How to select an item from a list with known percentages in Python

Question

I wish to select a random word from a list where the is a known chance for each word, for example:

Fruit with Probability

Orange 0.10 Apple 0.05 Mango 0.15 etc

How would be the best way of implementing this? The actual list I will take from is up to 100 items longs and the % do not all tally to 100 % they do fall short to account for the items that had a really low chance of occurrence. I would ideally like to take this from a CSV which is where I store this data. This is not a time critical task.

Thank you for any advice on how best to proceed.

Answer 1

You can pick items with weighted probabilities if you assign each item a number range proportional to its probability, pick a random number between zero and the sum of the ranges and find what item matches it. The following class does exactly that:

from random import random

class WeightedChoice(object):
    def __init__(self, weights):
        """Pick items with weighted probabilities.

            weights
                a sequence of tuples of item and it's weight.
        """
        self._total_weight = 0.
        self._item_levels = []
        for item, weight in weights:
            self._total_weight += weight
            self._item_levels.append((self._total_weight, item))

    def pick(self):
        pick = self._total_weight * random()
        for level, item in self._item_levels:
            if level >= pick:
                return item

You can then load the CSV file with the csv module and feed it to the WeightedChoice class:

import csv

weighed_items = [(item,float(weight)) for item,weight in csv.reader(open('file.csv'))]
picker = WeightedChoice(weighed_items)
print(picker.pick())

Answer 2

What you want is to draw from a multinomial distribution. Assuming you have two lists of items and probabilities, and the probabilities sum to 1 (if not, just add some default value to cover the extra):

def choose(items,chances):
    import random
    p = chances[0]
    x = random.random()
    i = 0
    while x > p :
        i = i + 1
        p = p + chances[i]
    return items[i]