Demo Python to put data into randomly-selected, previously-empty element of a source list.

find_empties.py

import random

# Initial list of references
source = [0, 1, 3, 4, 0, 5, 7, 0]
print source

# Build list of indices of source where content is zero
emptylist = [i for i, element in enumerate(source) if element == 0]
print emptylist

# Select one of those indices at random
select = random.choice(emptylist)
print select

# Assign a value to the previously-empty element of the source list
source[select] = "TADA!"
print source