (+ operator, join, etc.)

print(tuple(random.sample(('xxx', 'yyy', 'zzz'), 2)))
# ('yyy', 'xxx')

print(''.join(random.sample('abcde', 2)))
# de

Note that if the original list or tuple contains duplicate elements, the same value may be selected.

l_dup = [0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3]

print(random.sample(l_dup, 3))
# [3, 1, 1]

If you want to avoid duplicate values, use set() to convert lists and tuples to set, extract only unique elements, and then use sample().

• Remove/extract duplicate elements from list in Python

print(set(l_dup))
# {0, 1, 2, 3}

print(random.sample(set(l_dup), 3))
# [1, 3, 2]

Random sample with replacement: random.choices()

random.choices() randomly samples multiple elements from a list with replacement.

random.choices() was added in Python 3.6 and cannot be used in earlier versions.

• random.choices() — Generate pseudo-random numbers — Python 3.8.1 documentation

Specify the number of elements you want to get with the argument k. Since elements are chosen with replacement, k can be larger than the number of elements in the original list.

Since k is a keyword-only argument, it is necessary to specify like k=3.

import random

l = [0, 1, 2, 3, 4]

print(random.choices(l, k=3))
# [2, 1, 0]

print(random.choices(l, k=10))
# [3, 4, 1, 4, 4, 2, 0, 4, 2, 0]

k is set to 1 by default. If omitted, a list with one element is returned.

print(random.choices(l))
# [1]

You can specify the weight (probability) for each element to the weights argument. The type of the list element specified in weights can be either int or float. If set to 0, the element is not selected.

print(random.choices(l, k=3, weights=[1, 1, 1, 10, 1]))
# [0, 2, 3]

print(random.choices(l, k=3, weights=[1, 1, 0, 0, 0]))
# [0, 1, 1]

Cumulative weights can be specified in the argument cum_weights. cum_weights in the following sample code is equivalent to the first weights in the above code.

print(random.choices(l, k=3, cum_weights=[1, 2, 3, 13, 14]))
# [3, 2, 3]

By default, both weights and cum_weights are set to None, and each element is selected with the same probability.

If the length (number of elements) of weights or cum_weights is different from the original list, an error is raised.

# print(random.choices(l, k=3, weights=[1, 1, 1, 10, 1, 1, 1]))
# ValueError: The number of weights does not match the population_

Also, an error is raised if you specify weights and cum_weights at the same time.

# print(random.choices(l, k=3, weights=[1, 1, 1, 10, 1], cum_weights=[1, 2, 3, 13, 14]))
# TypeError: Cannot specify both weights and cumulative weights

In the sample code so far, a list was specified to the first argument, but the same applies to a tuple and string.

Initialize the random number generator: random.seed()

You can initialize a random number generator with random.seed().

After initializing with the same seed, the element is selected in the same way.

random.seed(0)
print(random.choice(l))
# 3

random.seed(0)
print(random.choice(l))
# 3

How do you randomly select an item from a list in Python?

How do you generate a random string in Python?

How do I randomly select multiple elements from a list in Python?

How do you randomly select N items from a list?