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?

Asked By: ulak blade

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Answer 1

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}

Answered By: Daniel Roseman

Answer 2

You’re right, but in your code python has to first get myDict[2000] and then do the assignment. Since that entry doesn’t exist it can’t assign to its elements

Answered By: Snakes and Coffee

Answer 3

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.

Answered By: OldGeeksGuide

Answer 4

What you want is to implement a nested dict:

I recommend this approach:

class Vividict(dict):
    def __missing__(self, key):
        value = self[key] = type(self)()
        return value

From the docs, under d[key]

New in version 2.5: If a subclass of dict defines a method
__missing__(), if the key key is not present, the d[key]
operation calls that method with the key key as argument

To try it:

myDict = Vividict()

myDict[2000]['hello'] = 50

and myDict now returns:

{2000: {'hello': 50}}

And this will work for any arbitrary depth you want:

myDict['foo']['bar']['baz']['quux']

just works.

Answered By: Russia Must Remove Putin

Answer 5

According to the below scenario, when you append type new_result into dict, you will get KeyError: 'result'

    dict = {}
    new_result = {'key1':'new_value1','key2':'new_value'}
    dict['result'].append(new_result)

Because key doesn’t exist in other words your dict doesn’t have a result key. I fixed this problem with defaultdict and their setdefault method.

To try it;

    from collections import defaultdict
    dict = defaultdict(dict)
    new_result = {'key1':'new_value1','key2':'new_value2'}
    dict.setdefault('result', []).append(new_result)

Answered By: Mehmet Sergen Ergen

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

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