import random

my_dict = {
    'animals' : ['dog', 'cat', 'mouse']
    }

# Makes a list of words.
my_str = 'john have some animals'.split()

# Finds a matching key in my_dict based on every item
# in my_str.
my_choice = [my_dict.get(x, x) for x in my_str]

# What happens is the 'animals' item in my_str matches
# the 'animals' key in my_dict, therefore replacing the
# 'animals' item to my_dict['animals'] (that is, ['dog',
# 'cat', 'mouse']).

# my_str is now ['john', 'have', 'some', ['dog', 'cat',
# 'mouse']].

# What you wanted is to choose an item on the list in
# my_str, making it either:
# 'john have some dog'
# 'john have some cat'
# 'john have some mouse'