I need a dictionary that is automatically filled with a default value for each accessed key that is missing. I've found defaultdict and some other ways to achieve this, but the problem in my case is that I want the default value for each key to be specific to the key itself.
For example, with defaultdict I can achieve something like this:
from collections import defaultdict
d = defaultdict(lambda: 5)
> d[1] = 3
> d[1]
> 3
> d[2]
> 5
But what if I need the default value for each accessed missing key to be for example key + 5? Something like:
from collections import defaultdict
d = defaultdict(lambda key: key + 5) # <-- This does not work as defaultdict expects lambda function to be without any parameters
> d[1] = 3
> d[1]
> 3
> d[2]
> 7 <- Calculated from accessed key + 5 (2+5)
> d[5]
> 10 <- Calculated from accessed key + 5 (5+5)
Is there a clean, builtin way to achieve what I need? I know that I can create a subclass of dict and implement this specific functionality at __getitem__ level, but I want to avoid that if possible.
I couldn't find a solution in other answers, so sorry if it is still a duplicate.
I don't think there is a builtin way of doing this. However, instead of subclassing dict and change getitem, you can subclass defaultdict itself to tell __missing__() to call the default_factory with an argument (the key), rather than without args. It is still a custom class, not a builtin, but it should be quite efficient still.
from collections import defaultdict
class DefaultDict(defaultdict):
def __missing__(self, key):
return self.default_factory(key)
Then:
d = DefaultDict(lambda key: key + 5)
d[2]
# 7