Choosing Random Items from a Collection

In the Select from a Collection section, we see how to use the random module to pick one item at random from a string or a list.

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import random

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
title = 'The Princess Bride'

# Select a random element from the colors list:
color_choice = random.choice(colors)

# Select a random character from the title string:
char_choice = random.choice(title)

Just like with random numbers, we can use the accumulator pattern to build a new collection by randomly selecting items from a different collection.

Try It!

Run the following program several times to see how it works!

Choose Random, Unique Values

Just like with generating a list of random numbers, if we run the program above several times, we will notice repeated characters in our collections.

To generate a set of unique characters (no repeats), we must use a while loop as we did before:

Try It!

Paste this code into the editor above to practice making unique choice collections.

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import string
import random

char_string = ''
collection_length = 10

while len(char_string) < collection_length:
   # Select a random letter from 'ABCDEFGHIJKLMNOPQRSTUVWXYZ':
   new_char = random.choice(string.ascii_uppercase)

   # Check if new_char is NOT already in char_string:
   if new_char not in char_string:
      char_string += new_char

print(char_string)

A slight modification to the code will append the random choice to a list instead.

Choose From a Dictionary

If we have a dictionary, we can select a random element from it using the following syntax:

random.choice(list(dictionary_name.keys()))
random.choice(list(dictionary_name.values()))

The first line selects a key from dictionary_name. Note that with this key we also get access to its value. The second line selects a value from dictionary_name, but NOT the key it is linked to.

Note the use of the list() function! When Python pulls from a collection, it does so by using an index value. Since dictionaries are unordered, they have no indexes. Thus, we need to convert the keys (or values) into a list format before making the random choice.

If we leave off the method name:

random.choice(list(dictionary_name))

Python defaults to selecting from the keys.

Try It!

Run the following program several times to see how the results change.

Now try this:

  1. Replace lines 16 & 17 with

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    item = random.choice(list(raffle_tickets.items()))
print(item)
print(item[0], item[1])

  2. What do you notice?