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?
This question is related to
python
list
dictionary
You could use this:
data[:1] = ['hello']
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]}.
You are populating your dictionaries with references to a single list so when you update it, the update is reflected across all the references. Try a dictionary comprehension instead. See Create a dictionary with list comprehension in Python
d = {k : v for k in blah blah blah}
Source: Stackoverflow.com