Python dictionary key error when assigning - how do I get around this?

Question

I have a dictionary that I create like this:

myDict = {} 

Then I like to add key in it that corresponds to another dictionary, in which I put another value:

myDict[2000]['hello'] = 50 

So when I pass myDict[2000]['hello'] somewhere, it would give 50.

Why isn't Python just creating those entries right there? What's the issue? I thought KeyError only occurs when you try to read an entry that doesn't exist, but I'm creating it right here?

Answer 1

KeyError occurs because you are trying to read a non-existant key when you try to access myDict[2000]. As an alternative, you could use defaultdict:

>>> from collections import defaultdict >>> myDict = defaultdict(dict) >>> myDict[2000]['hello'] = 50 >>> myDict[2000] {'hello': 50} 

defaultdict(dict) means that if myDict encounters an unknown key, it will return a default value, in this case whatever is returned by dict() which is an empty dictionary.

Answer 2

But you are trying to read an entry that doesn't exist: myDict[2000].

The exact translation of what you say in your code is "give me the entry in myDict with the key of 2000, and store 50 against the key 'hello' in that entry." But myDict doesn't have a key of 2000, hence the error.

What you actually need to do is to create that key. You can do that in one go:

myDict[2000] = {'hello': 50}