Fmrubio Fmrubio - 1 month ago 8
Python Question

Initialize List to a variable in a Dictionary inside a loop

I have been working for a while in Python and I have solved this issue using "try" and "except", but I was wondering if there is another method to solve it.

Basically I want to create a dictionary like this:

example_dictionary = {"red":[2,3,4],"blue":[6,7,8],"orange":[10,11,12]}


So if I have a variable with the following content:

root_values = [{"name":"red","value":2},{"name":"red","value":3},{"name":"red","value":4},{"blue":6}...]


My way to implement the example_dictionary was:

example_dictionary = {}
for item in root_values:
try:
example_dictionary[item.name].append(item.value)
except:
example_dictionary[item.name] =[item.value]


I hope my question is clear and someone can help me with this.

Thanks.

Answer

Your code is not appending elements to the lists; you are instead replacing the list with single elements. To access values in your existing dictionaries, you must use indexing, not attribute lookups (item['name'], not item.name).

Use collections.defaultdict():

from collections import defaultdict

example_dictionary = defaultdict(list)
for item in root_values:
    example_dictionary[item['name']].append(item['value'])

defaultdict is a dict subclass that uses the __missing__ hook on dict to auto-materialize values if the key doesn't yet exist in the mapping.

or use dict.setdefault():

example_dictionary = {}
for item in root_values:
    example_dictionary.setdefault(item['name'], []).append(item['value'])
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