Generate Random Integers
Problem to Solve
I want an integer randomly chosen from some range.
If you need your code to randomly pick an integer within some range, with all outcomes equally likely (like rolling a die), this is your recipe.
All of these recipes use the random module, which is part of the Python standard library. You can read about it in the Python documentation. For the sake of brevity, the recipes will assume that import random has already been executed.
Recipes
generating a random integer between 0 and \(N - 1\), inclusive
The random module's basic function for generating random integers is randrange(…). Like the range(...) function for iteration, randrange(N) produces a randomly-chosen integer between 0 and \(N - 1\), inclusive.
To select a random element from a list or other indexable sequence:
one_value = random.randrange(len(my_list))
If you want to specify a lower bound other than zero, call the function with two arguments. For example, to select any value from a list other than the first:
one_value = random.randrange(1, len(my_list))
The randrange() function can take an optional third argument that specifies an increment or step, so you could conceivably get it to produce a randomly-chosen integer from the set \(\{2, 5, 8, 11, \dots, 32\}\) by calling random.randrange(2, 33, 3). It doesn't take keyword arguments, though, so read the documentation if you want to do that.
generating an integer between \(a\) and \(b\), inclusive
A common annoyance and source of bugs is the fact that we often think "I want a random number from 1 to 20", and accidentally write random.randint(1, 20), which can never produce 20 as a result. Unless you need the weird step functionality of randrange, it's generally easier and safer to use randint(a, b), which produces a random integer between a and b, inclusive.
To simulate a six-sided die:
roll_value = random.randint(1, 6)
To generate random integer coordinates between -10 and +10 inclusive:
x = random.randint(-10, 10)
y = random.randint(-10, 10)
To simulate rolling two six-sided dice and adding their values:
craps_roll = random.randint(1, 6) + random.randint(1, 6)
Make sure not to use 2 * random.randint(1, 6) instead. That simulates rolling one die and doubling it.