如何从字典中的列表之一中随机选择一项，并获取列表的键？

Question

I have a dictionary of lists:

lists=dict(animals=["dog","cat","shark"],
           things=["desk","chair","pencil"],
           food=["spaghetti","ice-cream","potatoes"])

How can I get Python to randomly pick an item from one of the lists and tell me what list it was in? Or how do I pick a key from the dictionary and then a value from the list corresponding to that key?

For example:

dog - from animals
potatoes - from food

Answer 1

random.choice picks a random item from a sequence:

import random

Select the key to draw from your dict , which you've named lists :

which_list = random.choice(lists.keys())

Then, use that key to get a list from the dict :

item = random.choice(lists[which_list])

If you need equal weighting:

import random

which_list, item = random.choice([(name, value) 
                                     for name, values in lists.iteritems() 
                                         for value in values])

Answer 2

Two approaches I can think of right away:

• pick a list name (key) first, then pick an entry from it -- you'll just have to be careful if the lists have different lengths, if you want an uniform distribution
• flatten the dict of lists into one list of ('list-name','value') pairs (easier to get the uniform distribution right no matter how many entries there are per list, but requires more memory)

One way to do the former:

from itertools import chain
import random
weight_choices = list(chain(*([name] * len(values) for (name, values) in lists.iteritems()))) # generate a list of the form ("animals", "animals", "animals", ...)
list_name = random.choice(weight_choice) # The list it's chosen from...
chosen_item = random.choice(lists[list_name]) # and the item itself

(and if you don't care about getting an uniform distribution between lists:)

import random
list_name = random.choice(lists.keys())
chosen_item = random.choice(lists[list_name])

... and the latter method:

from itertools import chain, repeat
all_items = list(chain(*((zip(repeat(name), values) for (name, values) in lists.iteritems()))))
list_name, chosen_item = random.choice(all_items)

and a less itertools way for the latter:

all_items = []
for name, values in lists.iteritems():
  for value in values:
    all_items.append((name, value))
list_name, chosen_item = random.choice(all_items)