Question

Here’s a common situation when compiling data in dictionaries from different sources:

Say you have a dictionary that stores lists of things, such as things I like:

likes = {
    'colors': ['blue','red','purple'],
    'foods': ['apples', 'oranges']
}

and a second dictionary with some related values in it:

favorites = {
    'colors':'yellow',
    'desserts':'ice cream'
}

You then want to iterate over the “favorites” object and either append the items in that object to the list with the appropriate key in the “likes” dictionary or add a new key to it with the value being a list containing the value in “favorites”.

There are several ways to do this:

for key in favorites:
    if key in likes:
        likes[key].append(favorites[key])
    else:
        likes[key] = list(favorites[key])

or

for key in favorites:
    try:
        likes[key].append(favorites[key])
    except KeyError:
        likes[key] = list(favorites[key])

And many more as well…

I generally use the first syntax because it feels more pythonic, but if there are other, better ways, I’d love to know what they are. Thanks!

Answer 1

Use collections.defaultdict, where the default value is a new list instance.

>>> import collections
>>> mydict = collections.defaultdict(list)

In this way calling .append(...) will always succeed, because in case of a non-existing key append will be called on a fresh empty list.

You can instantiate the defaultdict with a previously generated list, in case you get the dict likes from another source, like so:

>>> mydict = collections.defaultdict(list, likes)

Note that using list as the default_factory attribute of a defaultdict is also discussed as an example in the documentation.