random text keys and class instances

'example code creates a dictionary of class instances with random text keys'

from random import choices, randint
import string

class Example():
    def __init__(self):
        self.num = randint(1, 100)

def unique_key(size, instances):
    while True:
        key = ''.join(choices(string.ascii_lowercase +
                             string.digits, k = size))
        if key not in instances:
            return key


size = 10  # number of instances I require

instances = {}
for _ in range(size):
    instances[unique_key(size, instances)] = Example()

for name, instance in instances.items():
    print(f'{name}: random number: {instance.num}')