Question

I’m trying to create a dictionary inside a dictionary with one statement. If the key doesn’t exist it should be created.

A code-snippet for what I have now:

self.data[self.stringvar1.get()] = { date : (int(self.total.get()), int(self.resources.get())) }

This doesn’t create a new key, but overrides the self.data, even if stringvar1 was different. I’ve tried a few options and couldn’t find what I want. Do I have to manually check if the key exists, or is there an easy idiom for this?

Answer 1

One option is to make self.data a collections.defaultdict(dict). Accessing non-existent keys on this defaultdict automatically creates a new dictionary for this key, and you can simply use

self.data[self.stringvar1.get()][date] = (
    int(self.total.get()), int(self.resources.get()))

Note that a defaultdict comes with the risk of hiding bugs, since you will never get a KeyError. An alternative is to use a plain dict and the setdefault() method:

self.data.setdefault(self.stringvar1.get(), {})[date] = (
    int(self.total.get()), int(self.resources.get()))