Python defaultdict but support key-wise default value

August 2, 2023

I want to define a defaultdict, so that if the key is missing, its value should be f'You can set {key} to inject configuration here

Consider the following code

from collections import defaultdict

# Valid, but the default value for all key will be empty string.
d1 = defaultdict(str)  
d1['post_simulation']  # return ''

# This is what I intend to do, 
# but it is invalid as the default_factory won't take any arguments.
d2 = defaultdict(lambda k: f'#You can inject text here by setting {k}')
d2['post_simulation'] 
# TypeError: <lambda>() missing 1 required positional argument: 'k'

Is there any tech that I can do this in Python by using defaultdict or other data structure?

>Solution :

Yes, by overriding the default __missing__ method:

from collections import defaultdict


class MyDefaultDict(defaultdict):
    def __missing__(self, key):
        self[key] = f'#You can inject text here by setting {key}'
        return self[key]


d = MyDefaultDict()
print(d["foo"])
# => #You can inject text here by setting foo

You can generalise it like this:

from collections import defaultdict


class LambdaDefaultDict(defaultdict):
    def __init__(self, missing, *args, **kwargs):
        super().__init__(None, *args, **kwargs)
        self.missing = missing

    def __missing__(self, key):
        self[key] = self.missing(key)
        return self[key]


d = LambdaDefaultDict(lambda k: f'#You can inject text here by setting {k}')
print(d["foo"])