Question

I have a while loop:

while parent != None:
     pathList.append(parentMap[parent])
     parent = parentMap[parent]

What I want to do with this list is simply add the parent value (already determined in other parts of the code) of a parent value to a list until I’ve reached the top of a tree and there are no more parents remaining. The while loop operates under the idea that once parentMap[parent] is provided with an invalid [parent] key, it will be assigned the value of “None” and the loop will terminate. This doesn’t happen. Instead, once I get to the topmost key, (5, 5), the program throws an error stating “invalid key.” Thoughts?

Answer 1

The problem, as others have also pointed out, is in the fact that a dict doesn’t return None if a key isn’t found but instead raises a KeyError exception (as seen in the documentation).

To overcome this you could use defaultdict from collections and set None to be the default value for that dict. I recommend you read the classes documentation.

In this case, instead of initializing your dict as parentMap = dict(some_initializer) or parentMap = {} you could use parentMap = defaultdict(lambda: None, some_initializer) or parentMap = defaultdict(lambda: None, {}) and missing keys will return None rather than raising KeyError exceptions.

In an unrelated issue, None is a singleton, It’s better to check if an item is not None rather than using !=. It generates clearer code, in my humble opinion.