Python dict how to create key or append an element to key?

Use dict.setdefault():

dic.setdefault(key,[]).append(value)

help(dict.setdefault):

    setdefault(...)        D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D

Here are the various ways to do this so you can compare how it looks and choose what you like. I've ordered them in a way that I think is most "pythonic", and commented the pros and cons that might not be obvious at first glance:

Using collections.defaultdict:

import collectionsdict_x = collections.defaultdict(list)...dict_x[key].append(value)

Pros: Probably best performance. Cons: Not available in Python 2.4.x.

Using dict().setdefault():

dict_x = {}...dict_x.setdefault(key, []).append(value)

Cons: Inefficient creation of unused list()s.

Using try ... except:

dict_x = {}...try:    values = dict_x[key]except KeyError:    values = dict_x[key] = []values.append(value)

Or:

try:    dict_x[key].append(value)except KeyError:    dict_x[key] = [value]

You can use a defaultdict for this.

from collections import defaultdictd = defaultdict(list)d['key'].append('mykey')

This is slightly more efficient than setdefault since you don't end up creating new lists that you don't end up using. Every call to setdefault is going to create a new list, even if the item already exists in the dictionary.