Here's a quick solution:
l = ['apple','bat','apple','car','pet','bat']
Create a dict that maps all unique strings to integers:
d = dict([(y,x+1) for x,y in enumerate(sorted(set(l)))])
Map each string in the original list to its respective integer:
print [d[x] for x in l]
# [1, 2, 1, 3, 4, 2]
