Calling a function in the dictionary in parallel with obtaining a value

Question

I want to pass a key to a dictonary, get an attached to this key value and also call a function attached to same key if the function is present. I'm able to create something like this d={'some_key':('value', func())}, but it calls the function when the dictonary is initialized. I know that I can get a tuple with value and fucntion, check the length of this tuple and if the length equals 2 then call a function, but isn't there a more elegant way? Can I somehow make the functon activate only when I input a certain key without any other syntax? Just write d[some_key], get a corresponding value and execute a function without any additional brackets.

Answer 1

You'd need to subclass dict to override the __getitem__ method:

class MyDict(dict):
    def __getitem__(self, index):
        a, b = super().__getitem__(index)
        return a, b()

def myfunc():
    return "Hello world"

mydict = MyDict({'a': (100, myfunc)})

print(mydict['a'])

outputs (100, 'Hello world')

If you want to call the function and return the value:

class MyDict(dict):
    def __getitem__(self, index):
        a, b = super().__getitem__(index)
        b()
        return a

Note that this is very unexpected behavior, so make sure your users know what will happen when they use this dictionary.