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

When [] is passed as the second argument to dict.fromkeys(), all values in the resulting dict will be the same list object.

In Python 2.7 or above, use a dict comprehension instead:

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

In earlier versions of Python, there is no dict comprehension, but a list comprehension can be passed to the dict constructor instead:

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

In 2.4-2.6, it is also possible to pass a generator expression to dict, and the surrounding parentheses can be dropped:

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