Slicing a dictionary

Question

I have a dictionary, and would like to pass a part of it to a function, that part being given by a list (or tuple) of keys. Like so:

# the dictionary
d = {1:         


        

Answer 1

Write a dict subclass that accepts a list of keys as an "item" and returns a "slice" of the dictionary:

class SliceableDict(dict):
    default = None
    def __getitem__(self, key):
        if isinstance(key, list):   # use one return statement below
            # uses default value if a key does not exist
            return {k: self.get(k, self.default) for k in key}
            # raises KeyError if a key does not exist
            return {k: self[k] for k in key}
            # omits key if it does not exist
            return {k: self[k] for k in key if k in self}
        return dict.get(self, key)

Usage:

d = SliceableDict({1:2, 3:4, 5:6, 7:8})
d[[1, 5]]   # {1: 2, 5: 6}

Or if you want to use a separate method for this type of access, you can use * to accept any number of arguments:

class SliceableDict(dict):
    def slice(self, *keys):
        return {k: self[k] for k in keys}
        # or one of the others from the first example

d = SliceableDict({1:2, 3:4, 5:6, 7:8})
d.slice(1, 5)     # {1: 2, 5: 6}
keys = 1, 5
d.slice(*keys)    # same