Why am I gettng a KeyError when adding a key to a dictonary?

Question

I am still trying to learn the ins and outs of Python dictionaries. When I run this:

#!/usr/bin/env python3
d = {}
d['foo']['bar'] = 1

I get KeyError: 'foo'. But in How can I add new keys to a dictionary? it says that "you create a new key\value pair on a dictionary by assigning a value to that key. If the key doesn't exist, it's added and points to that value. If it exists, the current value it points to is overwritten." So why am I getting the key error?

You're not assigning to `d['foo']`. You're assigning to `d['foo']['bar']`, which requires that `d['foo']` already exists. Just as assigning to `e['bar']` requires that `e` exists. — khelwood, Feb 05 '21 at 01:00

score 3 · Accepted Answer · answered Feb 05 '21 at 01:48

3

You have, at least, two options:

Create nested dictionaries in order:

d = {}
d['foo'] = {}
d['foo']['bar'] = 1

Use collections.defaultdict, passing the default factory as dict:

from collections import defaultdict

d = defaultdict(dict)
d['foo']['bar'] = 1

answered Feb 05 '21 at 01:48

Pedro Lobito

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Thanks all. I figured it was something like that. I do most of my scripting in Perl, where you can just do things like that on the fly. It's great. – Steve Feb 05 '21 at 15:14
#2 is such a nice lifehack. Glad i found this. – cowlinator Apr 20 '23 at 02:36
Note: it only works for 1 level of nesting. – cowlinator Apr 20 '23 at 03:47

score 0 · Answer 2 · answered Feb 05 '21 at 01:50

0

you need to assign d['key'] to be a dictionary first

d['a']={}
d['a']['b']= value

answered Feb 05 '21 at 01:50

mutong

31
1

Why am I gettng a KeyError when adding a key to a dictonary?

2 Answers2