How do I initialize a dictionary of empty lists in Python?

Question

My attempt to programmatically create a dictionary of lists is failing to allow me to individually address dictionary keys. Whenever I create the dictionary of lists and try to append to one key, all of them are updated. Here's a very simple test case:

data = {} data = data.fromkeys(range(2),[]) data[1].append('hello') print data 

Actual result: {0: ['hello'], 1: ['hello']}

Expected result: {0: [], 1: ['hello']}

Here's what works

data = {0:[],1:[]} data[1].append('hello') print data 

Actual and Expected Result: {0: [], 1: ['hello']}

Why is the fromkeys method not working as expected?

Answer 1

Passing [] as second argument to dict.fromkeys() gives a rather useless result – all values in the dictionary will be the same list object.

In Python 2.7 or above, you can use a dicitonary comprehension instead:

data = {k: [] for k in range(2)} 

In earlier versions of Python, you can use

data = dict((k, []) for k in range(2)) 

Answer 2

Use defaultdict instead:

from collections import defaultdict data = defaultdict(list) data[1].append('hello') 

This way you don't have to initialize all the keys you want to use to lists beforehand.

What is happening in your example is that you use one (mutable) list:

alist = [1] data = dict.fromkeys(range(2), alist) alist.append(2) print data 

would output {0: [1, 2], 1: [1, 2]}.