python - Can I have a dictionary with same-name keys?

Question

I need to have a dictionary which might have same names for some keys and return a list of values when referencing the key in that case.

For example

print mydict['key']
[1,2,3,4,5,6]

Answer 1

For consistency, you should have the dictionary map keys to lists (or sets) of values, of which some can be empty. There is a nice idiom for this:

from collections import defaultdict
d = defaultdict(set)

d["key"].add(...)

(A defaultdict is like a normal dictionary, but if a key is missing it will call the argument you passed in when you instantiated it and use the result as the default value. So this will automatically create an empty set of values if you ask for a key which isn't already present.)

---

If you need the object to look more like a dictionary (i.e. to set a value by d["key"] = ...) you can do the following. But this is probably a bad idea, because it goes against the normal Python syntax, and is likely to come back and bite you later. Especially if someone else has to maintain your code.

class Multidict(defaultdict):
    def __init__(self):
        super(Multidict, self).__init__(set)

    def __setitem__(self, key, value):
        if isinstance(value, (self.default_factory)): # self.default_factory is `set`
            super().__setitem__(key, value)
        else:
            self[key].append(value)

_{I haven't tested this.}