Keeping Lists in Dictionaries

Here's a cute technique I saw, for keeping lists inside dictionaries.

If you have a dictionary with list values, you can run into a problem when you need to add an item to a list value- does the list already exist?

Usually, we write it out like so:

   1 def add_value_to_keys_list(dict_, key, value):
   2     if key not in dict_:
   3         dict_[key] = [value]  # Construct list
   4     else:
   5         dict_[key].append(value)  # Append to list

This is okay, but we can do even better!

   1 def add_value_to_keys_list(dict_, key, value):
   2     dict_.setdefault(key, []).append(value)

How does it work?

.setdefault works like .get, except that when the item isn't found, the default isn't only returned, but they key is set to the default as well.

So, in effect, the line means, "Look in the dictionary for the key. If it's there, return it's associate. If it's not there, set it's associate to [], and then return it's new associate. The associate is (surprise, surprise) - a list. Now, append the value to the list."

Cute!