Updating Variables Pointing to the Same Dictionary in Python

Question

My question emerged when looking here to find how to create a trie in python. The following code was given in the top-voted answer:

>>> _end = '_end_'
>>> 
>>> def make_trie(*words):
...     root = dict()
...     for word in words:
...         current_dict = root
...         for letter in word:
...             current_dict = current_dict.setdefault(letter, {})
...         current_dict[_end] = _end
...     return root
... 
>>> make_trie('foo', 'bar', 'baz', 'barz')
{'b': {'a': {'r': {'_end_': '_end_', 'z': {'_end_': '_end_'}}, 
'z': {'_end_': '_end_'}}}, 'f': {'o': {'o': {'_end_': '_end_'}}}}

I don't understand what purpose the line "current_dict = root" serves; seems like deleting that line and substituting all current_dict with root would do the same thing. (This same thought is expressed in this reply but with no answer.) I know this actually doesn't work as I tried it and an empty dictionary was returned.

I also tried putting print statements in the second for loop to see how current_dict and root were updated. I thought that since they were set to be equal, they referred to the same dictionary and would be updated simultaneously, but that wasn't the case.

Clearly, I have a fundamental misunderstanding of this interaction. Help?

Answer 1

You have to reset current_dict = root for each single word, because current_dict = current_dict.setdefault(letter, {}) sets current_dict to a new empty dictionary or existing sub-dictionary of root if the key is already in the dict.

dict.setdefault(k, d) is a bit tricky because it does two things at the same time. It works just like dict.get and returns the value of the key k if it exists, otherwise the default value d . And if the key doesn't exist it also inserts it with d as the value.

So, as you see, current_dict is not always the root dict, but also refers to the sub-dicts when you're iterating over the letters in the word, and you have to reset it to root to start again at the top level.